[gcc r13-6670] c++: -Wreturn-type with if (true) throw [PR107310]

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:71b33f8fb8daa6a7a359f322b24365d9016438fc

commit r13-6670-g71b33f8fb8daa6a7a359f322b24365d9016438fc
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Mar 14 12:20:51 2023 -0400

    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.

Diff:
---
 gcc/cp/cp-gimplify.cc                                 |  6 ++++++
 gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c |  4 ++--
 gcc/testsuite/g++.dg/warn/Wreturn-6.C                 | 16 ++++++++++++++++
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index b995b4f81a5..4fecd5616bd 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);
diff --git a/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c b/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c
index da4aef3a318..d06bc0db5ec 100644
--- a/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c
+++ b/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-39.c
@@ -37,8 +37,8 @@ fn2 (int n)
   switch (n)
     {
     case 0:
-      if (1)	  /* { dg-warning "statement may fall through" "" { target c++ } } */
-       	n++;	  /* { dg-warning "statement may fall through" "" { target c } } */
+      if (1)
+       	n++;	  /* { dg-warning "statement may fall through" } */
     case 1:	  /* { dg-message "here" } */
       return -1;
     }
diff --git a/gcc/testsuite/g++.dg/warn/Wreturn-6.C b/gcc/testsuite/g++.dg/warn/Wreturn-6.C
new file mode 100644
index 00000000000..85fef0e16df
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wreturn-6.C
@@ -0,0 +1,16 @@
+// PR c++/107310
+
+struct f
+{
+  ~f();
+};
+
+int foo(int t) {
+  f g;
+  switch (t) {
+  case 1:
+    return 1;
+  }
+  if (true)
+    throw 1;
+} // { dg-bogus "control reaches end" }