public inbox for gcc-patches@gcc.gnu.org
 help / color / mirror / Atom feed
* [PATCH] c++: const_cast of null pointer in constant expr [PR99176]
@ 2021-02-24 22:34 Marek Polacek
  2021-02-25  3:32 ` Jason Merrill
  0 siblings, 1 reply; 4+ messages in thread
From: Marek Polacek @ 2021-02-24 22:34 UTC (permalink / raw)
  To: Jason Merrill, GCC Patches

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.
(I could perhaps limit the new check to only trigger for integer_zerop
and then remove it in GCC 12.)

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

gcc/cp/ChangeLog:

	DR 1312
	PR c++/99176
	* constexpr.c (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.
---
 gcc/cp/constexpr.c                            | 49 ++++++++++++-------
 gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C  | 16 ++++++
 gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C  | 14 ++++++
 .../g++.dg/cpp0x/constexpr-nullptr-2.C        |  4 +-
 4 files changed, 64 insertions(+), 19 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 377fe322ee8..adf575d3dc6 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6653,6 +6653,37 @@ 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 (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.  */
+	    && !(ctx->call
+		 && ctx->call->fundef
+		 && (is_std_construct_at (ctx->call->fundef->decl)
+		     || is_std_allocator_allocate (ctx->call->fundef->decl))))
+	  {
+	    /* 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 +6702,10 @@ 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
 	      {
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C
new file mode 100644
index 00000000000..b79e8a90131
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast2.C
@@ -0,0 +1,16 @@
+// DR 1312 - Simulated reinterpret_cast in constant expressions.
+// PR c++/99176
+// { dg-do compile { target c++11 } }
+
+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 *p2 = (int *) vpi; // { dg-error "cast from .void\\*. is not allowed" }
+constexpr int *p3 = static_cast<int *>(vp0); // { dg-error "cast from .void\\*. is not allowed" }
+constexpr int *p4 = static_cast<int *>(vpi); // { dg-error "cast from .void\\*. is not allowed" }
+constexpr void *p5 = vp0;
+constexpr void *p6 = vpi;
+
+constexpr int *pi = &i;
+constexpr bool b = ((int *)(void *) pi == pi); // { dg-error "cast from .void\\*. is not allowed" }
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C
new file mode 100644
index 00000000000..a330a99f7de
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-cast3.C
@@ -0,0 +1,14 @@
+// PR c++/99176
+// { dg-do compile { target c++11 } }
+
+constexpr const int *p = nullptr;
+constexpr int *q1 = const_cast<int*>(p);
+constexpr int *q2 = (int *)(const int *) nullptr;
+
+struct B { };
+struct D : B { };
+constexpr B *q3 = static_cast<B*>(nullptr);
+constexpr D *pd = nullptr;
+constexpr B *pb = nullptr;
+constexpr B *q4 = static_cast<B*>(pd);
+constexpr D *q5 = static_cast<D*>(pb);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C
index afb4b37be5a..92f3bbdc0a6 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nullptr-2.C
@@ -163,7 +163,7 @@ constexpr const void *pv2 = pv0;
 constexpr void *pv3 = pv2;      // { dg-error "invalid conversion|not a constant expression" }
 constexpr const void *pv4 = pv2;
 
-constexpr X *px4 = pv0;	 // { dg-error "invalid conversion|not a constant expression" }
+constexpr X *px4 = pv0;	 // { dg-error "cast from|invalid conversion|not a constant expression" }
 
 }
 
@@ -226,7 +226,7 @@ constexpr A *pa3 = pd0;		   // { dg-error "ambiguous base" }
 constexpr A *pa4 = static_cast<A*>(pd0);  // { dg-error "ambiguous base" }
 
 constexpr B *pb1 = pa0;		   // { dg-error "invalid conversion|not a constant expression" }
-constexpr B *pb2 = static_cast<B*>(pa0);  // { dg-error "not a constant expression" }
+constexpr B *pb2 = static_cast<B*>(pa0);
 
 constexpr C *pc1 = pa0;		   // { dg-error "invalid conversion|not a constant expression" }
 constexpr D *pd1 = pa0;		   // { dg-error "ambiguous base|invalid conversion" }

base-commit: 9a4eb720b343324f7f8fd2dceed5d0347e5a0153
-- 
2.29.2


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-02-25 21:24 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-02-24 22:34 [PATCH] c++: const_cast of null pointer in constant expr [PR99176] Marek Polacek
2021-02-25  3:32 ` Jason Merrill
2021-02-25 21:20   ` [PATCH v2] " Marek Polacek
2021-02-25 21:24     ` Jason Merrill

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).