https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98672
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-10 branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:8182cbe3fb2c2d20e8dff9d2476fb94046e560b3 commit r10-9315-g8182cbe3fb2c2d20e8dff9d2476fb94046e560b3 Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Jan 21 17:20:24 2021 +0100 c++: Fix up potential_constant_expression_1 FOR/WHILE_STMT handling [PR98672] The following testcase is rejected even when it is valid. The problem is that potential_constant_expression_1 doesn't have the accurate *jump_target tracking cxx_eval_* has, and when the loop has a condition that isn't guaranteed to be always true, the body isn't walked at all. That is mostly a correct conservative behavior, except that it doesn't detect if there are any return statements in the body, which means the loop might return instead of falling through to the next statement. We already have code for return stmt discovery in code snippets we don't try to evaluate for switches, so this patch reuses that for FOR_STMT and WHILE_STMT bodies. Note, I haven't touched FOR_EXPR, with statement expressions it could have return stmts in it too, or it could have break or continue statements that wouldn't bind to the current loop but to something outer. That case is clearly mishandled by potential_constant_expression_1 even when the condition is missing or is always true, and it wouldn't surprise me if cxx_eval_* didn't handle it right either, so I'm deferring that to separate PR for later. We'd need proper test coverage for all of that. > Hmm, IF_STMT probably also needs to check the else clause, if the condition > isn't a known constant. You're right, I thought it was ok because it recurses with tf_none, but if the then branch is potentially constant and only else returns, continues or breaks, then as the enhanced testcase shows we were mishandling it too. 2021-01-21 Jakub Jelinek <ja...@redhat.com> PR c++/98672 * constexpr.c (check_for_return_continue_data): Add break_stmt member. (check_for_return_continue): Also look for BREAK_STMT. Handle SWITCH_STMT by ignoring break_stmt from its body. (potential_constant_expression_1) <case FOR_STMT>, <case WHILE_STMT>: If the condition isn't constant true, check if the loop body can contain a return stmt. <case SWITCH_STMT>: Adjust check_for_return_continue_data initializer. <case IF_STMT>: If recursion with tf_none is successful, merge *jump_target from the branches - returns with highest priority, breaks or continues lower. If then branch is potentially constant and doesn't return, check the else branch if it could return, break or continue. * g++.dg/cpp1y/constexpr-98672.C: New test.
