Defining a method with a user-defined concept of class template outside its scope gives an error: "out-of-line definition of does not match any declaration in"

[akukh]

Well, trying to define a method with a user-defined concept of class template outside its scope gives an error:

error: out-of-line definition of '...' does not match any declaration in '...'

GCC, in turn, compiles such code just fine.
https://godbolt.org/z/PTnh6Y4ab

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[royjacobson]

This is solved in the Clang 16 branch. Since this concepts refers to the injected class name it's probably the deferred concepts change.

https://godbolt.org/z/ffa94TTjE

[hiraditya]

seems like clang 17 has regressed and this issue shows up again?

https://godbolt.org/z/56eYa4jnq

[shafik]

Confirmed fixed in 16 and then regressed in 17: https://godbolt.org/z/4nhKzahxc

CC @erichkeane @cor3ntin

[erichkeane]

Interestingly, it is fully specifying the template arguments to the requires clause that makes it not the same: https://godbolt.org/z/ExEj5xaWP

(Thats a minimized version, uncomment the to get it to break).

[erichkeane]

So this probably worked in Clang16 because we were doing a little bit of a 'dumb' and inaccurate comparison of constraints by just trying to make the levels 'even'. We fixed this in 17, which is unfortunately exposing a bunch of errors in how we 'instantiate' incomplete constraints.

I don't know what the exact issue is (and might not have a chance to take a look anytime soon), but it seems we're having trouble with setting up the template arguments for the out of line definition there. For some reason, even dumping the post-instantiated version fails with DeclTemplate.cpp:1646's llvm_unreachable of: "Unhandled templated declaration kind", so we're messing SOMETHING up in collecting types.

If anyone wnats to take a look, the best place to start is SemaConcept.cpp's AreConstraintExpressionsEqual. In that function we try to do the partial instantiation (SubstituteConstraintExpression collects the template arguments then tries to instantiate them) and compare using the FoldingSetNodeID stuff. I doubt the latter is the problem, but I suspect the 2nd call to SubstituteConstraintExpr (from the New side) is what is wrong.