[CSDiagnostics] Simplify MissingContextualConformanceFailure and avoid storing diagnostic

Carrying diagnostic through the method is becoming unmanagable and
prune to issues due to "in-flight" diagnostic replacement.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -5774,6 +5774,12 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
   auto eltType = getFromType();
   auto contextualType = getToType();
 
+  auto diagnoseSingleElement = [&](Diag<Type, Type> msg, Type eltType,
+                                   Type contextualType) {
+    auto diagnostic = emitDiagnostic(msg, eltType, contextualType);
+    (void)trySequenceSubsequenceFixIts(diagnostic);
+  };
+
   auto diagnoseAllOccurrences = [&](Diag<Type, Type> diagnostic) {
     assert(AffectedElements.size() > 1);
     for (auto *element : AffectedElements) {
@@ -5790,16 +5796,16 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
   };
 
   bool treatAsDictionary = false;
-  Optional<InFlightDiagnostic> diagnostic;
   if (auto *AE = getAsExpr<ArrayExpr>(anchor)) {
     if (!(treatAsDictionary = isFixedToDictionary(AE))) {
       if (AffectedElements.size() > 1) {
         diagnoseAllOccurrences(diag::cannot_convert_array_element);
         return true;
       }
 
-      diagnostic.emplace(emitDiagnostic(diag::cannot_convert_array_element,
-                                        eltType, contextualType));
+      diagnoseSingleElement(diag::cannot_convert_array_element, eltType,
+                            contextualType);
+      return true;
     }
   }
 
@@ -5812,9 +5818,9 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
         return true;
       }
 
-      diagnostic.emplace(emitDiagnostic(diag::cannot_convert_dict_key, eltType,
-                                        contextualType));
-      break;
+      diagnoseSingleElement(diag::cannot_convert_dict_key, eltType,
+                            contextualType);
+      return true;
     }
 
     case 1: { // value
@@ -5823,9 +5829,9 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
         return true;
       }
 
-      diagnostic.emplace(emitDiagnostic(diag::cannot_convert_dict_value,
-                                        eltType, contextualType));
-      break;
+      diagnoseSingleElement(diag::cannot_convert_dict_value, eltType,
+                            contextualType);
+      return true;
     }
 
     default:
@@ -5841,25 +5847,22 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
     if ((purpose == ContextualTypePurpose::CTP_ForEachStmt ||
          purpose == ContextualTypePurpose::CTP_ForEachSequence) &&
         contextualType->hasUnresolvedType()) {
-      diagnostic.emplace(emitDiagnostic(
+      auto diagnostic = emitDiagnostic(
           (contextualType->is<TupleType>() && !eltType->is<TupleType>())
               ? diag::cannot_match_expr_tuple_pattern_with_nontuple_value
               : diag::cannot_match_unresolved_expr_pattern_with_value,
-          eltType));
+          eltType);
+      (void)trySequenceSubsequenceFixIts(diagnostic);
     } else {
-      diagnostic.emplace(
-          emitDiagnostic(contextualType->isExistentialType()
-                             ? diag::cannot_convert_sequence_element_protocol
-                             : diag::cannot_convert_sequence_element_value,
-                         eltType, contextualType));
+      diagnoseSingleElement(contextualType->isExistentialType()
+                                ? diag::cannot_convert_sequence_element_protocol
+                                : diag::cannot_convert_sequence_element_value,
+                            eltType, contextualType);
     }
+    return true;
   }
 
-  if (!diagnostic)
-    return false;
-
-  (void)trySequenceSubsequenceFixIts(*diagnostic);
-  return true;
+  return false;
 }
 
 bool MissingContextualConformanceFailure::diagnoseAsError() {