https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:46fdced6a9f936ae4d5b42347d7d87f69875683a commit r11-4230-g46fdced6a9f936ae4d5b42347d7d87f69875683a Author: Patrick Palka <ppa...@redhat.com> Date: Thu Oct 22 07:33:58 2020 -0400 c++: constexpr evaluation and bare EMPTY_CLASS_EXPR [PR96575] In the testcase below, folding of the initializer for 'ret' inside the instantiated f<lambda>::lambda ends up yielding an initializer for which potential_constant_expression returns false. This causes finish_function to mark the lambda as non-constexpr, which ultimately causes us to reject 'f(g)' as a call to a non-constexpr function. The initializer for 'ret' inside f<lambda>::lambda, prior to folding, is the CALL_EXPR <lambda(S)>::operator() (&cb, ({}, <<< Unknown tree: empty_class_expr >>>;)) where the second argument is a COMPOUND_EXPR whose second operand is an EMPTY_CLASS_EXPR that was formed by build_class_a. cp_fully_fold_init is able to only partially fold this initializer: it gets rid of the side-effectless COMPOUND_EXPR to obtain <lambda(S)>::operator() (&cb, <<< Unknown tree: empty_class_expr >>>) as the final initializer for 'ret'. This initializer no longer satifies potential_constant_expression due to the bare EMPTY_CLASS_EXPR which is not wrapped in a COMPOUND_EXPR. (cp_fully_fold_init first tries maybe_constant_value on the original CALL_EXPR, but constexpr evaluation punts upon seeing __builtin_is_constant_evaluated, since manifestly_const_eval is false.) To fix this, it seems we could either make cp_fold preserve the COMPOUND_EXPR trees produced by build_call_a, or we could improve the constexpr machinery to treat EMPTY_CLASS_EXPR trees as first-class citizens. Assuming it's safe to continue folding away these COMPOUND_EXPRs, the second approach seems cleaner, so this patch implements the second approach. gcc/cp/ChangeLog: PR c++/96575 * constexpr.c (cxx_eval_constant_expression) <case EMPTY_CLASS_EXPR>: Lower it to a CONSTRUCTOR. (potential_constant_expression_1) <case COMPOUND_EXPR>: Remove now-redundant handling of COMPOUND_EXPR with EMPTY_CLASS_EXPR second operand. <case EMPTY_CLASS_EXPR>: Return true instead of false. gcc/testsuite/ChangeLog: PR c++/96575 * g++.dg/cpp1z/constexpr-96575.C: New test.
