https://gcc.gnu.org/g:bc8f5424977b74e107543b34af00768cdbb3a3cf
commit r16-1653-gbc8f5424977b74e107543b34af00768cdbb3a3cf
Author: Patrick Palka <ppa...@redhat.com>
Date: Tue Jun 24 09:33:25 2025 -0400
libstdc++: Unnecessary type completion in __is_complete_or_unbounded
[PR120717]
When checking __is_complete_or_unbounded on a reference to incomplete
type, we overeagerly try to instantiate/complete the referenced type
which besides being unnecessary may also produce an unexpected
-Wsfinae-incomplete warning (added in r16-1527) if the referenced type
is later defined.
This patch fixes this by effectively restricting the sizeof check to
object (except unknown-bound array) types. In passing simplify the
implementation by using is_object instead of is_function/reference/void
and introducing a __maybe_complete_object_type helper.
PR libstdc++/120717
libstdc++-v3/ChangeLog:
* include/std/type_traits (__maybe_complete_object_type): New
helper trait, factored out from ...
(__is_complete_or_unbounded): ... here. Only check sizeof on a
__maybe_complete_object_type type. Fix formatting.
* testsuite/20_util/is_complete_or_unbounded/120717.cc: New test.
Reviewed-by: Tomasz Kamiński <tkami...@redhat.com>
Co-authored-by: Jonathan Wakely <jwak...@redhat.com>
Reviewed-by: Jonathan Wakely <jwak...@redhat.com>
Diff:
---
libstdc++-v3/include/std/type_traits | 39 ++++++++++++----------
.../20_util/is_complete_or_unbounded/120717.cc | 20 +++++++++++
2 files changed, 42 insertions(+), 17 deletions(-)
diff --git a/libstdc++-v3/include/std/type_traits
b/libstdc++-v3/include/std/type_traits
index abff9f880001..055411195f17 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -280,11 +280,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// Forward declarations
template<typename>
- struct is_reference;
- template<typename>
- struct is_function;
- template<typename>
- struct is_void;
+ struct is_object;
template<typename>
struct remove_cv;
template<typename>
@@ -294,21 +290,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename>
struct __is_array_unknown_bounds;
+ // An object type which is not an unbounded array.
+ // It might still be an incomplete type, but if this is false_type
+ // then we can be certain it's not a complete object type.
+ template<typename _Tp>
+ using __maybe_complete_object_type
+ = __and_<is_object<_Tp>, __not_<__is_array_unknown_bounds<_Tp>>>;
+
// Helper functions that return false_type for incomplete classes,
// incomplete unions and arrays of known bound from those.
- template <typename _Tp, size_t = sizeof(_Tp)>
- constexpr true_type __is_complete_or_unbounded(__type_identity<_Tp>)
- { return {}; }
-
- template <typename _TypeIdentity,
- typename _NestedType = typename _TypeIdentity::type>
- constexpr typename __or_<
- is_reference<_NestedType>,
- is_function<_NestedType>,
- is_void<_NestedType>,
- __is_array_unknown_bounds<_NestedType>
- >::type __is_complete_or_unbounded(_TypeIdentity)
+ // More specialized overload for complete object types (returning true_type).
+ template<typename _Tp,
+ typename = __enable_if_t<__maybe_complete_object_type<_Tp>::value>,
+ size_t = sizeof(_Tp)>
+ constexpr true_type
+ __is_complete_or_unbounded(__type_identity<_Tp>)
+ { return {}; };
+
+ // Less specialized overload for reference and unknown-bound array types
+ // (returning true_type), and incomplete types (returning false_type).
+ template<typename _TypeIdentity,
+ typename _NestedType = typename _TypeIdentity::type>
+ constexpr typename __not_<__maybe_complete_object_type<_NestedType>>::type
+ __is_complete_or_unbounded(_TypeIdentity)
{ return {}; }
// __remove_cv_t (std::remove_cv_t for C++11).
diff --git a/libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
b/libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
new file mode 100644
index 000000000000..4c07683d494e
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/is_complete_or_unbounded/120717.cc
@@ -0,0 +1,20 @@
+// PR libstdc++/120717
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wsfinae-incomplete" }
+
+#include <type_traits>
+
+// Verify __is_complete_or_unbounded doesn't try to instantiate the underlying
+// type of a reference or array of unknown bound.
+template<class T> struct A { static_assert(false, "do not instantiate"); };
+static_assert(std::__is_complete_or_unbounded(std::__type_identity<A<int>&>{}),
"");
+static_assert(std::__is_complete_or_unbounded(std::__type_identity<A<int>&&>{}),
"");
+static_assert(std::__is_complete_or_unbounded(std::__type_identity<A<int>[]>{}),
"");
+
+// Verify __is_complete_or_unbounded doesn't produce unexpected
+// -Wsfinae-incomplete warnings.
+struct B;
+static_assert(std::__is_complete_or_unbounded(std::__type_identity<B&>{}), "");
+static_assert(std::__is_complete_or_unbounded(std::__type_identity<B&&>{}),
"");
+static_assert(std::__is_complete_or_unbounded(std::__type_identity<B[]>{}),
"");
+struct B { }; // { dg-bogus "-Wsfinae-incomplete" }
