Compilation Error using C++ Concepts

[ryanolson]

Full reproducer:
https://godbolt.org/z/oh1v7h64G

Compiles with gcc, but not clang.

This could also be user error and gcc should be throwing an error; however, I feel like the code is correct since clang doesn't fail the static_assert, but fails when applied to the lambda auto arguments.

[shafik]

Looks like MSVC also accepts this as well.

CC @erichkeane

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[erichkeane]

As far as I can tell just looking, this probably SHOULD compile, but the example is complicated enough I don't have a great idea of what the issue could be.

[ryanolson]

I think, but I am not sure, that the error results from using the concept where the first type is being inferred by the auto.

Explicitly using a type in the static_assert works correctly.

So what ever mechanism the compiler uses propagate the type passed into the the auto to then construct the constexpr of evaluating the concepts is where I would look.

I will also say, that using clangd extension in vscode with 15.0.6 results in many clangd process deaths as I was coding this. There were definitely frontend failures when the code was close to correct, but not quite correct.

[cor3ntin]

GCC now crashes on this, after diagnostics https://godbolt.org/z/oh1v7h64G
Could you provide a smaller program which you think should be valid?