[Bug c++/79083] New: GCC incorrectly compiles calls to constexpr static methods via a non-constexpr variable

[matt at godbolt dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79083

            Bug ID: 79083
           Summary: GCC incorrectly compiles calls to constexpr static
                    methods via a non-constexpr variable
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at godbolt dot org
  Target Milestone: ---

Consider the following (unusual) code:

--
struct Foo { static constexpr bool hasBar() { return true; } };

void test(const Foo &f) {
  static_assert(Foo::hasBar(), ""); // -is ok
  static_assert(f.hasBar(), ""); // fails on clang trunk, works in GCC
}
--

GCC incorrectly compiles the second static assert; clang gives the error:
"static_assert expression is not an integral constant expression".

I initially thought this was a bug in clang, but according to Richard Smith
it's not: https://llvm.org/bugs/show_bug.cgi?id=31624 :

[expr.const]/2.10:

"[An expression is not a constant expression if it would evaluate]  an
id-expression that refers to a variable or data member of reference type unless
the reference has a preceding initialization and either
— it is initialized with a constant expression or
— its lifetime began within the evaluation of e;"

Thus we believe GCC's behaviour is the incorrect in this case.