[layer8]

I hope there’s also some text in the standard prohibiting implementations from allowing any other expressions as a constant expression (which they otherwise could as a language extension), and thus requires compilation failure for such expressions?

[kps]

Pragmatically, you can't stop extensions; if the fine print for `--std=cool++23` says that this mode is not actually C++23-compliant, nearly nobody will ever notice or care. Pragmatically, if a popular compiler makes `--std=cool++23` the default, and requires `--std=C++23 --iso-eic-jtc1-sc22-wg21 --pompous` to get standard-conforming behaviour, nearly nobody will do that; instead they will complain that other compilers lack the extensions.

[layer8]

Extensions can be standard-compliant, in the sense that they don’t violate any prescription by the standard, and thus a program cannot assume their absence. My question was whether the standard actually takes care to render the acceptance of constexprs-with-UB non-standard-compliant. That is, in addition to “must accept X”, does it also say “must only accept X”?