[Bug c++/80460] Incorrect fallthrough warning after [[noreturn]] function inside always-true conditional

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80460

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Actually, nothing I imagined wouldn't work for the case when the else branch is
dead :(.

A patch that helps with bogus warning for dead then branches:
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -1912,6 +1912,7 @@ collect_fallthrough_labels (gimple_stmt_iterator *gsi_p,
       if (gimple_code (gsi_stmt (*gsi_p)) == GIMPLE_COND)
    {
      gcond *cond_stmt = as_a <gcond *> (gsi_stmt (*gsi_p));
+     bool false_p = gimple_cond_false_p (cond_stmt);
      tree false_lab = gimple_cond_false_label (cond_stmt);
      location_t if_loc = gimple_location (cond_stmt);

@@ -1956,7 +1957,9 @@ collect_fallthrough_labels (gimple_stmt_iterator *gsi_p,
        = gimple_call_internal_p (gsi_stmt (*gsi_p), IFN_FALLTHROUGH);
          gsi_next (gsi_p);
          tree goto_dest = gimple_goto_dest (gsi_stmt (*gsi_p));
-         if (!fallthru_before_dest)
+         /* If FALSE_P, the then branch is dead, do not collect
+        the label.  */
+         if (!fallthru_before_dest && !false_p)
        {
          struct label_entry l = { goto_dest, if_loc };
          labels->safe_push (l);