[Bug c++/103991] [12 Regression] Bogus -Wreturn-type with constexpr if and local var with destructor

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103991

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
For consteval if I guess we could do:
--- gcc/cp/cp-objcp-common.c.jj 2022-01-11 23:11:22.091294356 +0100
+++ gcc/cp/cp-objcp-common.c    2022-01-12 17:57:18.232202275 +0100
@@ -313,6 +313,13 @@ cxx_block_may_fallthru (const_tree stmt)
       return false;

     case IF_STMT:
+      if (IF_STMT_CONSTEXPR_P (stmt))
+       {
+         if (integer_nonzerop (IF_COND (stmt)))
+           return block_may_fallthru (THEN_CLAUSE (stmt));
+         if (integer_zerop (IF_COND (stmt)))
+           return block_may_fallthru (ELSE_CLAUSE (stmt));
+       }
       if (block_may_fallthru (THEN_CLAUSE (stmt)))
        return true;
       return block_may_fallthru (ELSE_CLAUSE (stmt));
--- gcc/cp/cp-gimplify.c.jj     2022-01-11 23:11:22.090294370 +0100
+++ gcc/cp/cp-gimplify.c        2022-01-12 17:53:57.431036236 +0100
@@ -166,8 +166,13 @@ genericize_if_stmt (tree *stmt_p)
      can contain unfolded immediate function calls, we have to discard
      the then_ block regardless of whether else_ has side-effects or not.  */
   if (IF_STMT_CONSTEVAL_P (stmt))
-    stmt = build3 (COND_EXPR, void_type_node, boolean_false_node,
-                  void_node, else_);
+    {
+      if (block_may_fallthru (then_))
+       stmt = build3 (COND_EXPR, void_type_node, boolean_false_node,
+                      void_node, else_);
+      else
+       stmt = else_;
+    }
   else
     stmt = build3 (COND_EXPR, void_type_node, cond, then_, else_);
   protected_set_expr_location_if_unset (stmt, locus);