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

[via cfe-commits]

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@@ -2494,6 +2506,58 @@ void MicrosoftCXXNameMangler::mangleAddressSpaceType(QualType T,
   mangleArtificialTagType(TagTypeKind::Struct, ASMangling, {"__clang"});
 }
 
+void MicrosoftCXXNameMangler::mangleAutoReturnType(QualType T,
+                                                   SourceRange Range,
+                                                   QualifierMangleMode QMM) {
+  assert(getASTContext().getLangOpts().isCompatibleWithMSVC(
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zmodem wrote:

I'm not sure asserting here is right. Wouldn't this break users targeting VS2017, whereas before they just got incorrect manglings (but most probably didn't notice)?

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