Break some cycles

Use the interface type of the underlying type in the GSB in as many
cases as possible except for one important one: requirement signature
computation.  There, use the structural type so we don't wind up
recursively validating the requirements of an incomplete protocol.

Author: CodaFi

lib/AST/GenericSignatureBuilder.cpp

@@ -3785,7 +3785,13 @@ PotentialArchetype *GenericSignatureBuilder::realizePotentialArchetype(
 
 static Type getStructuralType(TypeDecl *typeDecl) {
   if (auto typealias = dyn_cast<TypeAliasDecl>(typeDecl)) {
-    return typealias->getStructuralType();
+	// When we're computing requirement signatures, the structural type
+	// suffices.  Otherwise we'll potentially try to validate incomplete 
+	// requirements.
+	auto *proto = dyn_cast_or_null<ProtocolDecl>(typealias->getDeclContext()->getAsDecl());
+	if (proto && proto->isComputingRequirementSignature())
+		return typealias->getStructuralType();
+    return typealias->getUnderlyingType();
   }
 
   return typeDecl->getDeclaredInterfaceType();

test/Generics/protocol_type_aliases.swift

@@ -42,14 +42,14 @@ func concreteRequirementOnConcreteNestedTypeAlias<T>(_: T) where T: Q2, S<T.C> =
 protocol P3 {
     typealias T = Int
 }
-protocol Q3: P3 { // expected-error{{generic signature requires types 'P3.T' (aka 'Int')}}
+protocol Q3: P3 { // expected-error{{generic signature requires types 'Int'}}
     typealias T = Float
 }
 
 protocol P3_1 {
     typealias T = Float
 }
-protocol Q3_1: P3, P3_1 {} // expected-error{{generic signature requires types 'P3_1.T' (aka 'Float')}}
+protocol Q3_1: P3, P3_1 {} // expected-error{{generic signature requires types 'Float'}}
 
 
 // Subprotocols can force associated types in their parents to be concrete, and

validation-test/compiler_crashers_2_fixed/0145-sr7097.swift

@@ -9,7 +9,7 @@ protocol P2 {
 }
 
 // CHECK-LABEL: .P3@
-// CHECK-NEXT: Requirement signature: <Self where Self : P2, Self.Assoc == P3.Assoc>
+// CHECK-NEXT: Requirement signature: <Self where Self : P2, Self.Assoc == ConformsToP1>
 protocol P3 : P2 { }
 
 struct S0<M: P3> where M.Assoc: P1 { } // expected-warning{{redundant conformance constraint 'M.Assoc': 'P1'}}