[clang] return type deduction fails for function recursively invoked via intermediate constexpr function

[mcencora]

Following code fails to compile, unless constexpr qualifier is removed from visitType:

https://godbolt.org/z/899fWhdGe

struct visitor {
    template < typename T >
    static auto visit() {
        return visitType< T >();
    }

    template < typename T >
    static constexpr void visitType() {
        visit< typename T::next_type>();
    }
};

struct iface  {
    using next_type = iface;
};

void foo()
{
    visitor::visit<iface>();
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

Confirmed, I don't see why the presence of constexpr should cause clang to reject this.

[TheInferentialImp]

The issue is that the visitType() function is marked as constexpr but it is calling the non-constexpr visit() function. This makes the code fail to compile since constexpr functions can only call other constexpr functions.

I believe the only way to fix this issue is to either remove the constexpr qualifier from visitType() or make the visit() function constexpr.

[mcencora]

I may have reduced the code too much. With constexpr visit() the problem still occurs:

https://godbolt.org/z/a9j6d6dcc

struct visitor {
    template < typename T >
    static constexpr auto visit() {
        return visitType< T >();
    }

    template < typename T >
    static constexpr void visitType() {
        visit< typename T::next_type>();
    }
};

struct iface  {
    using next_type = iface;
};

void foo()
{
    visitor::visit<iface>();
}

[mcencora]

Still reproducible on clang-17 and trunk

[mcencora]

Still reproducible on clang-19 and trunk

[shafik]

CC @Fznamznon maybe you want to take a look