From: Jakub Jelinek <jakub@redhat.com>
To: Jason Merrill <jason@redhat.com>, Marek Polacek <polacek@redhat.com>
Cc: gcc-patches@gcc.gnu.org
Subject: [PATCH] c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]
Date: Fri, 5 Apr 2024 09:40:48 +0200 [thread overview]
Message-ID: <Zg+rAIt6bpyXb7QZ@tucnak> (raw)
Hi!
When looking at maybe_warn_for_constant_evaluated for the trivial
infinite loops patch, I've noticed that it can emit weird diagnostics
for if constexpr in templates, first warn that std::is_constant_evaluted()
always evaluates to false (because the function template is not constexpr)
and then during instantiation warn that std::is_constant_evaluted()
always evaluates to true (because it is used in if constexpr condition).
Now, only the latter is actually true, even when the if constexpr
is in a non-constexpr function, it will still always evaluate to true.
So, the following patch fixes it to call maybe_warn_for_constant_evaluated
always with IF_STMT_CONSTEXPR_P (if_stmt) as the second argument rather than
true if it is if constexpr with non-dependent condition etc.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2024-04-05 Jakub Jelinek <jakub@redhat.com>
PR c++/114580
* semantics.cc (finish_if_stmt_cond): Call
maybe_warn_for_constant_evaluated with IF_STMT_CONSTEXPR_P (if_stmt)
as the second argument, rather than true/false depending on if
it is if constexpr with non-dependent constant expression with
bool type.
* g++.dg/cpp2a/is-constant-evaluated15.C: New test.
--- gcc/cp/semantics.cc.jj 2024-04-03 09:58:33.407772541 +0200
+++ gcc/cp/semantics.cc 2024-04-04 12:11:36.203886572 +0200
@@ -1126,6 +1126,9 @@ tree
finish_if_stmt_cond (tree orig_cond, tree if_stmt)
{
tree cond = maybe_convert_cond (orig_cond);
+ maybe_warn_for_constant_evaluated (cond,
+ /*constexpr_if=*/
+ IF_STMT_CONSTEXPR_P (if_stmt));
if (IF_STMT_CONSTEXPR_P (if_stmt)
&& !type_dependent_expression_p (cond)
&& require_constant_expression (cond)
@@ -1134,16 +1137,11 @@ finish_if_stmt_cond (tree orig_cond, tre
converted to bool. */
&& TYPE_MAIN_VARIANT (TREE_TYPE (cond)) == boolean_type_node)
{
- maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/true);
cond = instantiate_non_dependent_expr (cond);
cond = cxx_constant_value (cond);
}
- else
- {
- maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/false);
- if (processing_template_decl)
- cond = orig_cond;
- }
+ else if (processing_template_decl)
+ cond = orig_cond;
finish_cond (&IF_COND (if_stmt), cond);
add_stmt (if_stmt);
THEN_CLAUSE (if_stmt) = push_stmt_list ();
--- gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C.jj 2024-04-04 12:23:36.706962932 +0200
+++ gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C 2024-04-04 12:22:29.915882859 +0200
@@ -0,0 +1,28 @@
+// PR c++/114580
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wtautological-compare" }
+
+namespace std {
+ constexpr inline bool
+ is_constant_evaluated () noexcept
+ {
+#if __cpp_if_consteval >= 202106L
+ if consteval { return true; } else { return false; }
+#else
+ return __builtin_is_constant_evaluated ();
+#endif
+ }
+}
+
+template <typename T>
+void foo ()
+{
+ if constexpr ((T) std::is_constant_evaluated ()) // { dg-warning "'std::is_constant_evaluated' always evaluates to true in 'if constexpr'" }
+ ; // { dg-bogus "'std::is_constant_evaluated' always evaluates to false in a non-'constexpr' function" }
+}
+
+void
+bar ()
+{
+ foo <bool> ();
+}
Jakub
next reply other threads:[~2024-04-05 7:41 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-05 7:40 Jakub Jelinek [this message]
2024-04-05 18:47 ` Marek Polacek
2024-04-08 22:55 ` Jason Merrill
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