Not new in C23, but I still think it's a glaring hole in the standard that there's still no standard way to ask the compiler which (if any) of "J.5 Common extensions" is supported.
For the C version you have __STDC_VERSION__, but there's no similar facility to check if e.g. J.5.7 is supported, which effectively makes the behavior that's explicitly omitted in 7.22.1.4 and 6.3.2.3 go from "undefined" to supported by C23 + the extension.
I understand why C can't have some generic "is this undefined?" test, but it seems weird not to be able to ask if extensions defined in the standard itself are in effect, as they define certain otherwise undefined behavior. The effect is that anyone using these extensions must be intimately familiar with all the compilers they're targeting.
For the C version you have __STDC_VERSION__, but there's no similar facility to check if e.g. J.5.7 is supported, which effectively makes the behavior that's explicitly omitted in 7.22.1.4 and 6.3.2.3 go from "undefined" to supported by C23 + the extension.
I understand why C can't have some generic "is this undefined?" test, but it seems weird not to be able to ask if extensions defined in the standard itself are in effect, as they define certain otherwise undefined behavior. The effect is that anyone using these extensions must be intimately familiar with all the compilers they're targeting.