std::source_location::function_name is ambiguous with function local types

[bebuch]

https://godbolt.org/z/4jx1dxE3E

#include <iostream>
#include <source_location>

template <typename T>
auto PrettyName() -> char const* {
    return std::source_location::current().function_name();
}

void normal() {
    struct A{};
    std::cout << "normal: " << PrettyName<A>() << '\n';
}

struct A;

int main() {
    auto lambda = []{
        struct A{};
        std::cout << "lambda: " << PrettyName<A>() << '\n';
    };

    std::cout << "global: " << PrettyName<A>() << '\n';
    normal();
    lambda();
}

clang output:

global: const char *PrettyName() [T = A]
normal: const char *PrettyName() [T = A]
lambda: const char *PrettyName() [T = A]

The GCC output is not ambiguous:

global: const char* PrettyName() [with T = A]
normal: const char* PrettyName() [with T = normal()::A]
lambda: const char* PrettyName() [with T = main()::<lambda()>::A]

I think it might be an good idea to change __PRETTY_FUNCTION__ output for function local types to something similar of what GCC does. Its annoying to look into a log file and see a type that exists globally while another function locale type was used. Thats pretty hard to debug.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Benjamin Buch (bebuch)

https://godbolt.org/z/4jx1dxE3E

#include <iostream>
#include <source_location>

template <typename T>
auto PrettyName() -> char const* {
    return std::source_location::current().function_name();
}

void normal() {
    struct A{};
    std::cout << "normal: " << PrettyName<A>() << '\n';
}

struct A;

int main() {
    auto lambda = []{
        struct A{};
        std::cout << "lambda: " << PrettyName<A>() << '\n';
    };

    std::cout << "global: " << PrettyName<A>() << '\n';
    normal();
    lambda();
}

clang output:

global: const char *PrettyName() [T = A]
normal: const char *PrettyName() [T = A]
lambda: const char *PrettyName() [T = A]

The GCC output is not ambiguous:

global: const char* PrettyName() [with T = A]
normal: const char* PrettyName() [with T = normal()::A]
lambda: const char* PrettyName() [with T = main()::<lambda()>::A]

I think it might be an good idea to change __PRETTY_FUNCTION__ output for function local types to something similar of what GCC does. Its annoying to look into a log file and see a type that exists globally while another function locale type was used. Thats pretty hard to debug.

[shafik]

CC @mizvekov

[mizvekov]

When printing the template arguments for PRETTY_FUNCTION, these appertain to the implicit instantiation, and not to any particular specialization, so it would make sense to print those fully qualified, the same as we do for canonical arguments in general.

So I agree this is something we could change, and this coincides with the user expectation here.

Note that for PRETTY_FUNCTION we print the function declaration itself as written, if that includes local types in the return type or parameters, so be it. But just to clarify this is different and not what this issue is about.

[bebuch]

std::source_location::function_name is implemented via PRETTY_FUNCTION. Therefore this is about both as far as I understand it. I chose std::source_location::function_name in the title because it's the standardized part of the pair.

[mizvekov]

I know it uses PRETTY_FUNCTION as the underlying thing, nothing I said previously contradicts that.

[bebuch]

Alright, then I misunderstood your comment. ;-) Sorry!