-Wunreachable-code-generic-assoc warnings on elver

Recently, buildfarm member elver has started spewing literally
thousands of $SUBJECT:

 elver         | 2022-12-10 01:17:29 | ../../src/include/utils/float.h:223:33: warning: due to lvalue conversion of the
controllingexpression, association of type 'volatile float' will never be selected because it is qualified
[-Wunreachable-code-generic-assoc]
 elver         | 2022-12-10 01:17:29 | ../../src/include/utils/float.h:223:33: warning: due to lvalue conversion of the
controllingexpression, association of type 'volatile double' will never be selected because it is qualified
[-Wunreachable-code-generic-assoc]
 elver         | 2022-12-10 01:17:29 | ../../src/include/utils/float.h:223:33: warning: due to lvalue conversion of the
controllingexpression, association of type 'volatile long double' will never be selected because it is qualified
[-Wunreachable-code-generic-assoc]
[ etc etc, about 9200 times per build ]

I have no idea what that means, and consulting the clang documentation
didn't leave me much wiser.  I do see that a lot of these seem to be
associated with isnan() calls, which makes me guess that clang does
not play nice with however isnan() is declared on that box.

            regards, tom lane