[abi] Declaring class of a member-like friend method is not mangled.

[VitaNuo]

The code sample

template <class>
concept True = true;

namespace test2 {
 template<typename T> struct A {
    template<typename U = void>
    friend void g(...) requires True<T> && True<U> {}
 };

 void call() {
    A<int> ai;
    A<bool> ab;
    g(ai);
    g(ab);
 }
}

does not compile with the following error

Output of x86-64 clang (trunk) (Compiler #1)
<source>:10:17: error: definition with same mangled name '_ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E' as another definition
   10 |     friend void g(...) requires True<T> && True<U> {}
      |                 ^
<source>:10:17: note: previous definition is here
1 error generated.
Compiler returned: 1

The reason appears to be that the enclosing class template specialization A<int> (or A<bool>, respectively) is not part of the mangled name _ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E, resulting in g(ai) and g(ab) mangled as the same string.

As per the discussion in itanium-cxx-abi/cxx-abi#24 (comment), the code above is valid and the solution is to include the declaring class in the mangling. From @rjmccall on the aforementioned issue:

The only viable implementation path I can see, given that nothing in the standard prevents these friend declarations from otherwise matching perfectly in signatures and requires-clauses, is to mangle the declaring class for these friends.

Note 1: gcc can compile the above snippet by including the declaring class in the mangling, which results in the mangled string _ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E for g(ai) (Note AIiE mangles the declaring class), and _ZN5test21AIbEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E for g(ab) (Note AIbE mangles the declaring class).

Note 2: this issue stands in the way of implementing the demangling of template parameters in constraints (https://github.com/llvm/llvm-project/blob/main/libcxxabi/src/demangle/ItaniumDemangle.h#L5795).
T_ in _ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E is impossible to demangle, since it is a substitution for the template argument of class A that is not part of the mangling.

[llvmbot]

@llvm/issue-subscribers-tools-llvm-cxxfilt

Author: Viktoriia Bakalova (VitaNuo)

The code sample

template <class>
concept True = true;

namespace test2 {
 template<typename T> struct A {
    template<typename U = void>
    friend void g(...) requires True<T> && True<U> {}
 };

 void call() {
    A<int> ai;
    A<bool> ab;
    g(ai);
    g(ab);
 }
}

does not compile with the following error

Output of x86-64 clang (trunk) (Compiler #<!-- -->1)
&lt;source&gt;:10:17: error: definition with same mangled name '_ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E' as another definition
   10 |     friend void g(...) requires True&lt;T&gt; &amp;&amp; True&lt;U&gt; {}
      |                 ^
&lt;source&gt;:10:17: note: previous definition is here
1 error generated.
Compiler returned: 1

The reason appears to be that the enclosing class template specialization A<int> (or A<bool>, respectively) is not part of the mangled name _ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E, resulting in g(ai) and g(ab) mangled as the same string.

As per the discussion in itanium-cxx-abi/cxx-abi#24 (comment), the code above is valid and the solution is to include the declaring class in the mangling. From @rjmccall on the aforementioned issue:

The only viable implementation path I can see, given that nothing in the standard prevents these friend declarations from otherwise matching perfectly in signatures and requires-clauses, is to mangle the declaring class for these friends.

Note 1: gcc can compile the above snippet by including the declaring class in the mangling, which results in the mangled string _ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E for g(ai) (Note AIiE mangles the declaring class), and _ZN5test21AIbEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E for g(ab) (Note AIbE mangles the declaring class).

Note 2: this issue stands in the way of implementing the demangling of template parameters in constraints (https://github.com/llvm/llvm-project/blob/main/libcxxabi/src/demangle/ItaniumDemangle.h#L5795).
T_ in _ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E is impossible to demangle, since it is a substitution for the template argument of class A that is not part of the mangling.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Viktoriia Bakalova (VitaNuo)

The code sample

template <class>
concept True = true;

namespace test2 {
 template<typename T> struct A {
    template<typename U = void>
    friend void g(...) requires True<T> && True<U> {}
 };

 void call() {
    A<int> ai;
    A<bool> ab;
    g(ai);
    g(ab);
 }
}

does not compile with the following error

Output of x86-64 clang (trunk) (Compiler #<!-- -->1)
&lt;source&gt;:10:17: error: definition with same mangled name '_ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E' as another definition
   10 |     friend void g(...) requires True&lt;T&gt; &amp;&amp; True&lt;U&gt; {}
      |                 ^
&lt;source&gt;:10:17: note: previous definition is here
1 error generated.
Compiler returned: 1

The reason appears to be that the enclosing class template specialization A<int> (or A<bool>, respectively) is not part of the mangled name _ZN5test2F1gIvEEvzQaa4TrueIT_E4TrueITL0__E, resulting in g(ai) and g(ab) mangled as the same string.

As per the discussion in itanium-cxx-abi/cxx-abi#24 (comment), the code above is valid and the solution is to include the declaring class in the mangling. From @rjmccall on the aforementioned issue:

The only viable implementation path I can see, given that nothing in the standard prevents these friend declarations from otherwise matching perfectly in signatures and requires-clauses, is to mangle the declaring class for these friends.

Note 1: gcc can compile the above snippet by including the declaring class in the mangling, which results in the mangled string _ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E for g(ai) (Note AIiE mangles the declaring class), and _ZN5test21AIbEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E for g(ab) (Note AIbE mangles the declaring class).

Note 2: this issue stands in the way of implementing the demangling of template parameters in constraints (https://github.com/llvm/llvm-project/blob/main/libcxxabi/src/demangle/ItaniumDemangle.h#L5795).
T_ in _ZN5test21AIiEF1gIvEEvzQaa4TrueIT_E4TrueITL0__E is impossible to demangle, since it is a substitution for the template argument of class A that is not part of the mangling.