https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66670
Bug ID: 66670
Summary: "template argument deduction/substitution failed" with
function pointers and multiple parameter packs
Product: gcc
Version: 5.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: michael at ensslin dot cc
Target Milestone: ---
Take this example:
$ cat t28.cpp
template<class ... Ts>
struct S {
template<class ... Us>
void foo(void (*)(Us..., Ts ...)) {}
};
void f(float, int) {}
int main() {
S<int>().foo<float>(f);
}
$ g++-4.9 -std=c++14 t28.cpp
t28.cpp: In function ‘int main()’:
t28.cpp:10:23: error: no matching function for call to ‘S<int>::foo(void
(&)(float, int))’
S<int>().foo<float>(f);
^
t28.cpp:10:23: note: candidate is:
t28.cpp:4:7: note: template<class ... Us> void S<Ts>::foo(void (*)(Us ..., Ts
...)) [with Us = {Us ...}; Ts = {int}]
void foo(void (*)(Us..., Ts ...)) {}
^
t28.cpp:4:7: note: template argument deduction/substitution failed:
t28.cpp:10:23: note: mismatched types ‘int’ and ‘float’
S<int>().foo<float>(f);
^
$ g++-4.9 --version
g++-4.9 (Debian 4.9.2-22) 4.9.2
I have reproduced the issue with g++-5.1.0 using https://gcc.godbolt.org/.
Note that clang++ has this issue as well, but it works with icc and supposedly
even MSVC.
The issue can be fixed by swapping the order of Us... and Ts... in the function
pointer type, or by replacing the function pointer type by some other type like
std::tuple.
Related: http://stackoverflow.com/questions/31040075
clang++ error, for completeness:
$ clang++-3.7 -std=c++14 t28.cpp
t28.cpp:10:11: error: no matching member function for call to 'foo'
S<int>().foo<float>(f);
~~~~~~~~~^~~~~~~~~~
t28.cpp:4:7: note: candidate template ignored: failed template argument
deduction
void foo(void (*)(Us..., Ts ...)) {}
^
1 error generated.
$ clang++-3.7 --version
Debian clang version 3.7.0-svn239806-1+b1 (trunk) (based on LLVM 3.7.0)
Target: x86_64-pc-linux-gnu
Thread model: posix
