https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127108
Bug ID: 127108
Summary: Array bound >= 2^31 is incorrectly sign-extended
during size_t non-type template parameter deduction
Product: gcc
Version: 16.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: my at rhanqtl dot com
Target Milestone: ---
g++ rejects below program (run in Compiler Explorer:
https://godbolt.org/z/xqc88xnno):
```
using size_t = decltype(sizeof(0));
template <typename T, size_t N>
constexpr size_t f(T (&)[N]) {
return N;
}
constexpr size_t M = 1ULL << 31;
extern char a[M];
static_assert(f(a) == M);
```
With errors:
```
<source>:12:16: error: no matching function for call to 'f(char [2147483648])'
12 | static_assert(f(a) == M);
| ~^~~
• there is 1 candidate
• candidate 1: 'template<class T, long unsigned int N> constexpr size_t f(T
(&)[N])'
<source>:4:18:
4 | constexpr size_t f(T (&)[N]) {
| ^
• template argument deduction/substitution failed:
• <source>: In substitution of 'template<class T, long unsigned int N>
constexpr size_t f(T (&)[N]) [with T = char; long unsigned int N =
18446744071562067968]':
• required from here
<source>:12:16:
12 | static_assert(f(a) == M);
| ~^~~
• error: size of array exceeds maximum object size
'9223372036854775807'
<source>:4:18:
4 | constexpr size_t f(T (&)[N]) {
| ^
• error: size of array exceeds maximum object size
'9223372036854775807'
<source>:4:20:
4 | constexpr size_t f(T (&)[N]) {
| ^~~~~~~~
```
Note that in the "in substitution ..." line `N` is `18446744071562067968`, i.e.
`0xffffffff80000000`.
clang++ accepts the program.
