https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120758
Bug ID: 120758
Summary: Empty initializer list (`{}`) incorrectly deduces an
array size for array reference parameter
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mital at mitalashok dot co.uk
Target Milestone: ---
Compiling the following with `-std=c++11 -fsyntax-only`
<https://godbolt.org/z/PoGsqTdqr>:
template<__SIZE_TYPE__ N, typename T>
void f(T(&)[N], T(&)[N]);
int main() {
const int arr[2] = {};
f<2>(arr, {}); // Compiles
f(arr, {}); // Fails
}
Results in the following error:
/tmp/test.cpp: In function 'int main()':
/tmp/test.cpp:7:6: error: no matching function for call to 'f(const int [2],
<brace-enclosed initializer list>)'
7 | f(arr, {}); // Fails
| ~^~~~~~~~~
/tmp/test.cpp:7:6: note: there is 1 candidate
/tmp/test.cpp:2:6: note: candidate 1: 'template<long unsigned int N, class T>
void f(T (&)[N], T (&)[N])'
2 | void f(T(&)[N], T(&)[N]);
| ^
/tmp/test.cpp:2:6: note: template argument deduction/substitution failed:
There are no more notes. Expected behaviour is that the two function calls
result in calling `f<2, const int>`, or if this is the intended behaviour, for
a note about why the template argument deduction/substitution failed.
The reason I believe this to be valid is because
<https://wg21.link/temp.deduct.call>:
> If removing references and cv-qualifiers from P gives
> std::initializer_list<P′> or P′[N] for some P′ and N and the argument is a
> non-empty initializer list, [...] Otherwise, an initializer list argument
> causes the parameter to be considered a non-deduced context.
And `{}` is not non-empty, so it should not affect deduction.
