Full name of submitter (unless configured in github; will be published with the issue): Benjamin Sch.

Reference (section label): [dcl.constexpr]

Link to reflector thread (if any):

Issue description:

[dcl.constexpr]/1 currently says:

If any declaration of a function or function template has a constexpr or consteval specifier, then all its declarations shall contain the same specifier.

This seems to imply that a mismatching constexpr on two declarations in different TU needs to be diagnosed.

Suggested resolution:

Insert after the quoted sentence:

No diagnostic is required if neither the declaration with the specifier nor the declaration without the specifier is reachable from the other.