[GSB] Clean up and audit uses of ArchetypeResolutionKind.

Use ArchetypeResolutionKind::CompleteWellFormed whenever we need to
ask questions about the potential archetype, and
ArchetypeResolutionKind::WellFormed when we need only evaluate whether
there is a legitimate type with that name (and possibly get a handle
to it).

Author: DougGregor

lib/AST/Decl.cpp

@@ -3292,8 +3292,7 @@ void ProtocolDecl::computeRequirementSignature() {
                                   LookUpConformanceInModule(module));
   builder.addGenericParameter(selfType);
   auto selfPA =
-      builder.resolveArchetype(selfType,
-                               ArchetypeResolutionKind::CompleteWellFormed);
+      builder.resolveArchetype(selfType, ArchetypeResolutionKind::WellFormed);
 
   builder.addRequirement(
          requirement,

lib/AST/GenericSignature.cpp

@@ -488,21 +488,25 @@ bool GenericSignature::requiresClass(Type type, ModuleDecl &mod) {
     builder.resolveArchetype(type, ArchetypeResolutionKind::CompleteWellFormed);
   if (!pa) return false;
 
-  pa = pa->getRepresentative();
+  if (pa->isConcreteType()) return false;
 
   // If this type was mapped to a concrete type, then there is no
   // requirement.
+  pa = pa->getRepresentative();
+
   if (pa->isConcreteType()) return false;
 
   // If there is a layout constraint, it might be a class.
-  if (auto layout = pa->getLayout())
-    if (layout->isClass())
-      return true;
+  if (auto layout = pa->getLayout()) {
+    if (layout->isClass()) return true;
+  }
 
   // If there is a superclass bound, then obviously it must be a class.
+  // FIXME: We shouldn't need this?
   if (pa->getSuperclass()) return true;
 
   // If any of the protocols are class-bound, then it must be a class.
+  // FIXME: We shouldn't need this?
   for (auto proto : pa->getConformsTo()) {
     if (proto->requiresClass()) return true;
   }
@@ -519,8 +523,6 @@ Type GenericSignature::getSuperclassBound(Type type, ModuleDecl &mod) {
     builder.resolveArchetype(type, ArchetypeResolutionKind::CompleteWellFormed);
   if (!pa) return nullptr;
 
-  pa = pa->getRepresentative();
-
   // If this type was mapped to a concrete type, then there is no
   // requirement.
   if (pa->isConcreteType()) return nullptr;
@@ -540,8 +542,6 @@ SmallVector<ProtocolDecl *, 2> GenericSignature::getConformsTo(Type type,
     builder.resolveArchetype(type, ArchetypeResolutionKind::CompleteWellFormed);
   if (!pa) return { };
 
-  pa = pa->getRepresentative();
-
   // If this type was mapped to a concrete type, then there are no
   // requirements.
   if (pa->isConcreteType()) return { };
@@ -567,8 +567,6 @@ bool GenericSignature::conformsToProtocol(Type type, ProtocolDecl *proto,
     builder.resolveArchetype(type, ArchetypeResolutionKind::CompleteWellFormed);
   if (!pa) return false;
 
-  pa = pa->getRepresentative();
-
   // FIXME: Deal with concrete conformances here?
   if (pa->isConcreteType()) return false;
 
@@ -596,9 +594,6 @@ Type GenericSignature::getConcreteType(Type type, ModuleDecl &mod) {
     builder.resolveArchetype(type, ArchetypeResolutionKind::CompleteWellFormed);
   if (!pa) return Type();
 
-  pa = pa->getRepresentative();
-  if (!pa->isConcreteType()) return Type();
-
   return pa->getConcreteType();
 }
 
@@ -611,7 +606,6 @@ LayoutConstraint GenericSignature::getLayoutConstraint(Type type,
     builder.resolveArchetype(type, ArchetypeResolutionKind::CompleteWellFormed);
   if (!pa) return LayoutConstraint();
 
-  pa = pa->getRepresentative();
   return pa->getLayout();
 }
 
@@ -882,7 +876,7 @@ ConformanceAccessPath GenericSignature::getConformanceAccessPath(
       auto pa =
         inProtoSigBuilder.resolveArchetype(
                                  storedType,
-                                 ArchetypeResolutionKind::WellFormed);
+                                 ArchetypeResolutionKind::CompleteWellFormed);
       auto equivClass = pa->getOrCreateEquivalenceClass();
 
       // Find the conformance of this potential archetype to the protocol in

lib/AST/GenericSignatureBuilder.cpp

@@ -1960,14 +1960,10 @@ PotentialArchetype *PotentialArchetype::updateNestedTypeForConformance(
     }
 
     case ArchetypeResolutionKind::AlreadyKnown:
-      break;
+        return nullptr;
     }
   }
 
-  // If we still don't have a result potential archetype, we're done.
-  if (!resultPA)
-    return nullptr;
-
   // If we have a potential archetype that requires more processing, do so now.
   if (shouldUpdatePA) {
     // For concrete types, introduce a same-type requirement to the aliased

lib/Sema/TypeCheckGeneric.cpp

@@ -132,9 +132,10 @@ Type CompleteGenericTypeResolver::resolveDependentMemberType(
   assert(basePA && "Missing potential archetype for base");
 
   // Retrieve the potential archetype for the nested type.
-  auto nestedPA = basePA->getNestedType(ref->getIdentifier(),
-                                        ArchetypeResolutionKind::WellFormed,
-                                        Builder);
+  auto nestedPA =
+    basePA->getNestedType(ref->getIdentifier(),
+                          ArchetypeResolutionKind::CompleteWellFormed,
+                          Builder);
 
   // If there was no such nested type, produce an error.
   if (!nestedPA) {