[ConstraintSystem] Use missing protocol fix for sequence element mismatches related to protocols

(cherry picked from commit e76ae9c)

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -456,7 +456,7 @@ ERROR(cannot_convert_sequence_element_value,none,
       "cannot convert sequence element type %0 to expected type %1",
       (Type, Type))
 ERROR(cannot_convert_sequence_element_protocol,none,
-      "sequence element type %0 does not conform to expected type %1",
+      "sequence element type %0 does not conform to expected protocol %1",
       (Type, Type))
 
 ERROR(throws_functiontype_mismatch,none,

lib/Sema/CSDiagnostics.cpp

@@ -2832,6 +2832,11 @@ bool MissingContextualConformanceFailure::diagnoseAsError() {
       diagnostic = getDiagnosticFor(Context);
       break;
 
+    case ConstraintLocator::SequenceElementType: {
+      diagnostic = diag::cannot_convert_sequence_element_protocol;
+      break;
+    }
+
     default:
       break;
     }

lib/Sema/CSSimplify.cpp

@@ -2192,6 +2192,11 @@ bool ConstraintSystem::repairFailures(
   }
 
   case ConstraintLocator::SequenceElementType: {
+    // This is going to be diagnosed as `missing conformance`,
+    // so no need to create duplicate fixes.
+    if (rhs->isExistentialType())
+      break;
+
     conversionsOrFixes.push_back(CollectionElementContextualMismatch::create(
         *this, lhs, rhs, getConstraintLocator(locator)));
     break;

test/stmt/foreach.swift

@@ -61,7 +61,7 @@ func patterns(gir: GoodRange<Int>, gtr: GoodTupleIterator) {
   for i : Int in gir { sum = sum + i }
   for i in gir { sum = sum + i }
   for f : Float in gir { sum = sum + f } // expected-error{{cannot convert sequence element type 'Int' to expected type 'Float'}}
-  for f : ElementProtocol in gir { } // expected-error {{sequence element type 'Int' does not conform to expected type 'ElementProtocol'}}
+  for f : ElementProtocol in gir { } // expected-error {{sequence element type 'Int' does not conform to expected protocol 'ElementProtocol'}}
 
   for (i, f) : (Int, Float) in gtr { sum = sum + i }