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

Lifetime extended temporary objects that are bound to references with
static storage duration may have external linkage and therefore require
mangled symbol names. Clang uses an extension of the Microsoft ABI to
give these symbols an implicit name of '$RT' followed by a discriminator
and then mangles them similarly to the variable they are bound to.
Clang's mangling scheme differs from the one used by MSVC.

Previously, the `$RT<discriminator>` portion of the name was not
registered as a back reference candidate and this resulted in incorrect
back references for enclosing class and/or namespace scopes that might
be referenced in the type of the object.

This is an ABI change and has the potential to cause backward
compatibility issues with previous Clang releases. Since MSVC uses a
different mangling scheme, this change does not affect compatibility
with MSVC.

This fixes one of the name mangling concerns reported in #85423.

Author: tahonermann

clang/docs/ReleaseNotes.rst

@@ -57,6 +57,12 @@ 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).
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 

clang/lib/AST/MicrosoftMangle.cpp

@@ -3911,7 +3911,8 @@ void MicrosoftMangleContextImpl::mangleReferenceTemporary(
   msvc_hashing_ostream MHO(Out);
   MicrosoftCXXNameMangler Mangler(*this, MHO);
 
-  Mangler.getStream() << "?$RT" << ManglingNumber << '@';
+  Mangler.getStream() << "?";
+  Mangler.mangleSourceName("$RT" + llvm::utostr(ManglingNumber));
   Mangler.mangle(VD, "");
 }
 

clang/test/CodeGenCXX/mangle-ms-back-references.cpp

@@ -1,5 +1,18 @@
 // RUN: %clang_cc1 -fms-extensions -fblocks -emit-llvm %s -o - -triple=i386-pc-win32 | FileCheck %s
 
+namespace NS {
+// The name "RT1" for the name of the class below has been specifically
+// chosen to ensure that back reference lookup does not match against the
+// implicitly generated "$RT1" name of the reference temporary symbol.
+struct RT1 {
+  static const RT1& singleton;
+  int i;
+};
+const RT1& RT1::singleton = RT1{1};
+}
+// CHECK: "?$RT1@singleton@RT1@NS@@2ABU23@B"
+// CHECK: "?singleton@RT1@NS@@2ABU12@B"
+
 void f1(const char* a, const char* b) {}
 // CHECK: "?f1@@YAXPBD0@Z"