https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114124
Bug ID: 114124
Summary: Rejected use of function parameter as non-type
template parameter in trailing-return-type
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Reduced from StackOverflow:
template <int V>
struct Constant {
constexpr operator int() const noexcept { return V; }
};
template <class T, int N>
struct Array { };
auto function(auto s) -> Array<int, s + s> {
return {};
}
auto const a = function(Constant<3>{});
gcc trunk rejects this example with:
<source>:10:42: error: template argument 2 is invalid
10 | auto function(auto s) -> Array<int, s + s> {
| ^
<source>:10:42: error: template argument 2 is invalid
<source>:10:42: error: template argument 2 is invalid
<source>:10:42: error: template argument 2 is invalid
<source>:10:26: error: invalid template-id
10 | auto function(auto s) -> Array<int, s + s> {
| ^~~~~
<source>:10:39: error: use of parameter outside function body before '+' token
10 | auto function(auto s) -> Array<int, s + s> {
| ^
<source>:10:42: error: use of parameter outside function body before '>' token
10 | auto function(auto s) -> Array<int, s + s> {
| ^
<source>:10:1: error: deduced class type 'Array' in function return type
10 | auto function(auto s) -> Array<int, s + s> {
| ^~~~
<source>:8:8: note: 'template<class T, int N> struct Array' declared here
8 | struct Array { };
| ^~~~~
<source>:14:16: error: 'function' was not declared in this scope; did you mean
'union'?
14 | auto const a = function(Constant<3>{});
| ^~~~~~~~
| union
But this exact equivalent formulation of function is accepted:
auto function(auto s) {
return Array<int, s + s>{};
}
In this case, we're not actually reading the value of s to form the non-type
template argument, so this should be valid.
