I haven't looked into it, but going through the release notes for tlsGuards showed this - though not directly a bug report https://learn.microsoft.com/en-us/cp

I haven't looked into it, but going through the release notes for tlsGuards showed this - though not directly a bug report https://learn.microsoft.com/en-us/cpp/overview/cpp-conformance-improvements-2019?view=msvc-170#tls-guard-changes https://learn.microsoft.com/en-us/cpp/overview/cpp-conforman... and also the implementation in "clang" (for "clang-cl" being conformant with MSVC) - https://reviews.llvm.org/D115456#3217595 https://reviews.llvm.org/D115456#3217595 then last year clang-cl also added ways to disable this (if need to), probably this hit some internal issue and had to be resolved. Maybe "thread_local" have become more widely used (unlike OS specific "TlsAlloc")