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>

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;
       });
     }

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;
       });
     }

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()
 {

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()
 {