c++: P2280R4, Using unknown refs in constant expr [PR106650]

c++: P2280R4, Using unknown refs in constant expr [PR106650]

This patch is an attempt to implement (part of?) P2280, Using unknown
pointers and references in constant expressions.  (Note that R4 seems to
only allow References to unknown/Accesses via this, but not Pointers to
unknown.)

This patch works to the extent that the test case added in [expr.const]
works as expected, as well as the test in
<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html#the-this-pointer>

Most importantly, the proposal makes this compile:

  template <typename T, size_t N>
  constexpr auto array_size(T (&)[N]) -> size_t {
      return N;
  }

  void check(int const (&param)[3]) {
      constexpr auto s = array_size(param);
      static_assert (s == 3);
  }

and I think it would be a pity not to have it in GCC 14.

What still doesn't work is the test in $3.2:

  struct A2 { constexpr int f() { return 0; } };
  struct B2 : virtual A2 {};
  void f2(B2 &b) { constexpr int k = b.f(); }

where we say
error: '* & b' is not a constant expression

This will be fixed in the future.

	PR c++/106650

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression) <case PARM_DECL>: Allow
	reference to unknown/this as per P2280.
	<case VAR_DECL>: Allow reference to unknown as per P2280.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-array-ptr6.C: Remove dg-error.
	* g++.dg/cpp0x/constexpr-ref12.C: Likewise.
	* g++.dg/cpp0x/constexpr-ref2.C: Adjust dg-error.
	* g++.dg/cpp0x/noexcept34.C: Remove dg-error.
	* g++.dg/cpp1y/lambda-generic-const10.C: Likewise.
	* g++.dg/cpp0x/constexpr-ref13.C: New test.
	* g++.dg/cpp1z/constexpr-ref1.C: New test.
	* g++.dg/cpp1z/constexpr-ref2.C: New test.
	* g++.dg/cpp2a/constexpr-ref1.C: New test.