On 2/24/21 5:34 PM, Marek Polacek wrote:
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.)
That sounds safest.
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))))
I wonder about adding overloads that take constexpr_call* so you don't
need the non-null checks here. Up to you.
+ {
+ /* 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
