AST: Add an extension point to TypeMatcher to always mismatch generic parameters

Author: slavapestov

include/swift/AST/TypeMatcher.h

@@ -195,9 +195,21 @@ class TypeMatcher {
     TRIVIAL_CASE(ModuleType)
     TRIVIAL_CASE(DynamicSelfType)
     TRIVIAL_CASE(ArchetypeType)
-    TRIVIAL_CASE(GenericTypeParamType)
     TRIVIAL_CASE(DependentMemberType)
 
+    bool visitGenericTypeParamType(CanGenericTypeParamType firstType,
+                                   Type secondType,
+                                   Type sugaredFirstType) {
+      /* If the types match, continue. */
+      if (!Matcher.asDerived().alwaysMismatchGenericParams() &&
+          firstType->isEqual(secondType))
+        return true;
+
+      /* Otherwise, let the derived class deal with the mismatch. */
+      return mismatch(firstType.getPointer(), secondType,
+                      sugaredFirstType);
+    }
+
     /// FIXME: Split this out into cases?
     bool visitAnyFunctionType(CanAnyFunctionType firstFunc, Type secondType,
                               Type sugaredFirstType) {
@@ -300,6 +312,8 @@ class TypeMatcher {
 #undef TRIVIAL_CASE
   };
 
+  bool alwaysMismatchGenericParams() const { return false; }
+
   ImplClass &asDerived() { return static_cast<ImplClass &>(*this); }
 
   const ImplClass &asDerived() const {