libstdc++: Make std::erase for linked lists convert to bool

LWG 4135 (approved in Wrocław, November 2024) fixes the lambda
expressions used by std::erase for std::list and std::forward_list.
Previously they attempted to copy something that isn't required to be
copyable. Instead they should convert it to bool right away.

The issue resolution also changes the lambda's parameter to be const, so
that it can't modify the elements while comparing them.

libstdc++-v3/ChangeLog:

	* include/std/forward_list (erase): Change lambda to have
	explicit return type and const parameter type.
	* include/std/list (erase): Likewise.
	* testsuite/23_containers/forward_list/erasure.cc: Check lambda
	is correct.
	* testsuite/23_containers/list/erasure.cc: Likewise.

Reviewed-by: Patrick Palka <ppalka@redhat.com>

4 files changed, 50 insertions, 4 deletions

diff --git a/libstdc++-v3/include/std/forward_list b/libstdc++-v3/include/std/forward_list
index 6c5ad11..166fdb0 100644
--- a/libstdc++-v3/include/std/forward_list
+++ b/libstdc++-v3/include/std/forward_list
@@ -83,8 +83,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline typename forward_list<_Tp, _Alloc>::size_type
     erase(forward_list<_Tp, _Alloc>& __cont, const _Up& __value)
     {
-      using __elem_type = typename forward_list<_Tp, _Alloc>::value_type;
-      return std::erase_if(__cont, [&](__elem_type& __elem) {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 4135. helper lambda of std::erase for list should specify return type
+      return std::erase_if(__cont, [&](const auto& __elem) -> bool {
 	  return __elem == __value;
       });
     }
diff --git a/libstdc++-v3/include/std/list b/libstdc++-v3/include/std/list
index 51f1bc0..170499d 100644
--- a/libstdc++-v3/include/std/list
+++ b/libstdc++-v3/include/std/list
@@ -107,8 +107,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline typename list<_Tp, _Alloc>::size_type
     erase(list<_Tp, _Alloc>& __cont, const _Up& __value)
     {
-      using __elem_type = typename list<_Tp, _Alloc>::value_type;
-      return std::erase_if(__cont, [&](__elem_type& __elem) {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 4135. helper lambda of std::erase for list should specify return type
+      return std::erase_if(__cont, [&](const auto& __elem) -> bool {
 	  return __elem == __value;
       });
     }
diff --git a/libstdc++-v3/testsuite/23_containers/forward_list/erasure.cc b/libstdc++-v3/testsuite/23_containers/forward_list/erasure.cc
index 37e5020..1ec11de 100644
--- a/libstdc++-v3/testsuite/23_containers/forward_list/erasure.cc
+++ b/libstdc++-v3/testsuite/23_containers/forward_list/erasure.cc
@@ -53,6 +53,28 @@ test02()
   VERIFY( num == 0 );
 }
 
+// LWG 4135.
+// The helper lambda of std::erase for list should specify return type as bool
+void
+test_lwg4135()
+{
+  struct Bool {
+    Bool() = default;
+    Bool(const Bool&) = delete;
+    operator bool() const { return false; }
+  };
+
+  static Bool b;
+
+  struct Int {
+    Bool& operator==(Int) const { return b; }
+    void operator==(Int) = delete;
+  };
+
+  std::forward_list<Int> l;
+  std::erase(l, Int{});
+}
+
 int
 main()
 {
diff --git a/libstdc++-v3/testsuite/23_containers/list/erasure.cc b/libstdc++-v3/testsuite/23_containers/list/erasure.cc
index 1a33eca..9f4f853 100644
--- a/libstdc++-v3/testsuite/23_containers/list/erasure.cc
+++ b/libstdc++-v3/testsuite/23_containers/list/erasure.cc
@@ -52,6 +52,28 @@ test02()
   VERIFY( num == 0 );
 }
 
+// LWG 4135.
+// The helper lambda of std::erase for list should specify return type as bool
+void
+test_lwg4135()
+{
+  struct Bool {
+    Bool() = default;
+    Bool(const Bool&) = delete;
+    operator bool() const { return false; }
+  };
+
+  static Bool b;
+
+  struct Int {
+    Bool& operator==(Int) const { return b; }
+    void operator==(Int) = delete;
+  };
+
+  std::list<Int> l;
+  std::erase(l, Int{});
+}
+
 int
 main()
 {