[Sema] Ensure that NameAliasTypes record the right substitutions.

lib/Sema/TypeCheckDecl.cpp

@@ -6938,29 +6938,26 @@ void TypeChecker::validateDecl(ValueDecl *D) {
         if (!aliasDecl->isGeneric()) {
           aliasDecl->setGenericEnvironment(proto->getGenericEnvironment());
 
-          // If the underlying alias declaration has a type parameter,
-          // we have unresolved dependent member types we will need to deal
-          // with. Wipe out the types and validate them again.
-          // FIXME: We never should have recorded such a type in the first
-          // place.
-          if (!aliasDecl->getUnderlyingTypeLoc().getType() ||
-              aliasDecl->getUnderlyingTypeLoc().getType()
-                ->findUnresolvedDependentMemberType()) {
-            aliasDecl->getUnderlyingTypeLoc().setType(Type(),
-                                                      /*validated=*/false);
-            validateAccessControl(aliasDecl);
-
-            // Check generic parameters, if needed.
-            bool validated = aliasDecl->hasValidationStarted();
+          // The generic environment didn't exist until now, we may have
+          // unresolved types we will need to deal with, and need to record the
+          // appropriate substitutions for that environment. Wipe out the types
+          // and validate them again.
+          aliasDecl->getUnderlyingTypeLoc().setType(Type(),
+                                                    /*validated=*/false);
+          aliasDecl->setInterfaceType(Type());
+
+          validateAccessControl(aliasDecl);
+
+          // Check generic parameters, if needed.
+          bool validated = aliasDecl->hasValidationStarted();
+          if (!validated)
+            aliasDecl->setIsBeingValidated();
+          SWIFT_DEFER {
             if (!validated)
-              aliasDecl->setIsBeingValidated();
-            SWIFT_DEFER {
-              if (!validated)
-                aliasDecl->setIsBeingValidated(false);
-            };
+              aliasDecl->setIsBeingValidated(false);
+          };
 
-            validateTypealiasType(*this, aliasDecl);
-          }
+          validateTypealiasType(*this, aliasDecl);
         }
       }
     }

lib/Sema/TypeCheckGeneric.cpp

@@ -193,6 +193,16 @@ Type CompleteGenericTypeResolver::resolveDependentMemberType(
   // base type into it.
   auto concrete = ref->getBoundDecl();
   tc.validateDeclForNameLookup(concrete);
+
+  if (auto typeAlias = dyn_cast<TypeAliasDecl>(concrete)) {
+    if (auto protocol = dyn_cast<ProtocolDecl>(typeAlias->getDeclContext())) {
+      // We need to make sure the generic environment of a surrounding protocol
+      // propagates to the typealias, since the former may not have existed when
+      // the typealiases type was first computed.
+      // FIXME: See the comment in the ProtocolDecl case of validateDecl().
+      tc.validateDecl(protocol);
+    }
+  }
   if (!concrete->hasInterfaceType())
     return ErrorType::get(tc.Context);
   if (baseTy->isTypeParameter()) {

validation-test/compiler_crashers_2_fixed/0156-rdar39636312.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend %s -emit-module
+
+protocol P2 {
+    func f1<T : P1>(_: T) -> T.T1
+}
+
+public struct S1<U> {}
+
+protocol P1 {
+    typealias T1 = S1<Self>
+}