[Clang]Clang rejects friend auto function inside class template when a same-named global function exists (accepted by GCC/MSVC/EDG) #163514
Description
When a class template defines a friend auto function and a global function with the same name exists, Clang rejects the code with a redefinition error, while GCC, MSVC, and EDG all accept it:
int g(){return 0;};
template<int M>
struct R {
friend auto g() {
return M;
}
};Clang output:
<source>:5:15: error: functions that differ only in their return type cannot be overloaded
5 | friend auto g() {
| ~~~~ ^
<source>:1:5: note: previous definition is here
1 | int g(){return 0;};
| ~~~ ^
1 error generated.
Compiler returned: 1
(See it live: https://godbolt.org/z/E5a1hsfsj)
This is strange. Maybe when a friend function inside a class template uses an auto return type and there exists a global function with the same name, Clang performs conflict checking too early.
Because the following code:
int g() { return 0; }
template<int M>
struct R {
friend decltype(M) g() {
return M;
}
};is accepted by all compilers, including Clang. https://godbolt.org/z/ecdYTc6vE
This suggests that Clang performs name lookup / overloading checks too early — before the return type of the friend auto function has been deduced.
Once deduced, the function should be considered a distinct friend, not an overload of the global g().