c++: Avoid some -Wreturn-type false positives with const{expr,eval} if [PR103991]

The changes done to genericize_if_stmt in order to improve
-Wunreachable-code* warning (which Richi didn't actually commit
for GCC 12) are I think fine for normal ifs, but for constexpr if
and consteval if we have two competing warnings.
The problem is that we replace the non-taken clause (then or else)
with void_node and keep the if (cond) { something } else {}
or if (cond) {} else { something }; in the IL.
This helps -Wunreachable-code*, if something can't fallthru but the
non-taken clause can, we don't warn about code after it because it
is still (in theory) reachable.
But if the non-taken branch can't fallthru, we can get false positive
-Wreturn-type warnings (which are enabled by default) if there is
nothing after the if and the taken branch can't fallthru either.

One possibility to fix this is revert at least temporarily
to the previous behavior for constexpr and consteval if, yes, we
can get false positive -Wunreachable-code* warnings but the warning
isn't present in GCC 12.
The patch below implements that for constexpr if which throws its
clauses very early (either during parsing or during instantiation),
and for consteval if it decides based on block_may_fallthru on the
non-taken (for constant evaluation only) clause - if the non-taken
branch may fallthru, it does what you did in genericize_if_stmt
for consteval if, if it can't fallthru, it uses the older way
of pretending there wasn't an if and just replacing it with the
taken clause.  There are some false positive risks with this though,
block_may_fallthru is optimistic and doesn't handle some statements
at all (like FOR_STMT, WHILE_STMT, DO_STMT - of course handling those
is quite hard).
For constexpr if (but perhaps for GCC 13?) we could try to
block_may_fallthru before we throw it away and remember it in some
flag on the IF_STMT, but am not sure how dangerous would it be to call
it on the discarded stmts.  Or if it is too dangerous e.g. just
remember whether the discarded block of consteval if wasn't present
or was empty, in that case assume fallthru, and otherwise assume
it can't fallthru (-Wunreachable-code possible false positives).

2022-01-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103991
	* cp-objcp-common.c (cxx_block_may_fallthru) <case IF_STMT>: For
	IF_STMT_CONSTEXPR_P with constant false or true condition only
	check if the taken clause may fall through.
	* cp-gimplify.c (genericize_if_stmt): For consteval if, revert
	to r12-5638^ behavior if then_ block can't fall through.  For
	constexpr if, revert to r12-5638^ behavior.

	* g++.dg/warn/Wreturn-type-13.C: New test.

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 8b097c4..b80010e 100644 (file)
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -166,8 +166,15 @@ 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 if (IF_STMT_CONSTEXPR_P (stmt))
+    stmt = integer_nonzerop (cond) ? then_ : else_;
   else
     stmt = build3 (COND_EXPR, void_type_node, cond, then_, else_);
   protected_set_expr_location_if_unset (stmt, locus);

diff --git a/gcc/cp/cp-objcp-common.c b/gcc/cp/cp-objcp-common.c
index fd6fe17..b31c9e3 100644 (file)
--- a/gcc/cp/cp-objcp-common.c
+++ b/gcc/cp/cp-objcp-common.c
@@ -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));

diff --git a/gcc/testsuite/g++.dg/warn/Wreturn-type-13.C b/gcc/testsuite/g++.dg/warn/Wreturn-type-13.C
new file mode 100644 (file)
index 0000000..fa2952b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wreturn-type-13.C
@@ -0,0 +1,35 @@
+// PR c++/103991
+// { dg-do compile { target c++17 } }
+
+struct S { ~S(); };
+int
+foo ()
+{
+  S s;
+  if constexpr (true)
+    return 0;
+  else
+    return 1;
+}                      // { dg-bogus "control reaches end of non-void function" }
+
+#if __cpp_if_consteval >= 202106L
+constexpr int
+bar ()
+{
+  S s;
+  if consteval
+    {
+      return 0;
+    }
+  else
+    {
+      return 1;
+    }
+}                      // { dg-bogus "control reaches end of non-void function" }
+
+int
+baz ()
+{
+  return bar ();
+}
+#endif