Overhaul the Overhauled Pattern Match Checkerwip/gadtpm

* Changed the representation of Value Set Abstractions. Instead
  of using a prefix tree, we now use a list of Value Vector
  Abstractions. The set of constraints Delta for every Value
  Vector Abstraction is the oracle state so that we solve
  everything only once.

* Instead of doing everything lazily, we prune at once (and in
  general everything is much stricter). A case writtern with
  pattern guards is not checked in almost the same time as the
  equivalent with pattern matching.

* Do not store the covered and the divergent sets at all. Since
  what we only need is a yes/no (does this clause cover anything?
  Does it force any thunk?) We just keep a boolean for each.

* Removed flags `-Wtoo-many-guards` and `-ffull-guard-reasoning`.
  Replaced with `fmax-pmcheck-iterations=n`. Still debatable what
  should the default `n` be.

* When a guard is for sure not going to contribute anything, we
  treat it as such: The oracle is not called and cases `CGuard`,
  `UGuard` and `DGuard` from the paper are not happening at all
  (the generation of a fresh variable, the unfolding of the
  pattern list etc.). his combined with the above seems to be
  enough to drop the memory increase for test T783 down to 18.7%.

* Do not export function `dsPmWarn` (it is now called directly
  from within `checkSingle` and `checkMatches`).

* Make `PmExprVar` hold a `Name` instead of an `Id`. The term
  oracle does not handle type information so using `Id` was a
  waste of time/space.

* Added testcases T11195, T11303b (data families) and T11374

The patch addresses at least the following:
    #11195, #11276, #11303, #11374, #11162

3 files changed, 0 insertions, 53 deletions

diff --git a/docs/users_guide/8.0.1-notes.rst b/docs/users_guide/8.0.1-notes.rst
index 41e6c2b579..771bc99669 100644
--- a/docs/users_guide/8.0.1-notes.rst
+++ b/docs/users_guide/8.0.1-notes.rst
@@ -291,15 +291,6 @@ Compiler
    warns in the case of unused term-level patterns. Both flags are implied by
    :ghc-flag:`-W`.
 
--  Added the :ghc-flag:`-Wtoo-many-guards` flag. When enabled, this will issue a
-   warning if a pattern match contains too many guards (over 20 at the
-   moment). Makes a difference only if pattern match checking is also enabled.
-
--  Added the :ghc-flag:`-ffull-guard-reasoning` flag. When enabled, pattern match
-   checking tries its best to reason about guards. Since the additional
-   expressivity may come with a high price in terms of compilation time and
-   memory consumption, it is turned off by default.
-
 -  :ghc-flag:`-this-package-key` has been renamed again (hopefully for the last time!)
    to :ghc-flag:`-this-unit-id`.  The renaming was motivated by the fact that
    the identifier you pass to GHC here doesn't have much to do with packages:
diff --git a/docs/users_guide/bugs.rst b/docs/users_guide/bugs.rst
index 8304e25e78..f0c522cf75 100644
--- a/docs/users_guide/bugs.rst
+++ b/docs/users_guide/bugs.rst
@@ -358,16 +358,6 @@ Bugs in GHC
    This flag ensures that yield points are inserted at every function entrypoint
    (at the expense of a bit of performance).
 
--  GHC's updated exhaustiveness and coverage checker (see
-   :ref:`options-sanity`) is quite expressive but with a rather high
-   performance cost (in terms of both time and memory consumption), mainly
-   due to guards. Two flags have been introduced to give more control to
-   the user over guard reasoning: :ghc-flag:`-Wtoo-many-guards`
-   and :ghc-flag:`-ffull-guard-reasoning` (see :ref:`options-sanity`).
-   When :ghc-flag:`-ffull-guard-reasoning` is on, pattern match checking for guards
-   runs in full power, which may run out of memory/substantially increase
-   compilation time.
-
 -  GHC does not allow you to have a data type with a context that
    mentions type variables that are not data type parameters. For
    example:
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index afcee5b5d7..7fd2019cc6 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -527,40 +527,6 @@ of ``-W(no-)*``.
     This option isn't enabled by default because it can be very noisy,
     and it often doesn't indicate a bug in the program.
 
-.. ghc-flag:: -Wtoo-many-guards
-              -Wno-too-many-guards
-
-    .. index::
-       single: too many guards, warning
-
-    The option :ghc-flag:`-Wtoo-many-guards` warns about places where a
-    pattern match contains too many guards (over 20 at the moment).
-    It has an effect only if any form of exhaustivness/overlapping
-    checking is enabled (one of
-    :ghc-flag:`-Wincomplete-patterns`,
-    :ghc-flag:`-Wincomplete-uni-patterns`,
-    :ghc-flag:`-Wincomplete-record-updates`,
-    :ghc-flag:`-Woverlapping-patterns`). When enabled, the warning can be
-    suppressed by enabling either :ghc-flag:`-Wno-too-many-guards`, which just
-    hides the warning, or :ghc-flag:`-ffull-guard-reasoning` which runs the
-    full check, independently of the number of guards.
-
-.. ghc-flag:: -ffull-guard-reasoning
-
-    :implies: :ghc-flag:`-Wno-too-many-guards`
-
-    .. index::
-       single: guard reasoning, warning
-
-    The option :ghc-flag:`-ffull-guard-reasoning` forces pattern match checking
-    to run in full. This gives more precise warnings concerning pattern
-    guards but in most cases increases memory consumption and
-    compilation time. Hence, it is off by default. Enabling
-    :ghc-flag:`-ffull-guard-reasoning` also implies :ghc-flag:`-Wno-too-many-guards`.
-    Note that (like :ghc-flag:`-Wtoo-many-guards`) :ghc-flag:`-ffull-guard-reasoning`
-    makes a difference only if pattern match checking is already
-    enabled.
-
 .. ghc-flag:: -Wmissing-fields
 
     .. index::