[llvm-bugs] [Bug 42005] New: clang-cl front end failure with xlocale from VS 2019 v16.1

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42005

            Bug ID: 42005
           Summary: clang-cl front end failure with xlocale from VS 2019
                    v16.1
           Product: new-bugs
           Version: 8.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: brian.apps at oxfordsemantic.tech
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Created attachment 22013
  --> https://bugs.llvm.org/attachment.cgi?id=22013&action=edit
Preprocessed source and script

I recently upgraded my system to Visual Studio 2019 16.1. When I build my
product with clang-cl v8.0.0, I now see the following error:

clang-cl: error: clang frontend command failed due to signal (use -v to see
invocation)

I've boiled the problem down to a few lines and the statement `return new
std::fstream(s, std::ofstream out | std::ofstream::trunc);` A script and
preprocessed source are attached.


Further digging around shows clang 7.0 works fine but both 8.0.1-rc1 and a
preview of 9.0.0 fail. I see different errors messages, which may aid
identification:

Assertion failed: Idx < size() && "Template parameter index out-of-range", file
C:\clang\llvm-project\clang\include\clang/AST/DeclTemplate.h, line 138

and

1.      <eof> parser at end of file
2.      Per-file LLVM IR generation
3.      C:\Program Files (x86)\Microsoft Visual
Studio\2019\Community\VC\Tools\MSVC\14.21.27702\include\xlocale:781:37:
Generating code for declaration 'std::codecvt<char, char,
_Mbstatet>::do_always_noconv'


The above error points at the xlocale header file. If I edit this function and
change line 783 from
return is_same_v<_Byte, _Elem>;
to
return is_same_v<_Byte, _Elem> ? true : false;
then my product compiles without error.

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