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

[Jakub Jelinek]

Hi!

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

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-07-03  Jakub Jelinek  <ja...@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.

--- gcc/cp/constexpr.cc.jj      2024-07-02 22:09:52.493176541 +0200
+++ gcc/cp/constexpr.cc 2024-07-03 12:46:57.255025849 +0200
@@ -8157,10 +8157,13 @@ cxx_eval_constant_expression (const cons
                    || 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)
                  {
--- gcc/testsuite/g++.dg/cpp26/constexpr-voidptr3.C.jj  2024-07-03 
12:35:56.301762995 +0200
+++ gcc/testsuite/g++.dg/cpp26/constexpr-voidptr3.C     2024-07-03 
12:35:51.577825446 +0200
@@ -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);
--- gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C.jj     2023-07-17 
09:07:42.104283529 +0200
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C        2024-07-03 
16:22:33.294916937 +0200
@@ -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;
--- gcc/testsuite/g++.dg/cpp0x/constexpr-cast4.C.jj     2023-11-02 
07:39:18.679201173 +0100
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-cast4.C        2024-07-03 
16:23:29.424197809 +0200
@@ -8,4 +8,3 @@ constexpr float* pf = static_cast<float*
 
 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 }

        Jakub