(From submission #683.)

Subclause 9.2.6 [dcl.constexpr] paragraph 1 specifies:

... 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 rule should not require a diagnostic if the declarations are not mutually reachable.

Possible resolution:

Change in 9.2.6 [dcl.constexpr] paragraph 1 as follows:

... If any declaration of a function or function template F has a constexpr or consteval specifier, then all its declarations shall contain the same specifier; a diagnostic is required only if a declaration of F reachable from another declaration of F deviates.