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* [gcc r14-9861] c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]
@ 2024-04-09 7:32 Jakub Jelinek
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From: Jakub Jelinek @ 2024-04-09 7:32 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:cfed80b9e4f562c99679739548df9369117dd791
commit r14-9861-gcfed80b9e4f562c99679739548df9369117dd791
Author: Jakub Jelinek <jakub@redhat.com>
Date: Tue Apr 9 09:31:42 2024 +0200
c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]
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.
2024-04-09 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.
Diff:
---
gcc/cp/semantics.cc | 10 +++-----
.../g++.dg/cpp2a/is-constant-evaluated15.C | 28 ++++++++++++++++++++++
2 files changed, 31 insertions(+), 7 deletions(-)
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index db0449b7786..a43ff6e2ab2 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -1126,6 +1126,7 @@ tree
finish_if_stmt_cond (tree orig_cond, tree if_stmt)
{
tree cond = maybe_convert_cond (orig_cond);
+ maybe_warn_for_constant_evaluated (cond, IF_STMT_CONSTEXPR_P (if_stmt));
if (IF_STMT_CONSTEXPR_P (if_stmt)
&& !type_dependent_expression_p (cond)
&& require_constant_expression (cond)
@@ -1134,16 +1135,11 @@ finish_if_stmt_cond (tree orig_cond, tree if_stmt)
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 ();
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C
new file mode 100644
index 00000000000..50a3cac6e07
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C
@@ -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> ();
+}
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