[TypeResolver] Fix a few corner cases in type resolution for explicit

existential types.

Use the correct type resolver context when resolving SIL metatypes
and function types, and diagnose an error if an explicit existential
type appears in a protocol composition.

Author: hborla

lib/Sema/TypeCheckType.cpp

@@ -2274,7 +2274,13 @@ TypeResolver::resolveAttributedType(TypeAttributes &attrs, TypeRepr *repr,
           Optional<MetatypeRepresentation> storedRepr;
           // The instance type is not a SIL type.
           auto instanceOptions = options;
-          instanceOptions.setContext(None);
+          TypeResolverContext context = TypeResolverContext::None;
+          if (isa<MetatypeTypeRepr>(repr)) {
+            context = TypeResolverContext::MetatypeBase;
+          } else if (isa<ProtocolTypeRepr>(repr)) {
+            context = TypeResolverContext::ProtocolMetatypeBase;
+          }
+          instanceOptions.setContext(context);
           instanceOptions -= TypeResolutionFlags::SILType;
 
           auto instanceTy = resolveType(base, instanceOptions);
@@ -3166,7 +3172,8 @@ NeverNullType TypeResolver::resolveSILFunctionType(
 
   ProtocolConformanceRef witnessMethodConformance;
   if (witnessMethodProtocol) {
-    auto resolved = resolveType(witnessMethodProtocol, options);
+    auto resolved = resolveType(witnessMethodProtocol,
+        options.withContext(TypeResolverContext::GenericRequirement));
     if (resolved->hasError())
       return resolved;
 
@@ -3443,9 +3450,7 @@ TypeResolver::resolveIdentifierType(IdentTypeRepr *IdType,
   }
 
   // FIXME: Don't use ExistentialType for AnyObject for now.
-  bool isConstraintType = (result->is<ProtocolType>() &&
-                           !result->isAnyObject());
-  if (isConstraintType &&
+  if (result->isConstraintType() && !result->isAnyObject() &&
       getASTContext().LangOpts.EnableExplicitExistentialTypes &&
       options.isConstraintImplicitExistential()) {
     return ExistentialType::get(result);
@@ -3807,7 +3812,7 @@ TypeResolver::resolveCompositionType(CompositionTypeRepr *repr,
       continue;
     }
 
-    if (ty->isExistentialType()) {
+    if (ty->isConstraintType()) {
       auto layout = ty->getExistentialLayout();
       if (auto superclass = layout.explicitSuperclass)
         if (checkSuperclass(tyR->getStartLoc(), superclass))