[clang] [clang] Correct Microsoft mangling of lifetime extended temporary objects. (PR #85529)

[via cfe-commits]

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@@ -54,6 +54,12 @@ ABI Changes in This Version
   inline member function that contains a static local variable with a dynamic
   initializer is declared with ``__declspec(dllimport)``. (#GH83616).
 
+- Fixed Microsoft name mangling of lifetime extended temporary objects. This
+  change corrects missing back reference registrations that could result in
+  incorrect back reference indexes and suprising demangled name results. Since
+  MSVC uses a different mangling for these objects, compatibility is not affected.
+  (#GH85423).
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zmodem wrote:

> Commit [e77de75](https://github.com/llvm/llvm-project/commit/e77de75d7e009daaadb03e9b1091c3a9a9c9d311) claims that Microsoft doesn't have a mangling for these objects and that the `$RT` mangling is a Clang invention. The claim that MSVC doesn't implement a mangling for these doesn't seem right though. Consider https://godbolt.org/z/n715zPbc6.

Looks like things have changed then. I tried the example code with MSVC 18.00.31101.0 (from VS2013) and there the object is just `_$S1`:

```
??__E?singleton at RT1@@2ABU1 at B@@YAXXZ PROC                ; ??__E?singleton at RT1@@2ABU1 at B@@YAXXZ, COMDAT
; File z:\tmp\a.cc
; Line 6
        push    ebp
        mov     ebp, esp
        push    ecx
        call    ?f@@YA?AURT1@@XZ                        ; f
        mov     DWORD PTR $T1[ebp], eax
        mov     eax, DWORD PTR $T1[ebp]
        mov     DWORD PTR _$S1, eax
        mov     DWORD PTR ?singleton at RT1@@2ABU1 at B, OFFSET _$S1 ; RT1::singleton
        mov     esp, ebp
        pop     ebp
        ret     0
```

https://github.com/llvm/llvm-project/pull/85529