On 9/19/23 20:55, Nathaniel Shead wrote:
On Tue, Sep 19, 2023 at 05:25:20PM -0400, Jason Merrill wrote:
On 9/1/23 08:22, Nathaniel Shead wrote:
On Wed, Aug 30, 2023 at 04:28:18PM -0400, Jason Merrill wrote:
On 8/29/23 09:35, Nathaniel Shead wrote:
This is an attempt to improve the constexpr machinery's handling of
union lifetime by catching more cases that cause UB. Is this approach
OK?
I'd also like some feedback on a couple of pain points with this
implementation; in particular, is there a good way to detect if a type
has a non-deleted trivial constructor? I've used 'is_trivially_xible' in
this patch, but that also checks for a trivial destructor which by my
reading of [class.union.general]p5 is possibly incorrect. Checking for a
trivial default constructor doesn't seem too hard but I couldn't find a
good way of checking if that constructor is deleted.
I guess the simplest would be
(TYPE_HAS_TRIVIAL_DFLT (t) && locate_ctor (t))
because locate_ctor returns null for a deleted default ctor. It would be
good to make this a separate predicate.
I'm also generally unsatisfied with the additional complexity with the
third 'refs' argument in 'cxx_eval_store_expression' being pushed and
popped; would it be better to replace this with a vector of some
specific structure type for the data that needs to be passed on?
Perhaps, but what you have here is fine. Another possibility would be to
just have a vec of the refs and extract the index from the ref later as
needed.
Jason
Thanks for the feedback. I've kept the refs as-is for now. I've also
cleaned up a couple of other typos I'd had with comments and diagnostics.
Bootstrapped and regtested on x86_64-pc-linux-gnu.
@@ -6192,10 +6197,16 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
type = reftype;
- if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
- && CONSTRUCTOR_ELT (*valp, 0)->index != index)
+ if (code == UNION_TYPE
+ && TREE_CODE (t) == MODIFY_EXPR
+ && (CONSTRUCTOR_NELTS (*valp) == 0
+ || CONSTRUCTOR_ELT (*valp, 0)->index != index))
{
- if (cxx_dialect < cxx20)
+ /* We changed the active member of a union. Ensure that this is
+ valid. */
+ bool has_active_member = CONSTRUCTOR_NELTS (*valp) != 0;
+ tree inner = strip_array_types (reftype);
+ if (has_active_member && cxx_dialect < cxx20)
{
if (!ctx->quiet)
error_at (cp_expr_loc_or_input_loc (t),
While we're looking at this area, this error message should really mention
that it's allowed in C++20.
@@ -6205,8 +6216,36 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
index);
*non_constant_p = true;
}
- else if (TREE_CODE (t) == MODIFY_EXPR
- && CONSTRUCTOR_NO_CLEARING (*valp))
+ else if (!is_access_expr
+ || (CLASS_TYPE_P (inner)
+ && !type_has_non_deleted_trivial_default_ctor (inner)))
+ {
+ /* Diagnose changing active union member after initialisation
+ without a valid member access expression, as described in
+ [class.union.general] p5. */
+ if (!ctx->quiet)
+ {
+ if (has_active_member)
+ error_at (cp_expr_loc_or_input_loc (t),
+ "accessing %qD member instead of initialized "
+ "%qD member in constant expression",
+ index, CONSTRUCTOR_ELT (*valp, 0)->index);
+ else
+ error_at (cp_expr_loc_or_input_loc (t),
+ "accessing uninitialized member %qD",
+ index);
+ if (is_access_expr)
+ {
+ inform (DECL_SOURCE_LOCATION (index),
+ "%qD does not implicitly begin its lifetime "
+ "because %qT does not have a non-deleted "
+ "trivial default constructor",
+ index, inner);
+ }
The !is_access_expr case could also use an explanatory message.
Thanks for the review, I've updated these messages and will send through
an updated patch once bootstrap/regtest is complete.
Also, I notice that this testcase crashes with the patch:
union U { int i; float f; };
constexpr auto g (U u) { return (u.i = 42); }
static_assert (g({.f = 3.14}) == 42);
This appears to segfault even without the patch since GCC 13.1.
https://godbolt.org/z/45sPh8WaK
I haven't done a bisect yet to work out what commit exactly caused this.
Should I aim to fix this first before coming back with this patch?
Ah, I was just assuming it was related, never mind. I'll fix it.
Jason
