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? Thanks, Nathaniel