[gcc r16-9275] c++: ICE with requires and -Wsequence-point [PR126066]

https://gcc.gnu.org/g:6d87c68f9904bab176c26359cdbb4a9001247cff

commit r16-9275-g6d87c68f9904bab176c26359cdbb4a9001247cff
Author: Marek Polacek <[email protected]>
Date:   Mon Jul 13 17:47:09 2026 -0400

    c++: ICE with requires and -Wsequence-point [PR126066]
    
    We trip on the assert in lvalue_kind/MODOP_EXPR whose comment says
    that we expect to see MODOP_EXPRs only during template processing.
    In this test we get there with processing_template_decl==0.  The
    MODOP_EXPR is created in:
    
        /* Parse the requirement body. */
        ++processing_template_decl;
        reqs = cp_parser_requirement_body (parser);
        --processing_template_decl;
    
    but we're not in a template when calling maybe_convert_cond which
    calls verify_sequence_points which ends up calling lvalue_p on
    the MODOP_EXPR.
    
    verify_sequence_points is walking the unevaluated operand of a requires
    because verify_tree assumes that all unknown expressions are evaluated, and
    REQUIRES_EXPR looks like a normal expression (tcc_expression).
    
    The trunk patch changed REQUIRES_EXPR to tcc_exceptional.  But in 16, we
    can just dial the assert down to _checking.
    
            PR c++/126066
    
    gcc/cp/ChangeLog:
    
            * tree.cc (lvalue_kind) <case MODOP_EXPR>: Use
            gcc_checking_assert instead of gcc_assert.
    
    Reviewed-by: Jason Merrill <[email protected]>

Diff:
---
 gcc/cp/tree.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 2bcbfa847079..1da3e7d6fc2a 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -280,7 +280,7 @@ lvalue_kind (const_tree ref)
     case MODOP_EXPR:
       /* We expect to see unlowered MODOP_EXPRs only during
 	 template processing.  */
-      gcc_assert (processing_template_decl);
+      gcc_checking_assert (processing_template_decl);
       if (CLASS_TYPE_P (TREE_TYPE (TREE_OPERAND (ref, 0))))
 	goto default_;
       else