c++: -Wreturn-type with if (true) throw [PR107310]

I removed this folding in GCC 12 because it was interfering with an
experiment of richi's, but that never went in and the change causes
regressions, so let's put it back.

This reverts commit r12-5638-ga3e75c1491cd2d.

	PR c++/107310

gcc/cp/ChangeLog:

	* cp-gimplify.cc (genericize_if_stmt): Restore folding
	of constant conditions.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wimplicit-fallthrough-39.c: Adjust warning.
	* g++.dg/warn/Wreturn-6.C: New test.

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index b995b4f..4fecd56 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -190,6 +190,12 @@ genericize_if_stmt (tree *stmt_p)
     }
   else if (IF_STMT_CONSTEXPR_P (stmt))
     stmt = integer_nonzerop (cond) ? then_ : else_;
+  /* ??? This optimization doesn't seem to belong here, but removing it
+     causes -Wreturn-type regressions (e.g. 107310).  */
+  else if (integer_nonzerop (cond) && !TREE_SIDE_EFFECTS (else_))
+    stmt = then_;
+  else if (integer_zerop (cond) && !TREE_SIDE_EFFECTS (then_))
+    stmt = else_;
   else
     stmt = build3 (COND_EXPR, void_type_node, cond, then_, else_);
   protected_set_expr_location_if_unset (stmt, locus);