Sema: StructuralTypeRequest wraps the resolved type in a TypeAliasType

This fixes an issue with protocol compositions using a typealias of
anoter protocol composition causing a difference between the flat
structural type and the nested underlying TypeRepr.

Author: xymus

lib/Sema/TypeCheckGeneric.cpp

@@ -1030,5 +1030,17 @@ swift::StructuralTypeRequest::evaluate(Evaluator &evaluator,
 
   auto typeRepr = D->getUnderlyingTypeLoc().getTypeRepr();
   auto resolution = TypeResolution::forStructural(D);
-  return resolution.resolveType(typeRepr, options);
+  auto resolvedType = resolution.resolveType(typeRepr, options);
+
+  // Wrap the resolved type in a TypeAliasType
+  auto *genericSig = D->getGenericSignature();
+  SubstitutionMap subs;
+  if (genericSig)
+    subs = genericSig->getIdentitySubstitutionMap();
+
+  Type parent;
+  auto parentDC = D->getDeclContext();
+  if (parentDC->isTypeContext())
+    parent = parentDC->getSelfInterfaceType();
+  return TypeAliasType::get(D, parent, subs, resolvedType);
 }

test/type/protocol_composition.swift

@@ -178,3 +178,10 @@ takesP1AndP2([Swift.DoesNotExist & P1 & P2]()) // expected-error {{module 'Swift
 typealias T08 = P1 & inout P2 // expected-error {{'inout' may only be used on parameters}}
 typealias T09 = P1 & __shared P2 // expected-error {{'__shared' may only be used on parameters}}
 typealias T10 = P1 & __owned P2 // expected-error {{'__owned' may only be used on parameters}}
+
+// Using a typealias of a protocol composition in a protocol composition
+protocol C { }
+protocol DA { }
+protocol DB { }
+typealias DD = DA & DB
+func composeTypealiasOfComposition<T: C & DD>(x: T) {}