https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115171
Bug ID: 115171
Summary: Constant evaluation of math.h functions in C++ against
constexpr.functions
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: accepts-invalid, rejects-valid, wrong-code
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hstong at ca dot ibm.com
Target Milestone: ---
It appears GCC implements a non-conforming extension that results in failure to
compile a well-formed program. In particular,
https://wg21.link/constexpr.functions states that standard library functions
are not to be made constexpr by implementations except where the Standard
explicitly requires constexpr.
Online compiler: https://godbolt.org/z/e8x5jhPMc
### SOURCE (<stdin>):
```
//#include <math.h>
extern "C" double fmin(double, double);
template <typename T> void f(T, char (*)[fmin(T{}, T{}) ? 1 : 2] = 0);
constexpr bool f(int) { return true; }
static_assert(f(0.));
```
### COMPILER INVOCATION:
```
g++ -fsyntax-only -std=c++20 -xc++ -
```
### ACTUAL OUTPUT:
```
<stdin>:7:16: error: could not convert 'f<double>(0.0, 0)' from 'void' to
'bool'
```
### EXPECTED OUTPUT:
(Clean compile)
### COMPILER VERSION INFO (g++ -v):
```
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/15.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240520 (experimental) (GCC)
```
