Resolve collisions when typealias used in associatedtype constraint

Uses Anthony's fix (3c87252) to resolve collisions

Author: zoecarver

lib/Sema/TypeCheckType.cpp

@@ -565,9 +565,7 @@ Type TypeChecker::resolveTypeInContext(TypeDecl *typeDecl, DeclContext *foundDC,
 
     if (selfType->is<GenericTypeParamType>()) {
       if (typeDecl->getDeclContext()->getSelfProtocolDecl()) {
-        if (isa<AssociatedTypeDecl>(typeDecl) ||
-            (isa<TypeAliasDecl>(typeDecl) &&
-             !cast<TypeAliasDecl>(typeDecl)->isGeneric())) {
+        if (isa<AssociatedTypeDecl>(typeDecl)) {
           // FIXME: We should use this lookup method for the Interface
           // stage too, but right now that causes problems with
           // Sequence.SubSequence vs Collection.SubSequence; the former
@@ -1295,10 +1293,23 @@ resolveTopLevelIdentTypeComponent(TypeResolution resolution,
     // Otherwise, check for an ambiguity.
     if (!resolution.areSameType(current, type)) {
       isAmbiguous = true;
-      break;
+      // If a typealias collides with an associatedtype in generic requirement
+      // position, prefer the associated type.
+      if (options.getContext() == TypeResolverContext::GenericRequirement) {
+        if (isa<TypeAliasDecl>(currentDecl) && isa<AssociatedTypeDecl>(typeDecl)) {
+          current = type;
+          currentDecl = typeDecl;
+          currentDC = foundDC;
+          isAmbiguous = false;
+        } else if (isa<AssociatedTypeDecl>(currentDecl) &&
+                   isa<TypeAliasDecl>(typeDecl)) {
+          isAmbiguous = false;
+        }
+      }
+      if (isAmbiguous) break;
     }
 
-    // We have a found multiple type aliases that refer to the same thing.
+    // We have found multiple type aliases that refer to the same thing.
     // Ignore the duplicate.
   }
 

validation-test/compiler_crashers_2_fixed/sr11639.swift

@@ -0,0 +1,16 @@
+// RUN: not %target-typecheck-verify-swift
+
+protocol ProtocolA {
+    associatedtype T1
+}
+
+struct S<T> : ProtocolA {
+  typealias T1 = T
+}
+
+protocol ProtocolB: ProtocolA {
+    associatedtype T2: ProtocolB where T2.T1 == T3.T1
+    associatedtype X
+    typealias T3 = S<X>
+}
+