[CS] Better diagnose conformance failures for EnumElementPatterns

Previously we could end up with a
ContextualMismatch fix and a MissingConformance fix
for different elements of the `matchTypes` disjunction,
leading to an ambiguity. Instead, avoid recording
the ContextualMismatch if we're matching an
existential, and tweak the MissingConformance
failure to have a custom diagnostic for
EnumElementPattern matching.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -208,6 +208,9 @@ ERROR(cannot_match_unresolved_expr_pattern_with_value,none,
 ERROR(cannot_match_value_with_pattern,none,
       "pattern of type %1 cannot match %0",
       (Type, Type))
+ERROR(pattern_does_not_conform_to_match,none,
+      "pattern of type %0 does not conform to expected match type %1",
+      (Type, Type))
 
 ERROR(cannot_reference_compare_types,none,
       "cannot check reference equality of functions; operands here have types "

lib/Sema/CSDiagnostics.cpp

@@ -6505,6 +6505,13 @@ bool MissingContextualConformanceFailure::diagnoseAsError() {
       break;
     }
 
+    case ConstraintLocator::EnumPatternImplicitCastMatch: {
+      emitDiagnostic(diag::pattern_does_not_conform_to_match, getFromType(),
+                     getToType())
+          .highlight(getSourceRange());
+      return true;
+    }
+
     default:
       break;
     }

lib/Sema/CSSimplify.cpp

@@ -6667,6 +6667,11 @@ bool ConstraintSystem::repairFailures(
     if (lhs->isPlaceholder() || rhs->isPlaceholder())
       return true;
 
+    // If we're converting to an existential, we'll diagnose failures in
+    // the conformance constraint.
+    if (hasConversionOrRestriction(ConversionRestrictionKind::Existential))
+      return false;
+
     conversionsOrFixes.push_back(ContextualMismatch::create(
         *this, lhs, rhs, getConstraintLocator(locator)));
     break;

test/Constraints/patterns.swift

@@ -791,3 +791,17 @@ do {
     }
   }
 }
+
+func testMatchingNonErrorConformingTypeInClosure(_ x: any Error) {
+  enum E {
+    case e
+  }
+  _ = {
+    switch x {
+    case E.e: // expected-error {{pattern of type 'E' does not conform to expected match type 'Error'}}
+      break
+    default:
+      break
+    }
+  }
+}

test/stmt/errors.swift

@@ -156,7 +156,7 @@ func twelve() {
   twelve_helper { (a, b) in // expected-error {{invalid conversion from throwing function of type '(Int, Int) throws -> ()' to non-throwing function type '(Int, Int) -> ()'}}
     do {
       try thrower()
-    } catch Twelve.Payload(a...b) {
+    } catch Twelve.Payload(a...b) { // expected-error {{pattern of type 'Twelve' does not conform to expected match type 'Error'}}
     }
   }
 }