From de07cff96abd43f6f65dcf333958899c2ec42598 Mon Sep 17 00:00:00 2001 From: Patrick Palka Date: Tue, 14 Sep 2021 11:22:12 -0400 Subject: c++: empty union member activation during constexpr [PR102163] Here, the union's constructor is defined to activate its empty data member _M_rest, but during constexpr evaluation of this constructor the subobject constructor call O::O(&_M_rest, 42) doesn't produce a side effect that actually activates the member, so the union still appears uninitialized after its constructor has run. This patch fixes this by using a dummy MODIFY_EXPR in this situation, whose evaluation ensures the member gets activated. PR c++/102163 gcc/cp/ChangeLog: * constexpr.c (cxx_eval_call_expression): After evaluating a subobject constructor call for an empty union member, produce a side effect that makes sure the member gets activated. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/constexpr-empty17.C: New test. --- gcc/cp/constexpr.c | 34 ++++++++++++++++++++++++++++------ 1 file changed, 28 insertions(+), 6 deletions(-) (limited to 'gcc/cp/constexpr.c') diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c index 0c2498a..d3c075c 100644 --- a/gcc/cp/constexpr.c +++ b/gcc/cp/constexpr.c @@ -2787,12 +2787,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t, &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; + { + /* 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); + } + } else if (VOID_TYPE_P (TREE_TYPE (res))) result = void_node; else -- cgit v1.1