https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89048
Bug ID: 89048
Summary: constant evaluation in trailing return type rejected
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
This program is rejected by gcc:
template <int V> struct X { };
template <typename F> constexpr auto f(F f) -> X<f()> { return {}; }
with:
source>:3:53: error: template argument 1 is invalid
3 | template <typename F> constexpr auto f(F f) -> X<f()> { return {}; }
| ^
<source>:3:53: error: template argument 1 is invalid
<source>:3:53: error: template argument 1 is invalid
<source>:3:53: error: template argument 1 is invalid
<source>:3:48: error: invalid template-id
3 | template <typename F> constexpr auto f(F f) -> X<f()> { return {}; }
| ^
<source>:3:51: error: use of parameter outside function body before '(' token
3 | template <typename F> constexpr auto f(F f) -> X<f()> { return {}; }
| ^
<source>:3:38: error: deduced class type 'X' in function return type
3 | template <typename F> constexpr auto f(F f) -> X<f()> { return {}; }
| ^
<source>:1:25: note: 'template<int V> struct X' declared here
1 | template <int V> struct X { };
| ^
Compiler returned: 1
On the other hand, this program is accepted:
template <int V> struct X { };
template <typename F> constexpr auto g(F f) { return X<f()>{}; }
Clang accepts both. I think both programs are valid.
