[clang] [Clang] [AST] Fix placeholder return type name mangling for MSVC 1920+ / VS2019+ (PR #102848)

[NAKAMURA Takumi via cfe-commits]

================
@@ -2900,17 +2958,51 @@ void MicrosoftCXXNameMangler::mangleFunctionType(const FunctionType *T,
       // can differ by their calling convention and are typically deduced.  So
       // we make sure that this type gets mangled properly.
       mangleType(ResultType, Range, QMM_Result);
-    } else if (const auto *AT = dyn_cast_or_null<AutoType>(
-                   ResultType->getContainedAutoType())) {
-      Out << '?';
-      mangleQualifiers(ResultType.getLocalQualifiers(), /*IsMember=*/false);
-      Out << '?';
+    } else if (IsInLambda) {
+      if (const auto *AT = ResultType->getContainedAutoType()) {
+        assert(AT->getKeyword() == AutoTypeKeyword::Auto &&
+               "should only need to mangle auto!");
+        Out << '?';
+        mangleQualifiers(ResultType.getLocalQualifiers(), /*IsMember=*/false);
+        Out << '?';
+        mangleSourceName("<auto>");
+        Out << '@';
+      } else {
+        Out << '@';
+      }
+    } else if (const auto *AT = ResultType->getContainedAutoType()) {
       assert(AT->getKeyword() != AutoTypeKeyword::GNUAutoType &&
              "shouldn't need to mangle __auto_type!");
-      mangleSourceName(AT->isDecltypeAuto() ? "<decltype-auto>" : "<auto>");
-      Out << '@';
-    } else if (IsInLambda) {
-      Out << '@';
+
+      // If we have any pointer types with the clang address space extension
+      // then defer to the custom clang mangling to keep backwards
+      // compatibility. See `mangleType(const PointerType *T, Qualifiers Quals,
+      // SourceRange Range)` for details.
+      auto UseClangMangling = [](QualType ResultType) {
+        QualType T = ResultType;
+        while (const auto *PT = dyn_cast<PointerType>(T.getTypePtr())) {
----------------
chapuni wrote:

PT is not used.

https://github.com/llvm/llvm-project/pull/102848