Fix unification of ConcreteTvs, removing IsRefl#

This patch fixes the unification of concrete type variables.
The subtlety was that unifying concrete metavariables is more subtle
than other metavariables, as decomposition is possible. See the Note
[Unifying concrete metavariables], which explains how we unify a
concrete type variable with a type 'ty' by concretising 'ty', using
the function 'GHC.Tc.Utils.Concrete.concretise'.

This can be used to perform an eager syntactic check for concreteness,
allowing us to remove the IsRefl# special predicate. Instead of emitting
two constraints `rr ~# concrete_tv` and `IsRefl# rr concrete_tv`, we
instead concretise 'rr'. If this succeeds we can fill 'concrete_tv',
and otherwise we directly emit an error message to the typechecker
environment instead of deferring. We still need the error message
to be passed on (instead of directly thrown), as we might benefit from
further unification in which case we will need to zonk the stored types.
To achieve this, we change the 'wc_holes' field of 'WantedConstraints'
to 'wc_errors', which stores general delayed errors. For the moement,
a delayed error is either a hole, or a syntactic equality error.

hasFixedRuntimeRep_MustBeRefl is now hasFixedRuntimeRep_syntactic, and
hasFixedRuntimeRep has been refactored to directly return the most
useful coercion for PHASE 2 of FixedRuntimeRep.

This patch also adds a field ir_frr to the InferResult datatype,
holding a value of type Maybe FRROrigin. When this value is not
Nothing, this means that we must fill the ir_ref field with a type
which has a fixed RuntimeRep.
When it comes time to fill such an ExpType, we ensure that the type
has a fixed RuntimeRep by performing a representation-polymorphism
check with the given FRROrigin
This is similar to what we already do to ensure we fill an Infer
ExpType with a type of the correct TcLevel.
This allows us to properly perform representation-polymorphism checks
on 'Infer' 'ExpTypes'.

The fillInferResult function had to be moved to GHC.Tc.Utils.Unify
to avoid a cyclic import now that it calls hasFixedRuntimeRep.

This patch also changes the code in matchExpectedFunTys to make use
of the coercions, which is now possible thanks to the previous change.
This implements PHASE 2 of FixedRuntimeRep in some situations.
For example, the test cases T13105 and T17536b are now both accepted.

Fixes #21239 and #21325

-------------------------
Metric Decrease:
    T18223
    T5631
-------------------------

1 files changed, 6 insertions, 3 deletions

diff --git a/testsuite/tests/rep-poly/RepPolyPatBind.stderr b/testsuite/tests/rep-poly/RepPolyPatBind.stderr
index 3914ea8e30..976e84e81e 100644
--- a/testsuite/tests/rep-poly/RepPolyPatBind.stderr
+++ b/testsuite/tests/rep-poly/RepPolyPatBind.stderr
@@ -1,8 +1,11 @@
 
 RepPolyPatBind.hs:18:5: error:
-    • The binder ‘x’ does not have a fixed runtime representation.
-      Its type is:
-        a :: TYPE rep
+    • • The binder ‘y’ does not have a fixed runtime representation.
+        Its type is:
+          a :: TYPE rep
+      • The binder ‘x’ does not have a fixed runtime representation.
+        Its type is:
+          a :: TYPE rep
     • In the expression:
         let
           x, y :: a