https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69139
Bug ID: 69139
Summary: [4.9/5/6 Regression] deduction failure with trailing
return type in function template argument
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rs2740 at gmail dot com
Target Milestone: ---
Test case (modified from http://stackoverflow.com/q/31229359/2756719):
auto get(int) -> int { return {}; }
template <class R> int f(auto (*)(int) -> R) { return {}; }
int i = f(get);
Compiles on 4.8.2, fails on 4.9 and later:
prog.cc:3:14: error: no matching function for call to 'f(int (&)(int))'
int i = f(get);
^
prog.cc:2:24: note: candidate: template<class R, class auto:1> int f(auto:1
(*)(int))
template <class R> int f(auto (*)(int) -> R) { return {}; }
^
prog.cc:2:24: note: template argument deduction/substitution failed:
prog.cc:3:14: note: couldn't deduce template parameter 'R'
int i = f(get);
^
It seems as if the 'auto' indicating the trailing return type is being treated
as introducing another template parameter, as if in abbreviated function
templates, and then the trailing return type is just ignored.
