[clang] [Clang] Diagnose invalid function types in dependent contexts (PR #138731)

[Yuxuan Chen via cfe-commits]

yuxuanchen1997 wrote:

Hi @cor3ntin, 

This patch seems to have made expression SFINAEs in variable template partial specialization an hard error: https://godbolt.org/z/3K6sh1cWM

Upstream clang rejects the program while clang-20, gcc, msvc accepted it. 

https://github.com/llvm/llvm-project/pull/138731