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.

1 files changed, 7 insertions, 1 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 344107d..b17e176 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7336,7 +7336,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       if (TREE_CODE (r) == TARGET_EXPR
 	  && TREE_CODE (TARGET_EXPR_INITIAL (r)) == CONSTRUCTOR)
 	r = TARGET_EXPR_INITIAL (r);
-      if (DECL_P (r))
+      if (DECL_P (r)
+	  /* P2280 allows references to unknown.  */
+	  && !(VAR_P (t) && TYPE_REF_P (TREE_TYPE (t))))
 	{
 	  if (!ctx->quiet)
 	    non_const_var_error (loc, r, /*fundef_p*/false);
@@ -7378,6 +7380,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  r = build_constructor (TREE_TYPE (t), NULL);
 	  TREE_CONSTANT (r) = true;
 	}
+      else if (TYPE_REF_P (TREE_TYPE (t)))
+	/* P2280 allows references to unknown...  */;
+      else if (is_this_parameter (t))
+	/* ...as well as the this pointer.  */;
       else
 	{
 	  if (!ctx->quiet)