[Bug c++/96111] checking type of attribute with concepts results in compilation error or ICE

[lts-rudolph at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96111

--- Comment #1 from Klaus Rudolph <lts-rudolph at gmx dot de> ---
The error massages are valid as 

as got by an answr on SO (
https://stackoverflow.com/questions/62791460/checking-type-of-attribute-with-concepts
)


    [expr.prim.req.compound]/1.3

        If the return-type-requirement is present, then:
            Substitution of template arguments (if any) into the
return-type-requirement is performed.
            The immediately-declared constraint ([temp.param]) of the
type-constraint for decltype((E)) shall be satisfied.

E is our expression, namely n.value.

Now, decltype(n.value) is char or int, that's because decltype has a special
rule for class member access and id expressions. But decltype((n.value)) is
char& or int&. The value category is encoded in the type of decltype when
dealing with a general expression (such as a parenthesized class member
access).

BUT: That we see an ICE on current trunk is a bug!