https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115900
Bug ID: 115900
Summary: [14 Regression] constexpr object modification during
construction gives "Modifying a const object is not
allowed in a constant expression"
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: valentin at tolmer dot fr
Target Milestone: ---
The smallest example I could make is:
```cpp
struct A {
char data;
constexpr explicit A() {
data = 0;
}
};
struct C {
constexpr C(int) {}
constexpr C() = default;
};
struct B : public C {
A a;
explicit constexpr B() : C(0) {}
};
struct D : public B {};
static constexpr A a;
static constexpr B b;
static constexpr D d;
```
Building `a` and `b` succeed, but `d` fails with:
```
$ g++ -std=c++20 <source>
<source>:22:20: in 'constexpr' expansion of 'D()'
<source>:18:8: in 'constexpr' expansion of '((D*)this)->D::B.B::B()'
<source>:15:31: in 'constexpr' expansion of '((B*)this)->B::a.A::A()'
<source>:4:14: error: modifying a const object '((A*)this)->A::data' is not
allowed in a constant expression
4 | data = 0;
| ~~~~~^~~
<source>:22:20: note: originally declared 'const' here
22 | static constexpr D d;
| ^
```
This compiles fine with GCC 13.3.0, but fails with GCC 14.1.0 or trunk (on
godbolt as of this writing, g++
(Compiler-Explorer-Build-gcc-88ff0504ab3286df57e27514065494a30c365ec5-binutils-2.42)
15.0.0 20240712 (experimental) ).
https://godbolt.org/z/TvjM4qjb6
AFAIK, a constexpr object only becomes const after the construction ended (the
constructor returns). Moreover, there's no reason why B works but not D.
Interestingly, calling C's default constructor instead of the one that takes an
`int` (inside B) makes the code compile.
