typecheck: Don't warn about "redundant" TypeError constraints

Summary: This fixes #15232, where we would warn about `TypeError` constraints being redundant.

Test Plan: Validate

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15232

Differential Revision: https://phabricator.haskell.org/D4808

compiler/typecheck/TcErrors.hs

@@ -459,9 +459,14 @@ warnRedundantConstraints ctxt env info ev_vars
    doc = text "Redundant constraint" <> plural redundant_evs <> colon
          <+> pprEvVarTheta redundant_evs
 
-   redundant_evs = case info of -- See Note [Redundant constraints in instance decls]
-                     InstSkol -> filterOut improving ev_vars
-                     _        -> ev_vars
+   redundant_evs =
+       filterOut is_type_error $
+       case info of -- See Note [Redundant constraints in instance decls]
+         InstSkol -> filterOut improving ev_vars
+         _        -> ev_vars
+
+   -- See #15232
+   is_type_error = isJust . userTypeError_maybe . idType
 
    improving ev_var = any isImprovementPred $
                       transSuperClasses (idType ev_var)

testsuite/tests/typecheck/should_compile/all.T

@@ -621,5 +621,4 @@ test('SplitWD', normal, compile, [''])
 test('T14441', omit_ways(['profasm']), compile, [''])
 test('T15050', [expect_broken(15050)], compile, [''])
 test('T14735', normal, compile, [''])
-test('T15232', expect_broken(15232), compile, [''])
-
+test('T15232', normal, compile, [''])