c++: Implement C++26 CWG2819 - Allow cv void * null pointer value conversion to object types in constant expressions

The following patch implements CWG2819 (which wasn't a DR because
it changes behavior of C++26 only).

2024-07-09  Jakub Jelinek  <jakub@redhat.com>

	* constexpr.cc (cxx_eval_constant_expression): CWG2819 - Allow
	cv void * null pointer value conversion to object types in constant
	expressions.

	* g++.dg/cpp26/constexpr-voidptr3.C: New test.
	* g++.dg/cpp0x/constexpr-cast2.C: Adjust expected diagnostics for
	C++26.
	* g++.dg/cpp0x/constexpr-cast4.C: Likewise.

4 files changed, 30 insertions, 25 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 0cdac0a..14bbdea 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8157,10 +8157,13 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 		    || DECL_NAME (decl) == heap_vec_uninit_identifier))
 	      /* OK */;
 	    /* P2738 (C++26): a conversion from a prvalue P of type "pointer to
-	       cv void" to a pointer-to-object type T unless P points to an
-	       object whose type is similar to T.  */
+	       cv void" to a pointer-to-object type T unless P is a null
+	       pointer value or points to an object whose type is similar to
+	       T.  */
 	    else if (cxx_dialect > cxx23)
 	      {
+		if (integer_zerop (sop))
+		  return build_int_cst (type, 0);
 		r = cxx_fold_indirect_ref (ctx, loc, TREE_TYPE (type), sop);
 		if (r)
 		  {
@@ -8169,26 +8172,16 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 		  }
 		if (!ctx->quiet)
 		  {
-		    if (TREE_CODE (sop) == ADDR_EXPR)
-		      {
-			auto_diagnostic_group d;
-			error_at (loc, "cast from %qT is not allowed in a "
-				  "constant expression because "
-				  "pointed-to type %qT is not similar to %qT",
-				  TREE_TYPE (op), TREE_TYPE (TREE_TYPE (sop)),
-				  TREE_TYPE (type));
-			tree obj = build_fold_indirect_ref (sop);
-			inform (DECL_SOURCE_LOCATION (obj),
-				"pointed-to object declared here");
-		      }
-		    else
-		      {
-			gcc_assert (integer_zerop (sop));
-			error_at (loc, "cast from %qT is not allowed in a "
-				  "constant expression because "
-				  "%qE does not point to an object",
-				  TREE_TYPE (op), oldop);
-		      }
+		    gcc_assert (TREE_CODE (sop) == ADDR_EXPR);
+		    auto_diagnostic_group d;
+		    error_at (loc, "cast from %qT is not allowed in a "
+			      "constant expression because "
+			      "pointed-to type %qT is not similar to %qT",
+			      TREE_TYPE (op), TREE_TYPE (TREE_TYPE (sop)),
+			      TREE_TYPE (type));
+		    tree obj = build_fold_indirect_ref (sop);
+		    inform (DECL_SOURCE_LOCATION (obj),
+			    "pointed-to object declared here");
 		  }
 		*non_constant_p = true;
 		return t;
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C
index 3efbd92..71ec08f 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C
@@ -5,9 +5,9 @@
 static int i;
 constexpr void *vp0 = nullptr;
 constexpr void *vpi = &i;
-constexpr int *p1 = (int *) vp0; // { dg-error "cast from .void\\*. is not allowed" }
+constexpr int *p1 = (int *) vp0; // { dg-error "cast from .void\\*. is not allowed" "" { target c++23_down } }
 constexpr int *p2 = (int *) vpi; // { dg-error "cast from .void\\*. is not allowed" "" { target c++23_down } }
-constexpr int *p3 = static_cast<int *>(vp0); // { dg-error "cast from .void\\*. is not allowed" }
+constexpr int *p3 = static_cast<int *>(vp0); // { dg-error "cast from .void\\*. is not allowed" "" { target c++23_down } }
 constexpr int *p4 = static_cast<int *>(vpi); // { dg-error "cast from .void\\*. is not allowed" "" { target c++23_down } }
 constexpr void *p5 = vp0;
 constexpr void *p6 = vpi;
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast4.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast4.C
index 884b6a5..324c8e3 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast4.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast4.C
@@ -8,4 +8,3 @@ constexpr float* pf = static_cast<float*>(vpr);  // { dg-error "cast from .void\
 
 constexpr void* vnp = nullptr;
 constexpr int* pi2 = static_cast<int*>(vnp);  // { dg-error "cast from .void\\*. is not allowed" "" { target c++23_down } }
-// { dg-error "cast from .void\\*. is not allowed in a constant expression because .vnp. does not point to an object" "" { target c++26 } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr3.C b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr3.C
new file mode 100644
index 0000000..67728b2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr3.C
@@ -0,0 +1,13 @@
+// CWG 2819 - Cast from null pointer value in a constant expression
+// { dg-do compile { target c++26 } }
+
+struct S { int s; };
+
+constexpr S *
+foo ()
+{
+  void *p = nullptr;
+  return static_cast<S *> (p);
+}
+
+static_assert (foo () == nullptr);