[clang] fix unresolved dependent template specialization mangling (llvm#135111)

This fixes a regression introduced in
llvm#133610 which was reported here
llvm#133610 (comment)

When mangling a dependent template specialization appearing within an
unresolved prefix, translate the dtst back to a dependent template name
including the prefix, and mangle following the nested unresolved-type
production.

There are no release notes, since this regression was never released.

Author: mizvekov

clang/lib/AST/ItaniumMangle.cpp

@@ -1404,9 +1404,19 @@ void CXXNameMangler::mangleUnresolvedPrefix(NestedNameSpecifier *qualifier,
     //   - a template type parameter
     //   - a template template parameter with arguments
     // In all of these cases, we should have no prefix.
-    if (qualifier->getPrefix()) {
-      mangleUnresolvedPrefix(qualifier->getPrefix(),
-                             /*recursive*/ true);
+    if (NestedNameSpecifier *Prefix = qualifier->getPrefix()) {
+      if (const auto *DTST =
+              dyn_cast<DependentTemplateSpecializationType>(type)) {
+        Out << "srN";
+        TemplateName Template = getASTContext().getDependentTemplateName(
+            {Prefix, DTST->getDependentTemplateName().getName(),
+             /*HasTemplateKeyword=*/true});
+        mangleTemplatePrefix(Template);
+        mangleTemplateArgs(Template, DTST->template_arguments());
+        break;
+      }
+      mangleUnresolvedPrefix(Prefix,
+                             /*recursive=*/true);
     } else {
       // Otherwise, all the cases want this.
       Out << "sr";

clang/test/CodeGenCXX/mangle-template.cpp

@@ -70,7 +70,7 @@ namespace test7 {
     static const unsigned value = sizeof(T);
   };
 
-  template<unsigned> struct int_c { 
+  template<unsigned> struct int_c {
     typedef float type;
   };
 
@@ -92,7 +92,7 @@ namespace test8 {
     };
   };
 
-  template<unsigned> struct int_c { 
+  template<unsigned> struct int_c {
     typedef float type;
   };
 
@@ -399,3 +399,20 @@ namespace type_qualifier {
   // CHECK: @_ZN14type_qualifier1gIPiEEvDTcmcvv_ELi1EE
   template void g<int*>(int);
 }
+
+namespace unresolved_template_specialization_type {
+  template <int> struct enable_if {};
+  struct Foo {
+    static const int value = true;
+  };
+  struct HashStateBase {
+    template <typename> using is_hashable = Foo;
+  };
+  template <class> struct raw_hash_set {
+    template <typename H>
+    static enable_if<H::template is_hashable<int>::value>
+        AbslHashValue() {}
+  };
+  template enable_if<true> raw_hash_set<int>::AbslHashValue<HashStateBase>();
+  // CHECH: @_ZN39unresolved_template_specialization_type12raw_hash_setIiE13AbslHashValueINS_13HashStateBaseEEENS_9enable_ifIXsrNT_11is_hashableIiEE5valueEEEv
+} // namespace unresolved_template_specialization_type