[AST] Dodge an annoying libc++ std::function quirk requiring complete result types

include/swift/AST/Type.h

@@ -166,8 +166,11 @@ enum class SubstFlags {
 
 /// Options for performing substitutions into a type.
 struct SubstOptions : public OptionSet<SubstFlags> {
-  typedef std::function<Type(const NormalProtocolConformance *,
-                             AssociatedTypeDecl *)>
+  // Note: The unfortunate use of TypeBase * here, rather than Type,
+  // is due to a libc++ quirk that requires the result type to be
+  // complete.
+  typedef std::function<TypeBase *(const NormalProtocolConformance *,
+                                   AssociatedTypeDecl *)>
     GetTentativeTypeWitness;
 
   /// Function that retrieves a tentative type witness for a protocol

lib/AST/ProtocolConformance.cpp

@@ -490,7 +490,8 @@ NormalProtocolConformance::getTypeWitnessAndDecl(AssociatedTypeDecl *assocType,
   if (getState() == ProtocolConformanceState::CheckingTypeWitnesses) {
     // If there is a tentative-type-witness function, use it.
     if (options.getTentativeTypeWitness) {
-      if (Type witnessType = options.getTentativeTypeWitness(this, assocType))
+     if (Type witnessType =
+           Type(options.getTentativeTypeWitness(this, assocType)))
         return { witnessType, nullptr };
     }
 

lib/Sema/TypeCheckProtocol.cpp

@@ -4204,10 +4204,10 @@ void ConformanceChecker::resolveTypeWitnesses() {
       SubstOptions options(None);
       options.getTentativeTypeWitness =
         [&](const NormalProtocolConformance *conformance,
-            AssociatedTypeDecl *assocType) -> Type {
-          if (conformance != Conformance) return Type(0);
+            AssociatedTypeDecl *assocType) -> TypeBase * {
+          if (conformance != Conformance) return nullptr;
 
-          return typeWitnesses.begin(assocType)->first;
+          return typeWitnesses.begin(assocType)->first.getPointer();
         };
 
       auto substitutions =