[libc++] Verify forward_list self-merging is a no-op (#129985)

https://wg21.link/LWG3088 requires that `forward_list::merge()` is a no-op when passed
`*this`, which aligns with the behavior of `list::merge`. Although libc++'s implementation
of `forward_list::merge()` already meets this requirement, there were no tests to verify
this behavior. This patch adds the necessary tests to ensure that self-merging remains a
no-op and prevents any future regressions on this.

Closes #104942.

Author: winner245

libcxx/docs/Status/Cxx23Issues.csv

@@ -137,7 +137,7 @@
 "`LWG3593 <https://wg21.link/LWG3593>`__","Several iterators' ``base() const &`` and ``lazy_split_view::outer-iterator::value_type::end()`` missing ``noexcept``","2021-10 (Virtual)","","",""
 "`LWG3595 <https://wg21.link/LWG3595>`__","Exposition-only classes proxy and postfix-proxy for ``common_iterator`` should be fully ``constexpr``","2021-10 (Virtual)","|Complete|","14",""
 "","","","","",""
-"`LWG3088 <https://wg21.link/LWG3088>`__","``forward_list::merge`` behaviour unclear when passed ``*this``","2022-02 (Virtual)","","",""
+"`LWG3088 <https://wg21.link/LWG3088>`__","``forward_list::merge`` behaviour unclear when passed ``*this``","2022-02 (Virtual)","|Complete|","Yes",""
 "`LWG3471 <https://wg21.link/LWG3471>`__","``polymorphic_allocator::allocate`` does not satisfy ``Cpp17Allocator`` requirements","2022-02 (Virtual)","","",""
 "`LWG3525 <https://wg21.link/LWG3525>`__","``uses_allocator_construction_args`` fails to handle types convertible to ``pair``","2022-02 (Virtual)","","",""
 "`LWG3598 <https://wg21.link/LWG3598>`__","``system_category().default_error_condition(0)`` is underspecified","2022-02 (Virtual)","","",""

libcxx/test/std/containers/sequences/forwardlist/forwardlist.ops/merge_lvalue.pass.cpp

@@ -109,5 +109,12 @@ int main(int, char**) {
   }
 #endif
 
+  { // LWG3088: Make sure self-merging does nothing.
+    int a[] = {1, 2, 3, 4, 5};
+    std::forward_list<int> c(std::begin(a), std::end(a));
+    c.merge(c);
+    assert(c == std::forward_list<int>(std::begin(a), std::end(a)));
+  }
+
   return 0;
 }

libcxx/test/std/containers/sequences/forwardlist/forwardlist.ops/merge_lvalue_pred.pass.cpp

@@ -110,5 +110,12 @@ int main(int, char**) {
   }
 #endif
 
+  { // LWG3088: Make sure self-merging does nothing.
+    int a[] = {5, 4, 3, 2, 1};
+    std::forward_list<int> c(std::begin(a), std::end(a));
+    c.merge(c, std::greater<int>());
+    assert(c == std::forward_list<int>(std::begin(a), std::end(a)));
+  }
+
   return 0;
 }

libcxx/test/std/containers/sequences/forwardlist/forwardlist.ops/merge_rvalue.pass.cpp

@@ -102,5 +102,12 @@ int main(int, char**) {
     assert(c1 == c3);
   }
 
+  { // LWG3088: Make sure self-merging does nothing.
+    int a[] = {1, 2, 3, 4, 5};
+    std::forward_list<int> c(std::begin(a), std::end(a));
+    c.merge(std::move(c));
+    assert(c == std::forward_list<int>(std::begin(a), std::end(a)));
+  }
+
   return 0;
 }

libcxx/test/std/containers/sequences/forwardlist/forwardlist.ops/merge_rvalue_pred.pass.cpp

@@ -103,5 +103,12 @@ int main(int, char**) {
     assert(c1 == c3);
   }
 
+  { // LWG3088: Make sure self-merging does nothing.
+    int a[] = {5, 4, 3, 2, 1};
+    std::forward_list<int> c(std::begin(a), std::end(a));
+    c.merge(std::move(c), std::greater<int>());
+    assert(c == std::forward_list<int>(std::begin(a), std::end(a)));
+  }
+
   return 0;
 }