c++: const_cast of null pointer in constant expr [PR99176]

Here we reject

  constexpr const int *p = nullptr;
  constexpr int *q = const_cast<int*>(p);

with "conversion of 'const int*' null pointer to 'int*' is not a
constant expression", which seems bogus.  This code has been rejected
since r238909 which added the can_convert check when converting a null
pointer.  I'm not finding any standard rule that this check was supposed
to enforce.  The original discussion was here
<https://gcc.gnu.org/legacy-ml/gcc-patches/2016-06/msg01447.html>
and here
<https://gcc.gnu.org/legacy-ml/gcc-patches/2016-07/msg00280.html>.

Since can_convert never assumes a C-style cast, it rejects casting
away constness as in the test above and in:

  constexpr int *q = (int *)(const int *) nullptr;

Removing the check only breaks constexpr-nullptr-2.C by not giving any
diagnostic for line 229:

  constexpr B *pb2 = static_cast<B*>(pa0);  // { dg-error "not a constant expression" }

but the cast seems to be valid: we do [expr.static.cast]/7, and
[expr.const] only says that a reinterpreter_cast and converting from
void* is invalid in constexpr.  The can_convert check rejected convering
from void *, but only when converting from a null pointer, so it's not
good enough.  So I've added a check to catch conversions from cv void*.
I realize it's not a great time to be adding additional checking, but
removing the can_convert check would then technically be a regression.

Let's limit the new check to only trigger for integer_zerop and then remove
it in GCC 12.

gcc/cp/ChangeLog:

	DR 1312
	PR c++/99176
	* constexpr.c (is_std_construct_at): New overload.
	(is_std_allocator_allocate): New overload.
	(cxx_eval_call_expression): Use the new overloads.
	(cxx_eval_constant_expression): Reject casting
	from void * as per DR 1312.  Don't check can_convert.

gcc/testsuite/ChangeLog:

	DR 1312
	PR c++/99176
	* g++.dg/cpp0x/constexpr-nullptr-2.C: Adjust dg-error.
	* g++.dg/cpp0x/constexpr-cast2.C: New test.
	* g++.dg/cpp0x/constexpr-cast3.C: New test.

1 files changed, 53 insertions, 23 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 377fe32..cd0a68e 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1837,6 +1837,16 @@ is_std_construct_at (tree fndecl)
   return name && id_equal (name, "construct_at");
 }
 
+/* Overload for the above taking constexpr_call*.  */
+
+static inline bool
+is_std_construct_at (const constexpr_call *call)
+{
+  return (call
+	  && call->fundef
+	  && is_std_construct_at (call->fundef->decl));
+}
+
 /* Return true if FNDECL is std::allocator<T>::{,de}allocate.  */
 
 static inline bool
@@ -1859,6 +1869,16 @@ is_std_allocator_allocate (tree fndecl)
   return decl_in_std_namespace_p (decl);
 }
 
+/* Overload for the above taking constexpr_call*.  */
+
+static inline bool
+is_std_allocator_allocate (const constexpr_call *call)
+{
+  return (call
+	  && call->fundef
+	  && is_std_allocator_allocate (call->fundef->decl));
+}
+
 /* Return true if FNDECL is __dynamic_cast.  */
 
 static inline bool
@@ -2313,9 +2333,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
       if (TREE_CODE (t) == CALL_EXPR
 	  && cxx_replaceable_global_alloc_fn (fun)
 	  && (CALL_FROM_NEW_OR_DELETE_P (t)
-	      || (ctx->call
-		  && ctx->call->fundef
-		  && is_std_allocator_allocate (ctx->call->fundef->decl))))
+	      || is_std_allocator_allocate (ctx->call)))
 	{
 	  const int nargs = call_expr_nargs (t);
 	  tree arg0 = NULL_TREE;
@@ -2423,9 +2441,7 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	 argument.  */
       if (TREE_CODE (t) == CALL_EXPR
 	  && cxx_placement_new_fn (fun)
-	  && ctx->call
-	  && ctx->call->fundef
-	  && is_std_construct_at (ctx->call->fundef->decl))
+	  && is_std_construct_at (ctx->call))
 	{
 	  const int nargs = call_expr_nargs (t);
 	  tree arg1 = NULL_TREE;
@@ -6653,6 +6669,36 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    return t;
 	  }
 
+	/* [expr.const]: a conversion from type cv void* to a pointer-to-object
+	   type cannot be part of a core constant expression as a resolution to
+	   DR 1312.  */
+	if (integer_zerop (op) /* FIXME: Remove in GCC 12.  */
+	    && TYPE_PTROB_P (type)
+	    && TYPE_PTR_P (TREE_TYPE (op))
+	    && VOID_TYPE_P (TREE_TYPE (TREE_TYPE (op)))
+	    /* Inside a call to std::construct_at or to
+	       std::allocator<T>::{,de}allocate, we permit casting from void*
+	       because that is compiler-generated code.  */
+	    && !is_std_construct_at (ctx->call)
+	    && !is_std_allocator_allocate (ctx->call))
+	  {
+	    /* Likewise, don't error when casting from void* when OP is
+	       &heap uninit and similar.  */
+	    tree sop = tree_strip_nop_conversions (op);
+	    if (TREE_CODE (sop) == ADDR_EXPR
+		&& VAR_P (TREE_OPERAND (sop, 0))
+		&& DECL_ARTIFICIAL (TREE_OPERAND (sop, 0)))
+	      /* OK */;
+	    else
+	      {
+		if (!ctx->quiet)
+		  error_at (loc, "cast from %qT is not allowed",
+			    TREE_TYPE (op));
+		*non_constant_p = true;
+		return t;
+	      }
+	  }
+
 	if (TREE_CODE (op) == PTRMEM_CST && !TYPE_PTRMEM_P (type))
 	  op = cplus_expand_constant (op);
 
@@ -6671,26 +6717,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 		if (TYPE_REF_P (type))
 		  {
 		    if (!ctx->quiet)
-		      error_at (loc,
-				"dereferencing a null pointer");
+		      error_at (loc, "dereferencing a null pointer");
 		    *non_constant_p = true;
 		    return t;
 		  }
-		else if (TYPE_PTR_P (TREE_TYPE (op)))
-		  {
-		    tree from = TREE_TYPE (op);
-
-		    if (!can_convert (type, from, tf_none))
-		      {
-			if (!ctx->quiet)
-			  error_at (loc,
-				    "conversion of %qT null pointer to %qT "
-				    "is not a constant expression",
-				    from, type);
-			*non_constant_p = true;
-			return t;
-		      }
-		  }
 	      }
 	    else
 	      {