Unexpected "reference to 'std' is ambiguous" error with C++20 modules

[MikailBag]

Reproducer: https://gist.github.com/MikailBag/1759e1c092649ee749fa0f620805fd62

Adding namespace std {} to GMF in connection.cpp fixes error.

Compiler version:

$ /home/mb/projects/llvm-project/build/bin/clang++ --version
clang version 21.0.0git (https://github.com/llvm/llvm-project.git 1276a5b368493cc73ef0febee35a6591b92464d5)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/mb/projects/llvm-project/build/bin
Build config: +assertions

Originally reported in #118137 (comment), this is minimized version.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Mikail Bagishov (MikailBag)

Reproducer: https://gist.github.com/MikailBag/1759e1c092649ee749fa0f620805fd62

Adding namespace std {} to GMF in connection.cpp fixes error.

Compiler version:

$ /home/mb/projects/llvm-project/build/bin/clang++ --version
clang version 21.0.0git (https://github.com/llvm/llvm-project.git 1276a5b368493cc73ef0febee35a6591b92464d5)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/mb/projects/llvm-project/build/bin
Build config: +assertions

Originally reported in #118137 (comment), this is minimized version.