https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100893
Bug ID: 100893
Summary: Template argument conversion fails for dependant
constant function pointer template parameters
Product: gcc
Version: 11.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jlegg at feralinteractive dot com
Target Milestone: ---
When compiling this C++ code:
void f();
struct S {using F = void (* const)();};
template <typename T, typename T::F f> void g() {}
void h() {g<S, &f>();}
GCC 11.1.1, and (much) earlier versions, reject it with:
<source>: In function 'void h()':
<source>:4:19: error: no matching function for call to 'g<S, f>()'
4 | void h() {g<S, &f>();}
| ~~~~~~~~^~
<source>:3:45: note: candidate: 'template<class T, typename T::F f> void g()'
3 | template <typename T, typename T::F f> void g() {}
| ^
<source>:3:45: note: template argument deduction/substitution failed:
<source>:4:16: error: could not convert template argument 'f' from 'void (*)()'
to 'void (* const)()'
4 | void h() {g<S, &f>();}
| ^~
My language lawyering isn't good enough to explain why, but I think the
template argument conversion should work, and the compiler should behave as it
does when the const is removed. Similar conversions seem to work if F is a
constant type but not a function pointer, or if the second template parameter
on g is not dependant on the first. MSVC and Clang also accept the example
code.
