Now that bootstrap has finished, I have gotten regressions in the
following libstdc++ tests:
Running libstdc++:libstdc++-dg/conformance.exp ...
FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess errors)
FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test for
excess errors)
FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test for
excess errors)
FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test for
excess errors)
FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test for
excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc -std=gnu++20
(test for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc -std=gnu++26
(test for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20 (test
for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26 (test
for excess errors)
FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess errors)
UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation failed
to produce executable
FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess errors)
UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation failed
to produce executable
On investigation though it looks like the issue might be with libstdc++
rather than the patch itself; running the failing tests using clang with
libstdc++ also produces similar errors, and my reading of the code
suggests that this is correct.
What's the way forward here? Should I look at creating a patch to fix
the libstdc++ issues before resubmitting this patch for the C++
frontend? Or should I submit a version of this patch without the
`std::construct_at` changes and wait till libstdc++ gets fixed for that?
On Sat, Sep 23, 2023 at 01:01:20AM +1000, Nathaniel Shead wrote:
> On Fri, Sep 22, 2023 at 02:21:15PM +0100, Jason Merrill wrote:
> > On 9/21/23 09:41, Nathaniel Shead wrote:
> > > I've updated the error messages, and also fixed another bug I found
> > > while retesting (value-initialised unions weren't considered to have any
> > > active member yet).
> > >
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > >
> > > -- >8 --
> > >
> > > This patch adds checks for attempting to change the active member of a
> > > union by methods other than a member access expression.
> > >
> > > To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
> > > patch redoes the solution for c++/59950 to avoid extranneous *&; it
> > > seems that the only case that needed the workaround was when copying
> > > empty classes.
> > >
> > > This patch also ensures that constructors for a union field mark that
> > > field as the active member before entering the call itself; this ensures
> > > that modifications of the field within the constructor's body don't
> > > cause false positives (as these will not appear to be member access
> > > expressions). This means that we no longer need to start the lifetime of
> > > empty union members after the constructor body completes.
> > >
> > > As a drive-by fix, this patch also ensures that value-initialised unions
> > > are considered to have activated their initial member for the purpose of
> > > checking stores, which catches some additional mistakes pre-C++20.
> > >
> > > PR c++/101631
> > >
> > > gcc/cp/ChangeLog:
> > >
> > > * call.cc (build_over_call): Fold more indirect refs for trivial
> > > assignment op.
> > > * class.cc (type_has_non_deleted_trivial_default_ctor): Create.
> > > * constexpr.cc (cxx_eval_call_expression): Start lifetime of
> > > union member before entering constructor.
> > > (cxx_eval_store_expression): Activate member for
> > > value-initialised union. Check for accessing inactive union
> > > member indirectly.
> > > * cp-tree.h (type_has_non_deleted_trivial_default_ctor):
> > > Forward declare.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > * g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
> > > * g++.dg/cpp1y/constexpr-union6.C: New test.
> > > * g++.dg/cpp2a/constexpr-union2.C: New test.
> > > * g++.dg/cpp2a/constexpr-union3.C: New test.
> > > * g++.dg/cpp2a/constexpr-union4.C: New test.
> > > * g++.dg/cpp2a/constexpr-union5.C: New test.
> > >
> > > Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
> > > ---
> > > gcc/cp/call.cc | 11 +-
> > > gcc/cp/class.cc | 8 ++
> > > gcc/cp/constexpr.cc | 129 +++++++++++++-----
> > > gcc/cp/cp-tree.h | 1 +
> > > .../g++.dg/cpp1y/constexpr-89336-3.C | 2 +-
> > > gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C | 13 ++
> > > gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C | 30 ++++
> > > gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C | 45 ++++++
> > > gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C | 29 ++++
> > > gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C | 71 ++++++++++
> > > 10 files changed, 296 insertions(+), 43 deletions(-)
> > > create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> > >
> > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > index e8dafbd8ba6..c1fb8807d3f 100644
> > > --- a/gcc/cp/call.cc
> > > +++ b/gcc/cp/call.cc
> > > @@ -10330,10 +10330,7 @@ build_over_call (struct z_candidate *cand, int
> > > flags, tsubst_flags_t complain)
> > > && DECL_OVERLOADED_OPERATOR_IS (fn, NOP_EXPR)
> > > && trivial_fn_p (fn))
> > > {
> > > - /* Don't use cp_build_fold_indirect_ref, op= returns an lvalue
> > > even if
> > > - the object argument isn't one. */
> > > - tree to = cp_build_indirect_ref (input_location, argarray[0],
> > > - RO_ARROW, complain);
> > > + tree to = cp_build_fold_indirect_ref (argarray[0]);
> > > tree type = TREE_TYPE (to);
> > > tree as_base = CLASSTYPE_AS_BASE (type);
> > > tree arg = argarray[1];
> > > @@ -10341,7 +10338,11 @@ build_over_call (struct z_candidate *cand, int
> > > flags, tsubst_flags_t complain)
> > > if (is_really_empty_class (type, /*ignore_vptr*/true))
> > > {
> > > - /* Avoid copying empty classes. */
> > > + /* Avoid copying empty classes, but ensure op= returns an lvalue even
> > > + if the object argument isn't one. This isn't needed in other cases
> > > + since MODIFY_EXPR is always considered an lvalue. */
> > > + to = cp_build_addr_expr (to, tf_none);
> > > + to = cp_build_indirect_ref (input_location, to, RO_ARROW, complain);
> > > val = build2 (COMPOUND_EXPR, type, arg, to);
> > > suppress_warning (val, OPT_Wunused);
> > > }
> > > diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> > > index b71333af1f8..e31aeb8e68b 100644
> > > --- a/gcc/cp/class.cc
> > > +++ b/gcc/cp/class.cc
> > > @@ -5688,6 +5688,14 @@ type_has_virtual_destructor (tree type)
> > > return (dtor && DECL_VIRTUAL_P (dtor));
> > > }
> > > +/* True iff class TYPE has a non-deleted trivial default
> > > + constructor. */
> > > +
> > > +bool type_has_non_deleted_trivial_default_ctor (tree type)
> > > +{
> > > + return TYPE_HAS_TRIVIAL_DFLT (type) && locate_ctor (type);
> > > +}
> > > +
> > > /* Returns true iff T, a class, has a move-assignment or
> > > move-constructor. Does not lazily declare either.
> > > If USER_P is false, any move function will do. If it is true, the
> > > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > > index a673a6022f1..5e50fc2c792 100644
> > > --- a/gcc/cp/constexpr.cc
> > > +++ b/gcc/cp/constexpr.cc
> > > @@ -3146,40 +3146,34 @@ cxx_eval_call_expression (const constexpr_ctx
> > > *ctx, tree t,
> > > cxx_set_object_constness (ctx, new_obj,
> > > /*readonly_p=*/false,
> > > non_constant_p, overflow_p);
> > > + /* If this is a constructor, we are beginning the lifetime of the
> > > + object we are initializing. */
> > > + if (new_obj
> > > + && DECL_CONSTRUCTOR_P (fun)
> > > + && TREE_CODE (new_obj) == COMPONENT_REF
> > > + && TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) ==
> > > UNION_TYPE)
> > > + {
> > > + tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
> > > + new_obj,
> > > + build_constructor (TREE_TYPE (new_obj),
> > > + NULL));
> > > + cxx_eval_constant_expression (ctx, activate,
> > > + lval, non_constant_p, overflow_p);
> > > + ggc_free (activate);
> > > + }
> > > +
> > > tree jump_target = NULL_TREE;
> > > cxx_eval_constant_expression (&call_ctx, body,
> > > vc_discard, non_constant_p,
> > > overflow_p,
> > > &jump_target);
> > > if (DECL_CONSTRUCTOR_P (fun))
> > > - {
> > > - /* This can be null for a subobject constructor call, in
> > > - which case what we care about is the initialization
> > > - side-effects rather than the value. We could get at the
> > > - value by evaluating *this, but we don't bother; there's
> > > - no need to put such a call in the hash table. */
> > > - result = lval ? ctx->object : ctx->ctor;
> > > -
> > > - /* If we've just evaluated a subobject constructor call for an
> > > - empty union member, it might not have produced a side effect
> > > - that actually activated the union member. So produce such a
> > > - side effect now to ensure the union appears initialized. */
> > > - if (!result && new_obj
> > > - && TREE_CODE (new_obj) == COMPONENT_REF
> > > - && TREE_CODE (TREE_TYPE
> > > - (TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> > > - && is_really_empty_class (TREE_TYPE (new_obj),
> > > - /*ignore_vptr*/false))
> > > - {
> > > - tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> > > - new_obj,
> > > - build_constructor (TREE_TYPE
> > > (new_obj),
> > > - NULL));
> > > - cxx_eval_constant_expression (ctx, activate, lval,
> > > - non_constant_p, overflow_p);
> > > - ggc_free (activate);
> > > - }
> > > - }
> > > + /* This can be null for a subobject constructor call, in
> > > + which case what we care about is the initialization
> > > + side-effects rather than the value. We could get at the
> > > + value by evaluating *this, but we don't bother; there's
> > > + no need to put such a call in the hash table. */
> > > + result = lval ? ctx->object : ctx->ctor;
> > > else if (VOID_TYPE_P (TREE_TYPE (res)))
> > > result = void_node;
> > > else
> > > @@ -6127,6 +6121,14 @@ cxx_eval_store_expression (const constexpr_ctx
> > > *ctx, tree t,
> > > mutable_p)
> > > && const_object_being_modified == NULL_TREE)
> > > const_object_being_modified = probe;
> > > +
> > > + /* Track named member accesses for unions to validate modifications
> > > + that change active member. */
> > > + if (!evaluated && TREE_CODE (probe) == COMPONENT_REF)
> > > + vec_safe_push (refs, probe);
> > > + else
> > > + vec_safe_push (refs, NULL_TREE);
> > > +
> > > vec_safe_push (refs, elt);
> > > vec_safe_push (refs, TREE_TYPE (probe));
> > > probe = ob;
> > > @@ -6135,6 +6137,7 @@ cxx_eval_store_expression (const constexpr_ctx
> > > *ctx, tree t,
> > > case REALPART_EXPR:
> > > gcc_assert (probe == target);
> > > + vec_safe_push (refs, NULL_TREE);
> > > vec_safe_push (refs, probe);
> > > vec_safe_push (refs, TREE_TYPE (probe));
> > > probe = TREE_OPERAND (probe, 0);
> > > @@ -6142,6 +6145,7 @@ cxx_eval_store_expression (const constexpr_ctx
> > > *ctx, tree t,
> > > case IMAGPART_EXPR:
> > > gcc_assert (probe == target);
> > > + vec_safe_push (refs, NULL_TREE);
> > > vec_safe_push (refs, probe);
> > > vec_safe_push (refs, TREE_TYPE (probe));
> > > probe = TREE_OPERAND (probe, 0);
> > > @@ -6230,6 +6234,7 @@ cxx_eval_store_expression (const constexpr_ctx
> > > *ctx, tree t,
> > > enum tree_code code = TREE_CODE (type);
> > > tree reftype = refs->pop();
> > > tree index = refs->pop();
> > > + bool is_access_expr = refs->pop() != NULL_TREE;
> > > if (code == COMPLEX_TYPE)
> > > {
> > > @@ -6270,21 +6275,72 @@ 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)
> > > + /* Probably a change of active member for a union. Ensure that
> > > + this is valid. */
> > > + no_zero_init = true;
> > > +
> > > + tree init = *valp;
> > > + if (!CONSTRUCTOR_NO_CLEARING (*valp) && CONSTRUCTOR_NELTS (*valp) ==
> > > 0)
> > > + /* This is a value-initialized union, process the initialization
> > > + to ensure that the active member is properly set. */
> > > + init = build_value_init (TREE_TYPE (*valp), tf_warning_or_error);
> > > +
> > > + bool has_active_member = CONSTRUCTOR_NELTS (init) != 0;
> > > + tree inner = strip_array_types (reftype);
> > > +
> > > + if (has_active_member && CONSTRUCTOR_ELT (init, 0)->index == index)
> > > + /* After handling value-initialization, this wasn't actually a
> > > + change of active member like we initially thought. */
> > > + no_zero_init = false;
> > > + else if (has_active_member && cxx_dialect < cxx20)
> > > {
> > > if (!ctx->quiet)
> > > error_at (cp_expr_loc_or_input_loc (t),
> > > "change of the active member of a union "
> > > - "from %qD to %qD",
> > > - CONSTRUCTOR_ELT (*valp, 0)->index,
> > > + "from %qD to %qD is not a constant expression "
> > > + "before C++20",
> > > + CONSTRUCTOR_ELT (init, 0)->index,
> > > 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 initialization
> > > + 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 (init, 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);
> >
> > It now occurs to me that this should suggest using placement new instead.
> > And that we should test using placement new to begin the lifetime of such
> > members.
> >
> > Jason
> >
>
> Good point. On testing placement new I found I wasn't checking for
> indirect change of active member during initialisation either, but this
> was pretty trivial to add. How does this look?
>
> Bootstrap + regtest ongoing, but dg.exp completed with no errors. I'm
> away from my computer next week so I thought I'd send this through now
> anyway.
>
> -- >8 --
>
> This patch adds checks for attempting to change the active member of a
> union by methods other than a member access expression.
>
> To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
> patch redoes the solution for c++/59950 to avoid extranneous *&; it
> seems that the only case that needed the workaround was when copying
> empty classes.
>
> This patch also ensures that constructors for a union field mark that
> field as the active member before entering the call itself; this ensures
> that modifications of the field within the constructor's body don't
> cause false positives (as these will not appear to be member access
> expressions). This means that we no longer need to start the lifetime of
> empty union members after the constructor body completes.
>
> As a drive-by fix, this patch also ensures that value-initialised unions
> are considered to have activated their initial member for the purpose of
> checking stores, which catches some additional mistakes pre-C++20.
>
> PR c++/101631
> PR c++/102286
>
> gcc/cp/ChangeLog:
>
> * call.cc (build_over_call): Fold more indirect refs for trivial
> assignment op.
> * class.cc (type_has_non_deleted_trivial_default_ctor): Create.
> * constexpr.cc (cxx_eval_call_expression): Start lifetime of
> union member before entering constructor.
> (cxx_eval_store_expression): Activate member for
> value-initialised union. Check for accessing inactive union
> member indirectly.
> * cp-tree.h (type_has_non_deleted_trivial_default_ctor):
> Forward declare.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
> * g++.dg/cpp1y/constexpr-union6.C: New test.
> * g++.dg/cpp2a/constexpr-union2.C: New test.
> * g++.dg/cpp2a/constexpr-union3.C: New test.
> * g++.dg/cpp2a/constexpr-union4.C: New test.
> * g++.dg/cpp2a/constexpr-union5.C: New test.
> * g++.dg/cpp2a/constexpr-union6.C: New test.
>
> Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
> ---
> gcc/cp/call.cc | 11 +-
> gcc/cp/class.cc | 8 ++
> gcc/cp/constexpr.cc | 135 +++++++++++++-----
> gcc/cp/cp-tree.h | 1 +
> .../g++.dg/cpp1y/constexpr-89336-3.C | 2 +-
> gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C | 13 ++
> gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C | 30 ++++
> gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C | 45 ++++++
> gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C | 29 ++++
> gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C | 71 +++++++++
> gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C | 43 ++++++
> 11 files changed, 345 insertions(+), 43 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index e8dafbd8ba6..c1fb8807d3f 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -10330,10 +10330,7 @@ build_over_call (struct z_candidate *cand, int
> flags, tsubst_flags_t complain)
> && DECL_OVERLOADED_OPERATOR_IS (fn, NOP_EXPR)
> && trivial_fn_p (fn))
> {
> - /* Don't use cp_build_fold_indirect_ref, op= returns an lvalue even if
> - the object argument isn't one. */
> - tree to = cp_build_indirect_ref (input_location, argarray[0],
> - RO_ARROW, complain);
> + tree to = cp_build_fold_indirect_ref (argarray[0]);
> tree type = TREE_TYPE (to);
> tree as_base = CLASSTYPE_AS_BASE (type);
> tree arg = argarray[1];
> @@ -10341,7 +10338,11 @@ build_over_call (struct z_candidate *cand, int
> flags, tsubst_flags_t complain)
>
> if (is_really_empty_class (type, /*ignore_vptr*/true))
> {
> - /* Avoid copying empty classes. */
> + /* Avoid copying empty classes, but ensure op= returns an lvalue even
> + if the object argument isn't one. This isn't needed in other cases
> + since MODIFY_EXPR is always considered an lvalue. */
> + to = cp_build_addr_expr (to, tf_none);
> + to = cp_build_indirect_ref (input_location, to, RO_ARROW, complain);
> val = build2 (COMPOUND_EXPR, type, arg, to);
> suppress_warning (val, OPT_Wunused);
> }
> diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> index b71333af1f8..e31aeb8e68b 100644
> --- a/gcc/cp/class.cc
> +++ b/gcc/cp/class.cc
> @@ -5688,6 +5688,14 @@ type_has_virtual_destructor (tree type)
> return (dtor && DECL_VIRTUAL_P (dtor));
> }
>
> +/* True iff class TYPE has a non-deleted trivial default
> + constructor. */
> +
> +bool type_has_non_deleted_trivial_default_ctor (tree type)
> +{
> + return TYPE_HAS_TRIVIAL_DFLT (type) && locate_ctor (type);
> +}
> +
> /* Returns true iff T, a class, has a move-assignment or
> move-constructor. Does not lazily declare either.
> If USER_P is false, any move function will do. If it is true, the
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index a673a6022f1..8e6f60d03d2 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -3146,40 +3146,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> tree t,
> cxx_set_object_constness (ctx, new_obj, /*readonly_p=*/false,
> non_constant_p, overflow_p);
>
> + /* If this is a constructor, we are beginning the lifetime of the
> + object we are initializing. */
> + if (new_obj
> + && DECL_CONSTRUCTOR_P (fun)
> + && TREE_CODE (new_obj) == COMPONENT_REF
> + && TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) ==
> UNION_TYPE)
> + {
> + tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
> + new_obj,
> + build_constructor (TREE_TYPE (new_obj),
> + NULL));
> + cxx_eval_constant_expression (ctx, activate,
> + lval, non_constant_p, overflow_p);
> + ggc_free (activate);
> + }
> +
> tree jump_target = NULL_TREE;
> cxx_eval_constant_expression (&call_ctx, body,
> vc_discard, non_constant_p, overflow_p,
> &jump_target);
>
> if (DECL_CONSTRUCTOR_P (fun))
> - {
> - /* This can be null for a subobject constructor call, in
> - which case what we care about is the initialization
> - side-effects rather than the value. We could get at the
> - value by evaluating *this, but we don't bother; there's
> - no need to put such a call in the hash table. */
> - result = lval ? ctx->object : ctx->ctor;
> -
> - /* If we've just evaluated a subobject constructor call for an
> - empty union member, it might not have produced a side effect
> - that actually activated the union member. So produce such a
> - side effect now to ensure the union appears initialized. */
> - if (!result && new_obj
> - && TREE_CODE (new_obj) == COMPONENT_REF
> - && TREE_CODE (TREE_TYPE
> - (TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> - && is_really_empty_class (TREE_TYPE (new_obj),
> - /*ignore_vptr*/false))
> - {
> - tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> - new_obj,
> - build_constructor (TREE_TYPE
> (new_obj),
> - NULL));
> - cxx_eval_constant_expression (ctx, activate, lval,
> - non_constant_p, overflow_p);
> - ggc_free (activate);
> - }
> - }
> + /* This can be null for a subobject constructor call, in
> + which case what we care about is the initialization
> + side-effects rather than the value. We could get at the
> + value by evaluating *this, but we don't bother; there's
> + no need to put such a call in the hash table. */
> + result = lval ? ctx->object : ctx->ctor;
> else if (VOID_TYPE_P (TREE_TYPE (res)))
> result = void_node;
> else
> @@ -6127,6 +6121,14 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
> mutable_p)
> && const_object_being_modified == NULL_TREE)
> const_object_being_modified = probe;
> +
> + /* Track named member accesses for unions to validate modifications
> + that change active member. */
> + if (!evaluated && TREE_CODE (probe) == COMPONENT_REF)
> + vec_safe_push (refs, probe);
> + else
> + vec_safe_push (refs, NULL_TREE);
> +
> vec_safe_push (refs, elt);
> vec_safe_push (refs, TREE_TYPE (probe));
> probe = ob;
> @@ -6135,6 +6137,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
>
> case REALPART_EXPR:
> gcc_assert (probe == target);
> + vec_safe_push (refs, NULL_TREE);
> vec_safe_push (refs, probe);
> vec_safe_push (refs, TREE_TYPE (probe));
> probe = TREE_OPERAND (probe, 0);
> @@ -6142,6 +6145,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
>
> case IMAGPART_EXPR:
> gcc_assert (probe == target);
> + vec_safe_push (refs, NULL_TREE);
> vec_safe_push (refs, probe);
> vec_safe_push (refs, TREE_TYPE (probe));
> probe = TREE_OPERAND (probe, 0);
> @@ -6230,6 +6234,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
> enum tree_code code = TREE_CODE (type);
> tree reftype = refs->pop();
> tree index = refs->pop();
> + bool is_access_expr = refs->pop() != NULL_TREE;
>
> if (code == COMPLEX_TYPE)
> {
> @@ -6270,21 +6275,78 @@ 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)
> + /* Check for implicit change of active member for a union. */
> + if (code == UNION_TYPE
> + && (TREE_CODE (t) == MODIFY_EXPR
> + /* Also check if initializations have implicit change of active
> + member earlier up the access chain. */
> + || !refs->is_empty())
> + && (CONSTRUCTOR_NELTS (*valp) == 0
> + || CONSTRUCTOR_ELT (*valp, 0)->index != index))
> {
> - if (cxx_dialect < cxx20)
> + no_zero_init = true;
> +
> + tree init = *valp;
> + if (TREE_CODE (t) == MODIFY_EXPR
> + && !CONSTRUCTOR_NO_CLEARING (*valp)
> + && CONSTRUCTOR_NELTS (*valp) == 0)
> + /* Modifying a value-initialized union, process the initialization
> + to ensure that the active member is properly set. */
> + init = build_value_init (TREE_TYPE (*valp), tf_warning_or_error);
> +
> + bool has_active_member = CONSTRUCTOR_NELTS (init) != 0;
> + tree inner = strip_array_types (reftype);
> +
> + if (has_active_member && CONSTRUCTOR_ELT (init, 0)->index == index)
> + /* After handling value-initialization, this wasn't actually a
> + change of active member like we initially thought. */
> + no_zero_init = false;
> + else if (has_active_member && cxx_dialect < cxx20)
> {
> if (!ctx->quiet)
> error_at (cp_expr_loc_or_input_loc (t),
> "change of the active member of a union "
> - "from %qD to %qD",
> - CONSTRUCTOR_ELT (*valp, 0)->index,
> + "from %qD to %qD is not a constant expression "
> + "before C++20",
> + CONSTRUCTOR_ELT (init, 0)->index,
> index);
> *non_constant_p = true;
> }
> - else if (TREE_CODE (t) == MODIFY_EXPR
> - && CONSTRUCTOR_NO_CLEARING (*valp))
> + else if (!is_access_expr
> + || (TREE_CODE (t) == MODIFY_EXPR
> + && CLASS_TYPE_P (inner)
> + && !type_has_non_deleted_trivial_default_ctor (inner)))
> + {
> + /* Diagnose changing active union member after initialization
> + 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 (init, 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, use "
> + "%<std::construct_at%> instead",
> + index, inner);
> + else
> + inform (DECL_SOURCE_LOCATION (index),
> + "initializing %qD requires a member access "
> + "expression as the left operand of the assignment",
> + index);
> + }
> + *non_constant_p = true;
> + }
> + else if (has_active_member && CONSTRUCTOR_NO_CLEARING (init))
> {
> /* Diagnose changing the active union member while the union
> is in the process of being initialized. */
> @@ -6292,11 +6354,10 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
> error_at (cp_expr_loc_or_input_loc (t),
> "change of the active member of a union "
> "from %qD to %qD during initialization",
> - CONSTRUCTOR_ELT (*valp, 0)->index,
> + CONSTRUCTOR_ELT (init, 0)->index,
> index);
> *non_constant_p = true;
> }
> - no_zero_init = true;
> }
>
> ctors.safe_push (valp);
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index fd6bf9fd4a0..40aa9f0be2b 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -6815,6 +6815,7 @@ extern bool trivial_default_constructor_is_constexpr
> (tree);
> extern bool type_has_constexpr_default_constructor (tree);
> extern bool type_has_constexpr_destructor (tree);
> extern bool type_has_virtual_destructor (tree);
> +extern bool type_has_non_deleted_trivial_default_ctor (tree);
> extern bool classtype_has_move_assign_or_move_ctor_p (tree, bool
> user_declared);
> extern bool classtype_has_non_deleted_move_ctor (tree);
> extern tree classtype_has_depr_implicit_copy (tree);
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> index 9d370ddefcf..6a60966f6f8 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> @@ -18,7 +18,7 @@ constexpr int
> bar ()
> {
> union U { int a[5]; long b; };
> - union V { union U u; short v; };
> + union V { short v; union U u; };
> V w {};
> w.v = 5;
> w.u.a[3] = w.u.a[1] = w.v; // { dg-error "change of the active
> member of a union from" "" { target c++17_down } }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> new file mode 100644
> index 00000000000..ff7ebf19f89
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> @@ -0,0 +1,13 @@
> +// { dg-do compile { target c++14 } }
> +
> +union U {
> + int a;
> + float b;
> +};
> +
> +constexpr bool foo() {
> + U u {};
> + u.b = 1.0f; // { dg-error "change of the active member" "" { target
> c++17_down } }
> + return u.b == 1.0f;
> +}
> +constexpr bool x = foo();
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> new file mode 100644
> index 00000000000..1712395d7e7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> @@ -0,0 +1,30 @@
> +// PR c++/101631
> +// { dg-do compile { target c++20 } }
> +
> +struct sso {
> + union {
> + int buf[10];
> + int* alloc;
> + };
> +};
> +
> +constexpr bool direct() {
> + sso val;
> + val.alloc = nullptr;
> + val.buf[5] = 42;
> + return true;
> +}
> +constexpr bool ok = direct();
> +
> +
> +constexpr void perform_assignment(int& left, int right) noexcept {
> + left = right; // { dg-error "accessing .+ member instead of initialized" }
> +}
> +
> +constexpr bool indirect() {
> + sso val;
> + val.alloc = nullptr;
> + perform_assignment(val.buf[5], 42); // { dg-message "in .constexpr.
> expansion" }
> + return true;
> +}
> +constexpr bool err = indirect(); // { dg-message "in .constexpr. expansion"
> }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> new file mode 100644
> index 00000000000..6d30bb2498f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> @@ -0,0 +1,45 @@
> +// { dg-do compile { target c++20 } }
> +
> +struct S
> +{
> + union {
> + char buf[8];
> + char* ptr;
> + };
> + unsigned len;
> +
> + constexpr S(const char* s, unsigned n)
> + {
> + char* p;
> + if (n > 7)
> + p = ptr = new char[n+1];
> + else
> + p = buf;
> + for (len = 0; len < n; ++len)
> + p[len] = s[len]; // { dg-error "accessing uninitialized member" }
> + p[len] = '\0';
> + }
> +
> + constexpr ~S()
> + {
> + if (len > 7)
> + delete[] ptr;
> + }
> +};
> +
> +constexpr bool test1()
> +{
> + S s("test", 4); // { dg-message "in .constexpr. expansion" }
> + return true;
> +}
> +
> +constexpr bool a = test1(); // { dg-message "in .constexpr. expansion" }
> +
> +
> +constexpr bool test2()
> +{
> + S s("hello world", 11);
> + return true;
> +}
> +
> +constexpr bool b = test2();
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> new file mode 100644
> index 00000000000..429ab203ac2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> @@ -0,0 +1,29 @@
> +// { dg-do compile { target c++20 } }
> +
> +// from [class.union.general] p5
> +
> +union A { int x; int y[4]; };
> +struct B { A a; };
> +union C { B b; int k; };
> +constexpr int f() {
> + C c; // does not start lifetime of any union member
> + c.b.a.y[3] = 4; // OK, S(c.b.a.y[3]) contains c.b and c.b.a.y;
> + // creates objects to hold union members c.b and
> c.b.a.y
> + return c.b.a.y[3]; // OK, c.b.a.y refers to newly created object (see
> [basic.life])
> +}
> +constexpr int a = f();
> +
> +struct X { const int a; int b; };
> +union Y { X x; int k; };// { dg-message "does not implicitly begin its
> lifetime" }
> +constexpr int g() {
> + Y y = { { 1, 2 } }; // OK, y.x is active union member ([class.mem])
> + int n = y.x.a;
> + y.k = 4; // OK, ends lifetime of y.x, y.k is active member of
> union
> +
> + y.x.b = n; // { dg-error "accessing .* member instead of
> initialized .* member" }
> + // undefined behavior: y.x.b modified outside its
> lifetime,
> + // S(y.x.b) is empty because X's default constructor
> is deleted,
> + // so union member y.x's lifetime does not
> implicitly start
> + return 0;
> +}
> +constexpr int b = g(); // { dg-message "in .constexpr. expansion" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> new file mode 100644
> index 00000000000..c94d7cc0fc9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> @@ -0,0 +1,71 @@
> +// { dg-do compile { target c++20 } }
> +
> +union U { int a; int b; int c[2]; };
> +
> +constexpr int test1() {
> + U u;
> + u.a = 10;
> + *&u.b = 20; // { dg-error "accessing" }
> + return u.b;
> +}
> +constexpr int x1 = test1(); // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test2() {
> + U u;
> + u.a = 10;
> + (0, u.b) = 20; // { dg-error "accessing" }
> + return u.b;
> +}
> +constexpr int x2 = test2(); // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test3() {
> + U u;
> + u.a = 0;
> + int* p = &u.b;
> + p[u.a] = 10; // { dg-error "accessing" }
> + return u.b;
> +}
> +constexpr int x3 = test3(); // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test4() {
> + U u;
> + u.a = 0;
> + int* p = &u.b;
> + u.a[p] = 10; // { dg-error "accessing" }
> + return u.b;
> +}
> +constexpr int x4 = test4(); // { dg-message "in .constexpr. expansion" }
> +
> +struct S { U u[10]; };
> +constexpr int test5() {
> + S s;
> + s.u[4].a = 10;
> + 6[s.u].b = 15;
> + return 4[s.u].a + s.u[6].b;
> +}
> +static_assert(test5() == 25);
> +
> +constexpr int test6() {
> + U u;
> + u.a = 5;
> + u.c[0] = 3;
> + 1[u.c] = 8;
> + return 1[u.c] + u.c[0];
> +}
> +static_assert(test6() == 11);
> +
> +constexpr int test7() {
> + U u {}; // value initialisation initialises first member
> + int* p = &u.a;
> + *p = 10;
> + return *p;
> +}
> +constexpr int x7 = test7();
> +
> +constexpr int test8() {
> + U u; // default initialisation leaves no member initialised
> + int* p = &u.a;
> + *p = 10; // { dg-error "accessing" }
> + return *p;
> +}
> +constexpr int x8 = test8(); // { dg-message "in .constexpr. expansion" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> new file mode 100644
> index 00000000000..7c37716892c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> @@ -0,0 +1,43 @@
> +// { dg-do compile { target c++20 } }
> +// PR c++/102286
> +
> +#include "construct_at.h"
> +
> +struct S { const int a; int b; };
> +union U { int k; S s; };
> +
> +constexpr int test1() {
> + U u {};
> + std::construct_at(&u.s, S{ 1, 2 });
> + return u.s.b;
> +}
> +static_assert(test1() == 2);
> +
> +constexpr int test2() {
> + U u {};
> + int* p = &u.s.b;
> + std::construct_at(p, 5); // { dg-message "in .constexpr. expansion" }
> + return u.s.b;
> +}
> +constexpr int x2 = test2(); // { dg-message "in .constexpr. expansion" }
> +
> +struct S2 { int a; int b; };
> +union U2 { int k; S2 s; };
> +constexpr int test3() {
> + U2 u;
> + int* p = &u.s.b;
> + std::construct_at(p, 8); // { dg-message "in .constexpr. expansion" }
> + return u.s.b;
> +};
> +constexpr int x3 = test3(); // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test4() {
> + union {
> + int data[1];
> + } u;
> + std::construct_at(u.data, 0); // { dg-message "in .constexpr. expansion" }
> + return 0;
> +}
> +constexpr int x4 = test4(); // { dg-message "in .constexpr. expansion" }
> +
> +// { dg-error "accessing uninitialized member" "" { target *-*-* } 0 }
> --
> 2.41.0
>
