From 6834793243091a478e7b5ee6328d544daed135b3 Mon Sep 17 00:00:00 2001
From: Marshall Clow <mclow3@bloomberg.net>
Date: Thu, 31 Oct 2024 13:48:45 -0700
Subject: [PATCH] Add more C++20 type traits {DRQS 177082446} (#5060)

---
 groups/bsl/bsl+bslhdrs/bsl_type_traits.h      | 11 +++++
 .../bslim/bslim_bslstandardheadertest.t.cpp   | 49 ++++++++++++++++++-
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/groups/bsl/bsl+bslhdrs/bsl_type_traits.h b/groups/bsl/bsl+bslhdrs/bsl_type_traits.h
index 3a8d1869f7..59c3c526ca 100644
--- a/groups/bsl/bsl+bslhdrs/bsl_type_traits.h
+++ b/groups/bsl/bsl+bslhdrs/bsl_type_traits.h
@@ -508,9 +508,15 @@ using std::is_nothrow_convertible;
 using std::is_nothrow_convertible_v;
 
 // 20.15.7.6, other transformations
+using std::remove_cvref;
+using std::remove_cvref_t;
 using std::common_reference;
 using std::common_reference_t;
 using std::basic_common_reference;
+using std::unwrap_reference;
+using std::unwrap_reference_t;
+using std::unwrap_ref_decay;
+using std::unwrap_ref_decay_t;
 #endif
 
 #ifdef BSLS_LIBRARYFEATURES_HAS_CPP20_IS_LAYOUT_COMPATIBLE
@@ -533,6 +539,11 @@ using std::is_pointer_interconvertible_with_class;
 using std::is_corresponding_member;
 #endif
 
+#ifdef BSLS_LIBRARYFEATURES_HAS_CPP20_BASELINE_LIBRARY
+// 20.15.10, constant evaluation context
+using std::is_constant_evaluated;
+#endif
+
 #if 0
     // These traits are provided by BDE, and have additional members for
     // Bloomberg legacy code still using the pre-standard interface.
diff --git a/groups/bsl/bslim/bslim_bslstandardheadertest.t.cpp b/groups/bsl/bslim/bslim_bslstandardheadertest.t.cpp
index 74965c8368..cd1f1000fc 100644
--- a/groups/bsl/bslim/bslim_bslstandardheadertest.t.cpp
+++ b/groups/bsl/bslim/bslim_bslstandardheadertest.t.cpp
@@ -1005,23 +1005,33 @@ int main(int argc, char *argv[])
         //    - `bsl::is_bounded_array`
         //    - `bsl::is_unbounded_array`
         //    - `bsl::is_nothrow_convertible`
+        //    - `bsl::is_constant_evaluated`
         //    - `bsl::is_layout_compatible`
         //    - `bsl::is_pointer_interconvertible_base_of`
         //    - `bsl::common_reference`
         //    - 'bsl::basic_common_reference
         //    - `bsl::is_pointer_interconvertible_with_class`
         //    - `bsl::is_corresponding_member`
+        //    - `bsl::remove_cvref`
+        //    - `bsl::unwrap_reference`
+        //    - `bsl::unwrap_ref_decay`
         //
         // 2. When the trait has a `*_v` form, that trait has the same value
         //   as the  non-`*_v` form.
         //
-        // 3. The feature test macros defined in `<version>` for the imported
+        // 3. When the trait has a `*_t` form, that trait has the same value
+        //   as the  non-`*_t` form.
+        //
+        // 4. The feature test macros defined in `<version>` for the imported
         //    features are available and have appropriate values.
         //
         // Plan:
         // 1. Verify that:
         //     o `__cpp_lib_is_layout_compatible        >= 201907L`
         //     o `__cpp_lib_is_pointer_interconvertible >= 201907L`
+        //     o `__cpp_lib_unwrap_ref                  >= 201811L`
+        //     o `__cpp_lib_remove_cvref                >= 201711L`
+        //     o `__cpp_lib_is_constant_evaluated       >= 201811L`
         //
         // 2. Verify that `__cpp_lib_is_layout_compatible` also also determines
         //    the availability of
@@ -1087,6 +1097,43 @@ int main(int argc, char *argv[])
             bool result = bsl::is_same<TU, UT>:: value;
             ASSERT(false == result);
         }
+
+        if (veryVerbose) {
+            Q('bsl::remove_cvref');
+        }
+        {
+            using T   = const int &;
+            using RET = int;
+
+            ASSERT(__cpp_lib_remove_cvref >= 201711L)
+            ASSERT((bsl::is_same_v<bsl::remove_cvref<T>::type, RET>));
+            ASSERT((bsl::is_same_v<bsl::remove_cvref_t<T>,     RET>));
+        }
+
+        if (veryVerbose) {
+            Q('bsl::unwrap_reference');
+            Q('bsl::unwrap_ref_decay');
+        }
+        {
+            using T    = bsl::reference_wrapper<int>;
+            using RET1 = int &;
+            using RET2 = int &;
+
+            ASSERT(__cpp_lib_unwrap_ref >= 201811L);
+            ASSERT((bsl::is_same_v<bsl::unwrap_reference<T>::type, RET1>));
+            ASSERT((bsl::is_same_v<bsl::unwrap_reference_t<T>,     RET1>));
+
+            ASSERT((bsl::is_same_v<bsl::unwrap_ref_decay<T>::type, RET2>));
+            ASSERT((bsl::is_same_v<bsl::unwrap_ref_decay_t<T>,     RET2>));
+        }
+        if (veryVerbose) {
+            Q('bsl::is_constant_evaluated');
+        }
+        {
+            ASSERT(__cpp_lib_is_constant_evaluated >= 201811L);
+            const bool b = bsl::is_constant_evaluated();
+            (void) b;
+        }
 #else
         if (veryVerbose) {
             puts("SKIPPED: "