From 21fd62e5ca9967bba8f97fd6244a8c6a564c2146 Mon Sep 17 00:00:00 2001 From: Patrick Palka Date: Wed, 11 Aug 2021 20:59:53 -0400 Subject: c++: constexpr std::construct_at on empty field [PR101663] Here during constexpr evaluation of std::construct_at(&a._M_value) we find ourselves in cxx_eval_store_expression where the target object is 'a._M_value' and the initializer is {}. Since _M_value is an empty [[no_unique_address]] member we don't create a sub-CONSTRUCTOR for it, so we end up in the early exit code path for empty stores with mismatched types and trip over the assert therein gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval); because lval is true. The reason it's true is because the INIT_EXPR in question is the LHS of a COMPOUND_EXPR, and evaluation of the LHS is always performed with lval=true (to indicate there's no lvalue-to-rvalue conversion). This patch makes the code path in question handle the lval=true case appropriately rather than asserting. In passing, it also consolidates the duplicate implementations of std::construct_at/destroy_at in some of the C++20 constexpr tests into a common header file. PR c++/101663 gcc/cp/ChangeLog: * constexpr.c (cxx_eval_store_expression): Handle the lval=true case in the early exit code path for empty stores with mismatched types. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/construct_at.h: New convenience header file that defines minimal implementations of std::construct_at/destroy_at, split out from ... * g++.dg/cpp2a/constexpr-new5.C: ... here. * g++.dg/cpp2a/constexpr-new6.C: Use the header. * g++.dg/cpp2a/constexpr-new14.C: Likewise. * g++.dg/cpp2a/constexpr-new20.C: New test. --- gcc/cp/constexpr.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'gcc/cp/constexpr.c') diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c index 1af365d..25d84a3 100644 --- a/gcc/cp/constexpr.c +++ b/gcc/cp/constexpr.c @@ -5588,8 +5588,8 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t, argument, which has the derived type rather than the base type. In this situation, just evaluate the initializer and return, since there's no actual data to store. */ - gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval); - return init; + gcc_assert (is_empty_class (TREE_TYPE (init))); + return lval ? target : init; } CONSTRUCTOR_ELTS (*valp) = CONSTRUCTOR_ELTS (init); TREE_CONSTANT (*valp) = TREE_CONSTANT (init); -- cgit v1.1