Sema: Clean up named opaque result type generic signature construction

We were building the signature twice, and adding the 'where' clause twice
each time.

The GSB magically uniqued them, whereas the Requirement Machine is not
so forgiving.

Author: slavapestov

lib/Sema/TypeCheckGeneric.cpp

@@ -122,7 +122,7 @@ OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
         originatingDC->getParentModule(),
         outerGenericSignature.getPointer(),
         genericParams,
-        WhereClauseOwner(originatingDC, genericParams),
+        WhereClauseOwner(),
         /*addedRequirements=*/{},
         /*inferenceSources=*/{},
         /*allowConcreteGenericParams=*/false};
@@ -208,6 +208,9 @@ OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
   opaqueDecl->copyFormalAccessFrom(originatingDecl);
   if (auto originatingSig = originatingDC->getGenericSignatureOfContext()) {
     opaqueDecl->setGenericSignature(originatingSig);
+  } else {
+    // Avoid kicking off GenericSignatureRequest for the OpaqueTypeDecl.
+    opaqueDecl->setGenericSignature(GenericSignature());
   }
 
   // Resolving in the context of `opaqueDecl` allows type resolution to create
@@ -572,6 +575,8 @@ static unsigned getExtendedTypeGenericDepth(ExtensionDecl *ext) {
 GenericSignature
 GenericSignatureRequest::evaluate(Evaluator &evaluator,
                                   GenericContext *GC) const {
+  assert(!isa<OpaqueTypeDecl>(GC));
+
   auto &ctx = GC->getASTContext();
 
   // The signature of a Protocol is trivial (Self: TheProtocol) so let's compute

lib/Serialization/Deserialization.cpp

@@ -3567,6 +3567,8 @@ class DeclDeserializer {
     auto genericSig = MF.getGenericSignature(genericSigID);
     if (genericSig)
       opaqueDecl->setGenericSignature(genericSig);
+    else
+      opaqueDecl->setGenericSignature(GenericSignature());
     if (underlyingTypeSubsID) {
       auto subMapOrError = MF.getSubstitutionMapChecked(underlyingTypeSubsID);
       if (!subMapOrError)

test/type/opaque_return_named.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-named-opaque-types -disable-availability-checking
+// RUN: %target-typecheck-verify-swift -enable-experimental-named-opaque-types -disable-availability-checking -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=on
 
 // Tests for experimental extensions to opaque return type support.
 
@@ -133,14 +133,10 @@ struct Generic<T: P1 & Equatable & DefaultInitializable>
     }
   }
 
-  // FIXME: expected-warning@+2{{redundant same-type constraint 'C.Element' == 'T.A'}}
-  // FIXME: expected-note@+1{{previous same-type constraint 'C.Element' == 'T.A' written here}}
   func sameTypeParams() -> <C: Collection where C.Element == T.A> C {
     [ T.A(), T.A() ]
   }
 
-  // FIXME: expected-warning@+2{{redundant same-type constraint 'C.Element' == 'T.A'}}
-  // FIXME: expected-note@+1{{previous same-type constraint 'C.Element' == 'T.A' written here}}
   func sameTypeParamsBad() -> <C: Collection where C.Element == T.A> C {
     [ T() ] // expected-error{{cannot convert value of type 'T' to expected element type 'T.A'}}
   }