diff --git a/LICENSE.md b/LICENSE.md
new file mode 100644
--- /dev/null
+++ b/LICENSE.md
@@ -0,0 +1,373 @@
+Mozilla Public License Version 2.0
+==================================
+
+1. Definitions
+--------------
+
+1.1. "Contributor"
+    means each individual or legal entity that creates, contributes to
+    the creation of, or owns Covered Software.
+
+1.2. "Contributor Version"
+    means the combination of the Contributions of others (if any) used
+    by a Contributor and that particular Contributor's Contribution.
+
+1.3. "Contribution"
+    means Covered Software of a particular Contributor.
+
+1.4. "Covered Software"
+    means Source Code Form to which the initial Contributor has attached
+    the notice in Exhibit A, the Executable Form of such Source Code
+    Form, and Modifications of such Source Code Form, in each case
+    including portions thereof.
+
+1.5. "Incompatible With Secondary Licenses"
+    means
+
+    (a) that the initial Contributor has attached the notice described
+        in Exhibit B to the Covered Software; or
+
+    (b) that the Covered Software was made available under the terms of
+        version 1.1 or earlier of the License, but not also under the
+        terms of a Secondary License.
+
+1.6. "Executable Form"
+    means any form of the work other than Source Code Form.
+
+1.7. "Larger Work"
+    means a work that combines Covered Software with other material, in
+    a separate file or files, that is not Covered Software.
+
+1.8. "License"
+    means this document.
+
+1.9. "Licensable"
+    means having the right to grant, to the maximum extent possible,
+    whether at the time of the initial grant or subsequently, any and
+    all of the rights conveyed by this License.
+
+1.10. "Modifications"
+    means any of the following:
+
+    (a) any file in Source Code Form that results from an addition to,
+        deletion from, or modification of the contents of Covered
+        Software; or
+
+    (b) any new file in Source Code Form that contains any Covered
+        Software.
+
+1.11. "Patent Claims" of a Contributor
+    means any patent claim(s), including without limitation, method,
+    process, and apparatus claims, in any patent Licensable by such
+    Contributor that would be infringed, but for the grant of the
+    License, by the making, using, selling, offering for sale, having
+    made, import, or transfer of either its Contributions or its
+    Contributor Version.
+
+1.12. "Secondary License"
+    means either the GNU General Public License, Version 2.0, the GNU
+    Lesser General Public License, Version 2.1, the GNU Affero General
+    Public License, Version 3.0, or any later versions of those
+    licenses.
+
+1.13. "Source Code Form"
+    means the form of the work preferred for making modifications.
+
+1.14. "You" (or "Your")
+    means an individual or a legal entity exercising rights under this
+    License. For legal entities, "You" includes any entity that
+    controls, is controlled by, or is under common control with You. For
+    purposes of this definition, "control" means (a) the power, direct
+    or indirect, to cause the direction or management of such entity,
+    whether by contract or otherwise, or (b) ownership of more than
+    fifty percent (50%) of the outstanding shares or beneficial
+    ownership of such entity.
+
+2. License Grants and Conditions
+--------------------------------
+
+2.1. Grants
+
+Each Contributor hereby grants You a world-wide, royalty-free,
+non-exclusive license:
+
+(a) under intellectual property rights (other than patent or trademark)
+    Licensable by such Contributor to use, reproduce, make available,
+    modify, display, perform, distribute, and otherwise exploit its
+    Contributions, either on an unmodified basis, with Modifications, or
+    as part of a Larger Work; and
+
+(b) under Patent Claims of such Contributor to make, use, sell, offer
+    for sale, have made, import, and otherwise transfer either its
+    Contributions or its Contributor Version.
+
+2.2. Effective Date
+
+The licenses granted in Section 2.1 with respect to any Contribution
+become effective for each Contribution on the date the Contributor first
+distributes such Contribution.
+
+2.3. Limitations on Grant Scope
+
+The licenses granted in this Section 2 are the only rights granted under
+this License. No additional rights or licenses will be implied from the
+distribution or licensing of Covered Software under this License.
+Notwithstanding Section 2.1(b) above, no patent license is granted by a
+Contributor:
+
+(a) for any code that a Contributor has removed from Covered Software;
+    or
+
+(b) for infringements caused by: (i) Your and any other third party's
+    modifications of Covered Software, or (ii) the combination of its
+    Contributions with other software (except as part of its Contributor
+    Version); or
+
+(c) under Patent Claims infringed by Covered Software in the absence of
+    its Contributions.
+
+This License does not grant any rights in the trademarks, service marks,
+or logos of any Contributor (except as may be necessary to comply with
+the notice requirements in Section 3.4).
+
+2.4. Subsequent Licenses
+
+No Contributor makes additional grants as a result of Your choice to
+distribute the Covered Software under a subsequent version of this
+License (see Section 10.2) or under the terms of a Secondary License (if
+permitted under the terms of Section 3.3).
+
+2.5. Representation
+
+Each Contributor represents that the Contributor believes its
+Contributions are its original creation(s) or it has sufficient rights
+to grant the rights to its Contributions conveyed by this License.
+
+2.6. Fair Use
+
+This License is not intended to limit any rights You have under
+applicable copyright doctrines of fair use, fair dealing, or other
+equivalents.
+
+2.7. Conditions
+
+Sections 3.1, 3.2, 3.3, and 3.4 are conditions of the licenses granted
+in Section 2.1.
+
+3. Responsibilities
+-------------------
+
+3.1. Distribution of Source Form
+
+All distribution of Covered Software in Source Code Form, including any
+Modifications that You create or to which You contribute, must be under
+the terms of this License. You must inform recipients that the Source
+Code Form of the Covered Software is governed by the terms of this
+License, and how they can obtain a copy of this License. You may not
+attempt to alter or restrict the recipients' rights in the Source Code
+Form.
+
+3.2. Distribution of Executable Form
+
+If You distribute Covered Software in Executable Form then:
+
+(a) such Covered Software must also be made available in Source Code
+    Form, as described in Section 3.1, and You must inform recipients of
+    the Executable Form how they can obtain a copy of such Source Code
+    Form by reasonable means in a timely manner, at a charge no more
+    than the cost of distribution to the recipient; and
+
+(b) You may distribute such Executable Form under the terms of this
+    License, or sublicense it under different terms, provided that the
+    license for the Executable Form does not attempt to limit or alter
+    the recipients' rights in the Source Code Form under this License.
+
+3.3. Distribution of a Larger Work
+
+You may create and distribute a Larger Work under terms of Your choice,
+provided that You also comply with the requirements of this License for
+the Covered Software. If the Larger Work is a combination of Covered
+Software with a work governed by one or more Secondary Licenses, and the
+Covered Software is not Incompatible With Secondary Licenses, this
+License permits You to additionally distribute such Covered Software
+under the terms of such Secondary License(s), so that the recipient of
+the Larger Work may, at their option, further distribute the Covered
+Software under the terms of either this License or such Secondary
+License(s).
+
+3.4. Notices
+
+You may not remove or alter the substance of any license notices
+(including copyright notices, patent notices, disclaimers of warranty,
+or limitations of liability) contained within the Source Code Form of
+the Covered Software, except that You may alter any license notices to
+the extent required to remedy known factual inaccuracies.
+
+3.5. Application of Additional Terms
+
+You may choose to offer, and to charge a fee for, warranty, support,
+indemnity or liability obligations to one or more recipients of Covered
+Software. However, You may do so only on Your own behalf, and not on
+behalf of any Contributor. You must make it absolutely clear that any
+such warranty, support, indemnity, or liability obligation is offered by
+You alone, and You hereby agree to indemnify every Contributor for any
+liability incurred by such Contributor as a result of warranty, support,
+indemnity or liability terms You offer. You may include additional
+disclaimers of warranty and limitations of liability specific to any
+jurisdiction.
+
+4. Inability to Comply Due to Statute or Regulation
+---------------------------------------------------
+
+If it is impossible for You to comply with any of the terms of this
+License with respect to some or all of the Covered Software due to
+statute, judicial order, or regulation then You must: (a) comply with
+the terms of this License to the maximum extent possible; and (b)
+describe the limitations and the code they affect. Such description must
+be placed in a text file included with all distributions of the Covered
+Software under this License. Except to the extent prohibited by statute
+or regulation, such description must be sufficiently detailed for a
+recipient of ordinary skill to be able to understand it.
+
+5. Termination
+--------------
+
+5.1. The rights granted under this License will terminate automatically
+if You fail to comply with any of its terms. However, if You become
+compliant, then the rights granted under this License from a particular
+Contributor are reinstated (a) provisionally, unless and until such
+Contributor explicitly and finally terminates Your grants, and (b) on an
+ongoing basis, if such Contributor fails to notify You of the
+non-compliance by some reasonable means prior to 60 days after You have
+come back into compliance. Moreover, Your grants from a particular
+Contributor are reinstated on an ongoing basis if such Contributor
+notifies You of the non-compliance by some reasonable means, this is the
+first time You have received notice of non-compliance with this License
+from such Contributor, and You become compliant prior to 30 days after
+Your receipt of the notice.
+
+5.2. If You initiate litigation against any entity by asserting a patent
+infringement claim (excluding declaratory judgment actions,
+counter-claims, and cross-claims) alleging that a Contributor Version
+directly or indirectly infringes any patent, then the rights granted to
+You by any and all Contributors for the Covered Software under Section
+2.1 of this License shall terminate.
+
+5.3. In the event of termination under Sections 5.1 or 5.2 above, all
+end user license agreements (excluding distributors and resellers) which
+have been validly granted by You or Your distributors under this License
+prior to termination shall survive termination.
+
+************************************************************************
+*                                                                      *
+*  6. Disclaimer of Warranty                                           *
+*  -------------------------                                           *
+*                                                                      *
+*  Covered Software is provided under this License on an "as is"       *
+*  basis, without warranty of any kind, either expressed, implied, or  *
+*  statutory, including, without limitation, warranties that the       *
+*  Covered Software is free of defects, merchantable, fit for a        *
+*  particular purpose or non-infringing. The entire risk as to the     *
+*  quality and performance of the Covered Software is with You.        *
+*  Should any Covered Software prove defective in any respect, You     *
+*  (not any Contributor) assume the cost of any necessary servicing,   *
+*  repair, or correction. This disclaimer of warranty constitutes an   *
+*  essential part of this License. No use of any Covered Software is   *
+*  authorized under this License except under this disclaimer.         *
+*                                                                      *
+************************************************************************
+
+************************************************************************
+*                                                                      *
+*  7. Limitation of Liability                                          *
+*  --------------------------                                          *
+*                                                                      *
+*  Under no circumstances and under no legal theory, whether tort      *
+*  (including negligence), contract, or otherwise, shall any           *
+*  Contributor, or anyone who distributes Covered Software as          *
+*  permitted above, be liable to You for any direct, indirect,         *
+*  special, incidental, or consequential damages of any character      *
+*  including, without limitation, damages for lost profits, loss of    *
+*  goodwill, work stoppage, computer failure or malfunction, or any    *
+*  and all other commercial damages or losses, even if such party      *
+*  shall have been informed of the possibility of such damages. This   *
+*  limitation of liability shall not apply to liability for death or   *
+*  personal injury resulting from such party's negligence to the       *
+*  extent applicable law prohibits such limitation. Some               *
+*  jurisdictions do not allow the exclusion or limitation of           *
+*  incidental or consequential damages, so this exclusion and          *
+*  limitation may not apply to You.                                    *
+*                                                                      *
+************************************************************************
+
+8. Litigation
+-------------
+
+Any litigation relating to this License may be brought only in the
+courts of a jurisdiction where the defendant maintains its principal
+place of business and such litigation shall be governed by laws of that
+jurisdiction, without reference to its conflict-of-law provisions.
+Nothing in this Section shall prevent a party's ability to bring
+cross-claims or counter-claims.
+
+9. Miscellaneous
+----------------
+
+This License represents the complete agreement concerning the subject
+matter hereof. If any provision of this License is held to be
+unenforceable, such provision shall be reformed only to the extent
+necessary to make it enforceable. Any law or regulation which provides
+that the language of a contract shall be construed against the drafter
+shall not be used to construe this License against a Contributor.
+
+10. Versions of the License
+---------------------------
+
+10.1. New Versions
+
+Mozilla Foundation is the license steward. Except as provided in Section
+10.3, no one other than the license steward has the right to modify or
+publish new versions of this License. Each version will be given a
+distinguishing version number.
+
+10.2. Effect of New Versions
+
+You may distribute the Covered Software under the terms of the version
+of the License under which You originally received the Covered Software,
+or under the terms of any subsequent version published by the license
+steward.
+
+10.3. Modified Versions
+
+If you create software not governed by this License, and you want to
+create a new license for such software, you may create and use a
+modified version of this License if you rename the license and remove
+any references to the name of the license steward (except to note that
+such modified license differs from this License).
+
+10.4. Distributing Source Code Form that is Incompatible With Secondary
+Licenses
+
+If You choose to distribute Source Code Form that is Incompatible With
+Secondary Licenses under the terms of this version of the License, the
+notice described in Exhibit B of this License must be attached.
+
+Exhibit A - Source Code Form License Notice
+-------------------------------------------
+
+  This Source Code Form is subject to the terms of the Mozilla Public
+  License, v. 2.0. If a copy of the MPL was not distributed with this
+  file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+If it is not possible or desirable to put the notice in a particular
+file, then You may include the notice in a location (such as a LICENSE
+file in a relevant directory) where a recipient would be likely to look
+for such a notice.
+
+You may add additional accurate notices of copyright ownership.
+
+Exhibit B - "Incompatible With Secondary Licenses" Notice
+---------------------------------------------------------
+
+  This Source Code Form is "Incompatible With Secondary Licenses", as
+  defined by the Mozilla Public License, v. 2.0.
diff --git a/README.rst b/README.rst
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--- /dev/null
+++ b/README.rst
@@ -0,0 +1,307 @@
+GHC Plugs Out
+=============
+Type checker plugins without the type checking.
+
+|cabal-ci|
+
+Introduction
+------------
+When getting ready to launch, one of the steps is the plugs-out test. Can the
+spacecraft function on its own without power or fuel from all cables and
+umbilicals?
+
+When debugging GHC plugins, I've added tracing and changed the wiring.  Rather
+than throw those edits away, I've collected them in `ghc-plugs-out`_, a package
+of tests that don't supply any power or fuel for typechecking. It is the first
+multiple library package I've put together [#]_.
+
+Wiring Diagram
+--------------
+Here's a type checker plugin that doesn't do any solving but instead writes its
+call count.
+
+.. code-block:: haskell
+
+    {-# LANGUAGE QuasiQuotes, NamedFieldPuns #-}
+
+    module CallCount.TcPlugin (callCount) where
+
+    import Language.Haskell.Printf (s)
+    import Data.IORef (IORef)
+    import IOEnv (newMutVar, readMutVar, writeMutVar)
+    import TcPluginM (tcPluginIO)
+    import TcRnTypes (TcPluginResult(..), TcPlugin(..), unsafeTcPluginTcM)
+
+    newtype State = State { callref :: IORef Int }
+
+    callCount :: TcPlugin
+    callCount =
+        TcPlugin
+            { tcPluginInit = return . State =<< (unsafeTcPluginTcM $ newMutVar 1)
+
+            , tcPluginSolve = \State{callref = c} _ _ _ -> do
+                n <- unsafeTcPluginTcM $ readMutVar c
+                tcPluginIO . putStrLn $ [s|>>> GHC-TcPlugin #%d|] n
+                unsafeTcPluginTcM $ writeMutVar c (n + 1)
+                return $ TcPluginOk [] []
+
+            , tcPluginStop = const $ return ()
+            }
+
+Plugins are flagged for recompilation in their ``pluginRecompile`` field.
+Let's now wire up and test the pure ``CallCount.Pure.Plugin`` and the impure
+``CallCount.Impure.Plugin``. The recommended way to wire up a plugin is with
+a pragma, only in source files that need the plugin.
+
+.. code-block:: haskell
+
+    {-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+    module Main where
+
+    main :: IO a
+    main = undefined
+
+The call count prints on first build but not when there's no work to do.
+
+.. code-block:: pre
+
+    > cabal build test-wireup-pure-by-pragma
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+
+    > cabal build test-wireup-pure-by-pragma
+    Up to date
+
+A plugin can also be wired up with an option, say in a cabal file. This is
+probably fine if all your modules need a plugin.
+
+.. code-block:: yaml
+
+    test-suite test-wireup-pure-by-option
+      import: opts
+      type: exitcode-stdio-1.0
+      main-is: Main.hs
+      hs-source-dirs: test-suites/wireup-pure-by-option
+      ghc-options: -Wall -fplugin CallCount.Pure.Plugin
+      build-depends: base, call-count-plugin
+
+.. code-block:: pre
+
+    > cabal build test-wireup-pure-by-option
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+
+If you mix and match both ways of doing the wiring you'll end up with two
+instances of the plugin in the compilation.
+
+.. code-block:: pre
+
+    > cabal build test-wireup-pure-by-both
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+    >>> GHC-TcPlugin #1
+
+If your plugin is impure, it's going to force a recompilation.
+
+.. code-block:: pre
+
+    > cabal build test-wireup-impure-by-pragma
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+    [1 of 1] Compiling Main [Impure plugin forced recompilation]
+    >>> GHC-TcPlugin #1
+
+    > cabal build test-wireup-impure-by-option
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+    [1 of 1] Compiling Main [Impure plugin forced recompilation]
+    >>> GHC-TcPlugin #1
+
+    > cabal build test-wireup-impure-by-both
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+    >>> GHC-TcPlugin #1
+    [1 of 1] Compiling Main [Impure plugin forced recompilation]
+    >>> GHC-TcPlugin #1
+    >>> GHC-TcPlugin #1
+
+Modularity
+----------
+GHC compiles modules. We see the counter plugin is called on twice when
+functions ``foo`` and ``bar`` are in module ``Main``.
+
+.. code-block:: haskell
+
+    {-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+    module Main where
+
+    foo :: IO a
+    foo = undefined
+
+    bar :: IO a
+    bar = undefined
+
+    main :: IO ()
+    main = return ()
+
+.. code-block:: pre
+
+    > cabal build test-counter-main
+    [1 of 1] Compiling Main
+    >>> GHC-TcPlugin #1
+    >>> GHC-TcPlugin #2
+
+Moving ``foo`` and ``bar`` to module ``FooBar`` and the counter plugin reports
+two calls again.
+
+.. code-block:: pre
+
+    > cabal build test-counter-foobar-main
+    [1 of 2] Compiling FooBar
+    >>> GHC-TcPlugin #1
+    >>> GHC-TcPlugin #2
+    [2 of 2] Compiling Main
+
+Move these functions into separate modules and we count one call for each module.
+
+.. code-block:: pre
+
+    > cabal build test-counter-foo-bar-main
+    [1 of 3] Compiling Bar
+    >>> GHC-TcPlugin #1
+    [2 of 3] Compiling Foo
+    >>> GHC-TcPlugin #1
+    [3 of 3] Compiling Main
+
+Undefined is not a Function
+---------------------------
+If your plugin behaves badly it is going to hurt. GHC panics when any one of
+the functions required of a type checker plugin is implemented undefined.
+
+.. code-block:: haskell
+
+    plugin :: Plugin
+    plugin = mkPureTcPlugin undefSolve
+
+    undefSolve :: TcPlugin
+    undefSolve = noOp { tcPluginSolve = \_ _ _ _ -> undefined }
+
+    noOp :: TcPlugin
+    noOp =
+        TcPlugin
+            { tcPluginInit = return ()
+            , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
+            , tcPluginStop = const $ return ()
+            }
+
+    mkPureTcPlugin :: TcPlugin -> Plugin
+    mkPureTcPlugin p =
+        defaultPlugin
+            { tcPlugin = const $ Just p
+            , pluginRecompile = purePlugin
+            }
+
+.. code-block:: pre
+
+    > cabal build test-undefined-solve
+    [1 of 1] Compiling Undefined.Solve.Plugin
+    [1 of 1] Compiling Main
+    ghc: panic! (the 'impossible' happened)
+    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
+
+I would have liked to use record update syntax for undefSolve as shown above
+but this is not yet possible [#]_ with GHC when the data type has an
+existential qualifier and that is how TcPlugin is defined [#]_.
+
+.. code-block:: haskell
+
+    data TcPlugin = forall s. TcPlugin
+        { tcPluginInit  :: TcPluginM s
+        -- ^ Initialize plugin, when entering type-checker.
+        , tcPluginSolve :: s -> TcPluginSolver
+        -- ^ Solve some constraints.
+        -- TODO: WRITE MORE DETAILS ON HOW THIS WORKS.
+        , tcPluginStop  :: s -> TcPluginM ()
+        -- ^ Clean up after the plugin, when exiting the type-checker.
+        }
+
+Care Free
+---------
+Type checker plugins are of course called on by GHC to resolve constraints.
+Some need solving and others don't. GHC knows that it can get an ``a`` from
+``undefined`` but maybe a plugin can do better so we get called.
+
+.. code-block:: haskell
+
+    {-# OPTIONS_GHC -fplugin Undefined.Solve.Plugin #-}
+    module Main where
+
+    main :: IO a
+    main = undefined
+
+Going from ``()`` to ``()`` needs no further resolution. GHC can handle this
+by itself. The ``test-undefined-*-carefree`` test suites have these mains.
+The ones without carefree in their name don't. They have the ``a`` from
+``undefined`` mains.
+
+.. code-block:: haskell
+
+    {-# OPTIONS_GHC -fplugin Undefined.Solve.Plugin #-}
+    module Main where
+
+    main :: IO ()
+    main = return ()
+
+So we've seen that a typechecker plugin's solve function **may** be called but
+its init and stop functions are **always** called.
+
+.. code-block:: ascii
+
+  +-------------------------------+------------+
+  | Test Suite                    | GHC Panics |
+  +===============================+============+
+  | test-undefined-init           |     x      |
+  +-------------------------------+------------+
+  | test-undefined-init-carefree  |     x      |
+  +-------------------------------+------------+
+  | test-undefined-solve          |     x      |
+  +-------------------------------+------------+
+  | test-undefined-solve-carefree |            |
+  +-------------------------------+------------+
+  | test-undefined-stop           |     x      |
+  +-------------------------------+------------+
+  | test-undefined-stop-carefree  |     x      |
+  +-------------------------------+------------+
+
+Takeaways
+---------
+* We should wire up type checker plugins with pragmas only in modules that need
+  it.
+* Don't forget to flag pure plugins as such.
+* If GHC doesn't need help resolving constraints then it won't call out to your plugin.
+* Modules are the units of compilation.
+
+License
+-------
+.. code-block:: ascii
+
+    Copyright © Phil de Joux 2017-2020
+    Copyright © Block Scope Limited 2017-2020
+
+This software is subject to the terms of the Mozilla Public License, v2.0. If
+a copy of the MPL was not distributed with this file, you can obtain one at
+http://mozilla.org/MPL/2.0/.
+
+.. _ghc-plugs-out: https://github.com/BlockScope/ghc-plugs-out
+
+.. _ghc-2595: https://gitlab.haskell.org/ghc/ghc/issues/2595
+
+.. _fgaz-GSoC-2018: https://fgaz.me/posts/2019-11-14-cabal-multiple-libraries/
+
+.. [#] Multiple libraries were added to cabal 3.0, see fgaz-GSoC-2018_.
+.. [#] The error if you try is "Record update for insufficiently polymorphic field", see ghc-2595_.
+.. [#] These field haddock comments are verbatim from the GHC source.
+
+.. |cabal-ci| image:: https://github.com/BlockScope/ghc-plugs-out/workflows/cabal/badge.svg
diff --git a/changelog.md b/changelog.md
new file mode 100644
--- /dev/null
+++ b/changelog.md
@@ -0,0 +1,5 @@
+# Versions of ghc-plugs-out
+
+## 1.0.0.0 Initial Release
+Add tests that show how typechecker plugins interact with GHC depending on
+their purity, how they're wired up and whether GHC needs help typechecking.
diff --git a/ghc-plugs-out.cabal b/ghc-plugs-out.cabal
new file mode 100644
--- /dev/null
+++ b/ghc-plugs-out.cabal
@@ -0,0 +1,246 @@
+cabal-version: 3.0
+
+name:           ghc-plugs-out
+version:        1.0.0.0
+synopsis:       Type checker plugins without the type checking.
+description:
+  Tests that show how typechecker plugins interact with GHC depending on their
+  purity, how they're wired up and whether GHC needs help typechecking.
+category:       Compiler Plugin
+author:         Phil de Joux
+maintainer:     phil.dejoux@blockscope.com
+copyright:      © 2020 Phil de Joux, © 2020 Block Scope Limited
+homepage:       https://github.com/blockscope/ghc-plugs-out
+license:        MPL-2.0
+license-file:   LICENSE.md
+tested-with:    GHC == 8.8.3, GHC == 8.10.1
+build-type:     Simple
+extra-source-files:
+  changelog.md, README.rst
+
+source-repository head
+  type: git
+  location: https://github.com/blockscope/ghc-plugs-out
+
+common opts
+  ghc-options:
+    -Wall
+    -Wincomplete-uni-patterns
+    -Wcompat
+    -Widentities
+    -Wredundant-constraints
+    -fhide-source-paths
+  default-language: Haskell2010
+
+library no-op-plugin
+  import: opts
+  exposed-modules:
+      NoOp.Plugin
+  hs-source-dirs:
+      plugins/noop
+  build-depends:
+      base >= 4.13 && < 5
+    , ghc
+
+library undefined-init-plugin
+  import: opts
+  exposed-modules:
+      Undefined.Init.Plugin
+  hs-source-dirs:
+      plugins/init
+  build-depends:
+      base >= 4.13 && < 5
+    , ghc
+    , no-op-plugin
+
+library undefined-solve-plugin
+  import: opts
+  exposed-modules:
+      Undefined.Solve.Plugin
+  hs-source-dirs:
+      plugins/solve
+  build-depends:
+      base >= 4.13 && < 5
+    , ghc
+    , no-op-plugin
+
+library undefined-stop-plugin
+  import: opts
+  exposed-modules:
+      Undefined.Stop.Plugin
+  hs-source-dirs:
+      plugins/stop
+  build-depends:
+      base >= 4.13 && < 5
+    , ghc
+    , no-op-plugin
+
+library call-count-plugin
+  import: opts
+  exposed-modules:
+      CallCount.TcPlugin
+      CallCount.Pure.Plugin
+      CallCount.Impure.Plugin
+  hs-source-dirs:
+      plugins/call-count
+  build-depends:
+      base >= 4.13 && < 5
+    , ghc
+    , th-printf
+    , no-op-plugin
+
+test-suite test-undefined-init
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/undef-init
+  build-depends:
+      base >= 4.13 && < 5
+    , undefined-init-plugin
+
+test-suite test-undefined-init-carefree
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/undef-init-carefree
+  build-depends:
+      base >= 4.13 && < 5
+    , undefined-init-plugin
+
+test-suite test-undefined-solve
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/undef-solve
+  build-depends:
+      base >= 4.13 && < 5
+    , undefined-solve-plugin
+
+test-suite test-undefined-solve-carefree
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/undef-solve-carefree
+  build-depends:
+      base >= 4.13 && < 5
+    , undefined-solve-plugin
+
+test-suite test-undefined-stop
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/undef-stop
+  build-depends:
+      base >= 4.13 && < 5
+    , undefined-stop-plugin
+
+test-suite test-undefined-stop-carefree
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/undef-stop-carefree
+  build-depends:
+      base >= 4.13 && < 5
+    , undefined-stop-plugin
+
+test-suite test-wireup-pure-by-option
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/wireup-pure-by-option
+  ghc-options: -Wall -fplugin CallCount.Pure.Plugin
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-wireup-pure-by-pragma
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/wireup-pure-by-pragma
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-wireup-pure-by-both
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/wireup-pure-by-pragma
+  ghc-options: -Wall -fplugin CallCount.Pure.Plugin
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-wireup-impure-by-option
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/wireup-impure-by-option
+  ghc-options: -Wall -fplugin CallCount.Impure.Plugin
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-wireup-impure-by-pragma
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/wireup-impure-by-pragma
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-wireup-impure-by-both
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/wireup-impure-by-pragma
+  ghc-options: -Wall -fplugin CallCount.Impure.Plugin
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-counter-main
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:
+      test-suites/counter-main
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-counter-foo-bar-main
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  other-modules: Foo Bar
+  hs-source-dirs:
+      test-suites/counter-foo-bar-main
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
+
+test-suite test-counter-foobar-main
+  import: opts
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  other-modules: FooBar
+  hs-source-dirs:
+      test-suites/counter-foobar-main
+  build-depends:
+      base >= 4.13 && < 5
+    , call-count-plugin
diff --git a/plugins/call-count/CallCount/Impure/Plugin.hs b/plugins/call-count/CallCount/Impure/Plugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/call-count/CallCount/Impure/Plugin.hs
@@ -0,0 +1,8 @@
+module CallCount.Impure.Plugin (plugin) where
+
+import Plugins (Plugin)
+import NoOp.Plugin (mkImpureTcPlugin)
+import CallCount.TcPlugin (callCount)
+
+plugin :: Plugin
+plugin = mkImpureTcPlugin callCount
diff --git a/plugins/call-count/CallCount/Pure/Plugin.hs b/plugins/call-count/CallCount/Pure/Plugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/call-count/CallCount/Pure/Plugin.hs
@@ -0,0 +1,8 @@
+module CallCount.Pure.Plugin (plugin) where
+
+import Plugins (Plugin)
+import NoOp.Plugin (mkPureTcPlugin)
+import CallCount.TcPlugin (callCount)
+
+plugin :: Plugin
+plugin = mkPureTcPlugin callCount
diff --git a/plugins/call-count/CallCount/TcPlugin.hs b/plugins/call-count/CallCount/TcPlugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/call-count/CallCount/TcPlugin.hs
@@ -0,0 +1,25 @@
+{-# LANGUAGE QuasiQuotes, NamedFieldPuns #-}
+
+module CallCount.TcPlugin (callCount) where
+
+import Language.Haskell.Printf (s)
+import Data.IORef (IORef)
+import IOEnv (newMutVar, readMutVar, writeMutVar)
+import TcPluginM (tcPluginIO)
+import TcRnTypes (TcPluginResult(..), TcPlugin(..), unsafeTcPluginTcM)
+
+newtype State = State{callref :: IORef Int}
+
+callCount :: TcPlugin
+callCount =
+    TcPlugin
+        { tcPluginInit = return . State =<< (unsafeTcPluginTcM $ newMutVar 1)
+
+        , tcPluginSolve = \State{callref = c} _ _ _ -> do
+            n <- unsafeTcPluginTcM $ readMutVar c
+            tcPluginIO . putStrLn $ [s|>>> GHC-TcPlugin #%d|] n
+            unsafeTcPluginTcM $ writeMutVar c (n + 1)
+            return $ TcPluginOk [] []
+
+        , tcPluginStop = const $ return ()
+        }
diff --git a/plugins/init/Undefined/Init/Plugin.hs b/plugins/init/Undefined/Init/Plugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/init/Undefined/Init/Plugin.hs
@@ -0,0 +1,7 @@
+module Undefined.Init.Plugin (plugin) where
+
+import Plugins (Plugin)
+import NoOp.Plugin (undefInit, mkPureTcPlugin)
+
+plugin :: Plugin
+plugin = mkPureTcPlugin undefInit
diff --git a/plugins/noop/NoOp/Plugin.hs b/plugins/noop/NoOp/Plugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/noop/NoOp/Plugin.hs
@@ -0,0 +1,70 @@
+{-# LANGUAGE RankNTypes #-}
+
+module NoOp.Plugin
+    ( plugin
+    , noOp, undefInit, undefSolve, undefStop
+    , mkPureTcPlugin, mkImpureTcPlugin
+    ) where
+
+import Plugins (Plugin(..), tcPlugin, defaultPlugin, purePlugin, impurePlugin)
+import TcRnTypes (TcPluginResult(..), TcPlugin(..))
+
+plugin :: Plugin
+plugin = mkPureTcPlugin noOp
+
+noOp :: TcPlugin
+noOp =
+    TcPlugin
+        { tcPluginInit = return ()
+        , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
+        , tcPluginStop = const $ return ()
+        }
+
+-- WARNING: Can't use record update syntax for existentially qualified types.
+-- SEE: https://gitlab.haskell.org/ghc/ghc/issues/2595
+-- data TcPlugin = forall s. TcPlugin
+--   { tcPluginInit  :: TcPluginM s
+--   , tcPluginSolve :: s -> TcPluginSolver
+--   , tcPluginStop  :: s -> TcPluginM ()
+--   }
+
+-- noOp { tcPluginInit = undefined }
+undefInit :: TcPlugin
+undefInit =
+    TcPlugin
+        { tcPluginInit = undefined
+        , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
+        , tcPluginStop = const $ return ()
+        }
+
+-- noOp { tcPluginSolve = \_ _ _ _ -> undefined }
+undefSolve :: TcPlugin
+undefSolve =
+    TcPlugin
+        { tcPluginInit = return ()
+        , tcPluginSolve = \_ _ _ _ -> undefined
+        , tcPluginStop = const $ return ()
+        }
+
+-- noOp { tcPluginStop = const undefined }
+undefStop :: TcPlugin
+undefStop =
+    TcPlugin
+        { tcPluginInit = return ()
+        , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
+        , tcPluginStop = const undefined
+        }
+
+mkPureTcPlugin :: TcPlugin -> Plugin
+mkPureTcPlugin p =
+    defaultPlugin
+        { tcPlugin = const $ Just p
+        , pluginRecompile = purePlugin
+        }
+
+mkImpureTcPlugin :: TcPlugin -> Plugin
+mkImpureTcPlugin p =
+    defaultPlugin
+        { tcPlugin = const $ Just p
+        , pluginRecompile = impurePlugin
+        }
diff --git a/plugins/solve/Undefined/Solve/Plugin.hs b/plugins/solve/Undefined/Solve/Plugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/solve/Undefined/Solve/Plugin.hs
@@ -0,0 +1,7 @@
+module Undefined.Solve.Plugin (plugin) where
+
+import Plugins (Plugin)
+import NoOp.Plugin (undefSolve, mkPureTcPlugin)
+
+plugin :: Plugin
+plugin = mkPureTcPlugin undefSolve
diff --git a/plugins/stop/Undefined/Stop/Plugin.hs b/plugins/stop/Undefined/Stop/Plugin.hs
new file mode 100644
--- /dev/null
+++ b/plugins/stop/Undefined/Stop/Plugin.hs
@@ -0,0 +1,7 @@
+module Undefined.Stop.Plugin (plugin) where
+
+import Plugins (Plugin)
+import NoOp.Plugin (undefStop, mkPureTcPlugin)
+
+plugin :: Plugin
+plugin = mkPureTcPlugin undefStop
diff --git a/test-suites/counter-foo-bar-main/Bar.hs b/test-suites/counter-foo-bar-main/Bar.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/counter-foo-bar-main/Bar.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module Bar where
+
+bar :: IO a
+bar = undefined
diff --git a/test-suites/counter-foo-bar-main/Foo.hs b/test-suites/counter-foo-bar-main/Foo.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/counter-foo-bar-main/Foo.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module Foo where
+
+foo :: IO a
+foo = undefined
diff --git a/test-suites/counter-foo-bar-main/Main.hs b/test-suites/counter-foo-bar-main/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/counter-foo-bar-main/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module Main where
+
+main :: IO ()
+main = return ()
diff --git a/test-suites/counter-foobar-main/FooBar.hs b/test-suites/counter-foobar-main/FooBar.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/counter-foobar-main/FooBar.hs
@@ -0,0 +1,9 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module FooBar where
+
+foo :: IO a
+foo = undefined
+
+bar :: IO a
+bar = undefined
diff --git a/test-suites/counter-foobar-main/Main.hs b/test-suites/counter-foobar-main/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/counter-foobar-main/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module Main where
+
+main :: IO ()
+main = return ()
diff --git a/test-suites/counter-main/Main.hs b/test-suites/counter-main/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/counter-main/Main.hs
@@ -0,0 +1,12 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module Main where
+
+foo :: IO a
+foo = undefined
+
+bar :: IO a
+bar = undefined
+
+main :: IO ()
+main = return ()
diff --git a/test-suites/undef-init-carefree/Main.hs b/test-suites/undef-init-carefree/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/undef-init-carefree/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin Undefined.Init.Plugin #-}
+
+module Main where
+
+main :: IO ()
+main = return ()
diff --git a/test-suites/undef-init/Main.hs b/test-suites/undef-init/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/undef-init/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin Undefined.Init.Plugin #-}
+
+module Main where
+
+main :: IO a
+main = undefined
diff --git a/test-suites/undef-solve-carefree/Main.hs b/test-suites/undef-solve-carefree/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/undef-solve-carefree/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin Undefined.Solve.Plugin #-}
+
+module Main where
+
+main :: IO ()
+main = return ()
diff --git a/test-suites/undef-solve/Main.hs b/test-suites/undef-solve/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/undef-solve/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin Undefined.Solve.Plugin #-}
+
+module Main where
+
+main :: IO a
+main = undefined
diff --git a/test-suites/undef-stop-carefree/Main.hs b/test-suites/undef-stop-carefree/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/undef-stop-carefree/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin Undefined.Stop.Plugin #-}
+
+module Main where
+
+main :: IO ()
+main = return ()
diff --git a/test-suites/undef-stop/Main.hs b/test-suites/undef-stop/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/undef-stop/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin Undefined.Stop.Plugin #-}
+
+module Main where
+
+main :: IO a
+main = undefined
diff --git a/test-suites/wireup-impure-by-option/Main.hs b/test-suites/wireup-impure-by-option/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/wireup-impure-by-option/Main.hs
@@ -0,0 +1,4 @@
+module Main where
+
+main :: IO a
+main = undefined
diff --git a/test-suites/wireup-impure-by-pragma/Main.hs b/test-suites/wireup-impure-by-pragma/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/wireup-impure-by-pragma/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin CallCount.Impure.Plugin #-}
+
+module Main where
+
+main :: IO a
+main = undefined
diff --git a/test-suites/wireup-pure-by-option/Main.hs b/test-suites/wireup-pure-by-option/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/wireup-pure-by-option/Main.hs
@@ -0,0 +1,4 @@
+module Main where
+
+main :: IO a
+main = undefined
diff --git a/test-suites/wireup-pure-by-pragma/Main.hs b/test-suites/wireup-pure-by-pragma/Main.hs
new file mode 100644
--- /dev/null
+++ b/test-suites/wireup-pure-by-pragma/Main.hs
@@ -0,0 +1,6 @@
+{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
+
+module Main where
+
+main :: IO a
+main = undefined