https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121670
Bug ID: 121670
Summary: Replacement of integer constant with sizeof(int) makes
an expression not constexpr
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: fchelnokov at gmail dot com
Target Milestone: ---
In the following program GCC behaves inconsistently for two seemingly identical
expressions:
```
static_assert( sizeof(int) == 4 );
char a[2*sizeof(int)];
int& p = *reinterpret_cast<int*>(a);
int& q0 = *reinterpret_cast<int*>(a + 4);
static_assert( &p + 1 == &q0 ); //ok in GCC
int& q1 = *reinterpret_cast<int*>(a + sizeof(int));
static_assert( &p + 1 == &q1 ); //fail in GCC
```
The error in the second case is
> error: the value of 'q1' is not usable in a constant expression
> note: 'q1' was not initialized with a constant expression
Online demo: https://gcc.godbolt.org/z/ja73eEG7o
