[GSB] Provide a root potential archetype for requirement-signature sources.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -617,8 +617,8 @@ class GenericSignatureBuilder::RequirementSource : public llvm::FoldingSetNode {
   /// Retrieve a requirement source representing the requirement signature
   /// computation for a protocol.
   static const RequirementSource *forRequirementSignature(
-                                              GenericSignatureBuilder &builder,
-                                              ProtocolDecl *protocol);
+                                                      PotentialArchetype *root,
+                                                      ProtocolDecl *protocol);
 
   /// Retrieve a requirement source for nested type name matches.
   static const RequirementSource *forNestedTypeNameMatch(

lib/AST/Decl.cpp

@@ -3104,9 +3104,10 @@ void ProtocolDecl::computeRequirementSignature() {
   GenericSignatureBuilder builder(getASTContext(),
                                   LookUpConformanceInModule(module));
   builder.addGenericParameter(selfType);
-  builder.addRequirement(requirement,
-                         GenericSignatureBuilder::RequirementSource
-                          ::forRequirementSignature(builder, this));
+  builder.addRequirement(
+         requirement,
+         GenericSignatureBuilder::RequirementSource
+          ::forRequirementSignature(builder.resolveArchetype(selfType), this));
   builder.finalize(SourceLoc(), { selfType });
 
   RequirementSignature = builder.getGenericSignature();

lib/AST/GenericSignatureBuilder.cpp

@@ -278,8 +278,10 @@ const RequirementSource *RequirementSource::forInferred(
 }
 
 const RequirementSource *RequirementSource::forRequirementSignature(
-                                              GenericSignatureBuilder &builder,
+                                              PotentialArchetype *root,
                                               ProtocolDecl *protocol) {
+  auto &builder = *root->getBuilder();
+  // FIXME: Store the root
   REQUIREMENT_SOURCE_FACTORY_BODY(RequirementSignatureSelf, nullptr, protocol);
 }