[CSDiagnostics] Teach generic parameter restoration logic about pack expansions

Just like in any other position, let's restore unresolved type variables
that represent generic parameters to their generic parameter type when
they are found in a pack expansion pattern type.

Author: xedin

lib/Sema/CSDiagnostics.h

@@ -99,20 +99,34 @@ class FailureDiagnostic {
   /// Resolve type variables present in the raw type, if any.
   Type resolveType(Type rawType, bool reconstituteSugar = false,
                    bool wantRValue = true) const {
-    auto &cs = getConstraintSystem();
-
     if (rawType->hasTypeVariable() || rawType->hasPlaceholder()) {
-      rawType = rawType.transform([&](Type type) {
+      rawType = rawType.transform([&](Type type) -> Type {
         if (auto *typeVar = type->getAs<TypeVariableType>()) {
           auto resolvedType = S.simplifyType(typeVar);
+
+          if (!resolvedType->hasUnresolvedType())
+            return resolvedType;
+
+          // If type variable was simplified to an unresolved pack expansion
+          // type, let's examine its original pattern type because it could
+          // contain type variables replaceable with their generic parameter
+          // types.
+          if (auto *expansion = resolvedType->getAs<PackExpansionType>()) {
+            auto *locator = typeVar->getImpl().getLocator();
+            auto *openedExpansionTy =
+                locator->castLastElementTo<LocatorPathElt::PackExpansionType>()
+                    .getOpenedType();
+            auto patternType = resolveType(openedExpansionTy->getPatternType());
+            return PackExpansionType::get(patternType,
+                                          expansion->getCountType());
+          }
+
           Type GP = typeVar->getImpl().getGenericParameter();
-          return resolvedType->is<UnresolvedType>() && GP
-                     ? GP
-                     : resolvedType;
+          return resolvedType->is<UnresolvedType>() && GP ? GP : resolvedType;
         }
 
         return type->isPlaceholder()
-                   ? Type(cs.getASTContext().TheUnresolvedType)
+                   ? Type(type->getASTContext().TheUnresolvedType)
                    : type;
       });
     }