Type mismatch for pointer to template template function

[pavelkryukov]

Bugzilla Link	49079
Version	11.0
OS	All
CC	@dwblaikie,@GabrielRavier,@zygoloid

Extended Description

If a function is a template template function (template<template class> void foo(int&)), and a template template parameter has a default value, it does not fit to function pointer type.

Example:

template<typename, typename = int> class A { };

template<template class> void foo(int&);
void bar(void(*)(int&));

int main()
{
bar(foo);
}
Output:

:8:5: error: no matching function for call to 'bar' bar(foo); ^~~ :4:6: note: candidate function not viable: no overload of 'foo' matching 'void (*)(int &)' for 1st argument void bar(void(*)(int&));

Godbolt: https://godbolt.org/z/enhYs8

With GCC and MSVC, example compiles fine.

[zygoloid]

Clang implements the language change that makes this code valid, but doesn't enable it by default. As a workaround, you can use -frelaxed-template-template-args to accept this code.

[pavelkryukov]

I see... Thank you!

[zygoloid]

*** Bug llvm/llvm-bugzilla-archive#49224 has been marked as a duplicate of this bug. ***

[GabrielRavier]

Is there any particular reason why Clang doesn't enable this by default ?

[pavelkryukov]

See https://clang.llvm.org/cxx_status.html#p0522

[zygoloid]

mentioned in issue llvm/llvm-bugzilla-archive#49224

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[mizvekov]

Duplicate of #42224