Kill derived constraints

Co-authored by: Sam Derbyshire

Previously, GHC had three flavours of constraint:
Wanted, Given, and Derived. This removes Derived constraints.

Though serving a number of purposes, the most important role
of Derived constraints was to enable better error messages.
This job has been taken over by the new RewriterSets, as explained
in Note [Wanteds rewrite wanteds] in GHC.Tc.Types.Constraint.

Other knock-on effects:
 - Various new Notes as I learned about under-described bits of GHC

 - A reshuffling around the AST for implicit-parameter bindings,
   with better integration with TTG.

 - Various improvements around fundeps. These were caused by the
   fact that, previously, fundep constraints were all Derived,
   and Derived constraints would get dropped. Thus, an unsolved
   Derived didn't stop compilation. Without Derived, this is no
   longer possible, and so we have to be considerably more careful
   around fundeps.

 - A nice little refactoring in GHC.Tc.Errors to center the work
   on a new datatype called ErrorItem. Constraints are converted
   into ErrorItems at the start of processing, and this allows for
   a little preprocessing before the main classification.

 - This commit also cleans up the behavior in generalisation around
   functional dependencies. Now, if a variable is determined by
   functional dependencies, it will not be quantified. This change
   is user facing, but it should trim down GHC's strange behavior
   around fundeps.

 - Previously, reportWanteds did quite a bit of work, even on an empty
   WantedConstraints. This commit adds a fast path.

 - Now, GHC will unconditionally re-simplify constraints during
   quantification. See Note [Unconditionally resimplify constraints when
   quantifying], in GHC.Tc.Solver.

Close #18398.
Close #18406.
Solve the fundep-related non-confluence in #18851.
Close #19131.
Close #19137.
Close #20922.
Close #20668.
Close #19665.

-------------------------
Metric Decrease:
    LargeRecord
    T9872b
    T9872b_defer
    T9872d
    TcPlugin_RewritePerf
-------------------------

1 files changed, 37 insertions, 0 deletions

diff --git a/docs/users_guide/9.4.1-notes.rst b/docs/users_guide/9.4.1-notes.rst
index 85662049c4..594d1035c2 100644
--- a/docs/users_guide/9.4.1-notes.rst
+++ b/docs/users_guide/9.4.1-notes.rst
@@ -3,6 +3,39 @@
 Version 9.4.1
 ==============
 
+Language
+~~~~~~~~
+
+- A small change has been made to the way GHC infers types for definitions
+  with no type signature: GHC will no longer generalize a function over
+  a type variable determined by a functional dependency. For example::
+
+    class C a b | a -> b where
+      op :: a -> b -> ()
+    f x = op True x
+
+  Previously, GHC inferred ``f :: C Bool b => b -> ()``. However, the functional
+  dependency says that only one type could ever be used for ``b``: this function
+  is hardly valid "for all" ``b``\ s. With the change, GHC will reject, looking
+  for the (non-existent) instance for ``C Bool b``.
+
+  If you want to retain the old behavior, add a (backward-compatible) type signature,
+  explicitly requesting this unusual quantification.
+
+- GHC no longer checks for ``-XGADTs`` or ``-XTypeFamilies`` in order to use
+  an equality constraint in a type. This is part of accepted proposal
+  `#371 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`_.
+
+- There were previously cases around functional dependencies and injective
+  type families where the result of type inference would depend on the order
+  of constraints, as written in a source file. These cases are fundamentally ambiguous.
+  While GHC previously made an arbitrary decision, it now notices the ambiguity
+  and rejects the program. This means that some previously accepted programs are
+  now rejected. The solution is to add a type annotation or type application to
+  resolve the ambiguity.
+
+  This is the fix for :ghc-ticket:`18851`.
+
 Compiler
 ~~~~~~~~
 
@@ -313,3 +346,7 @@ Compiler
   For authors of type-checking plugins, this means you don't need to wrap
   a call to ``newWanted`` in ``setCtLocM`` to create a new Wanted constraint
   with the provided ``CtLoc``.
+
+- GHC no longer carries ``Derived`` constraints. Accordingly, several functions
+  in the plugin architecture that previously passed or received three sets of
+  constraints (givens, deriveds, and wanteds) now work with two such sets.
\ No newline at end of file