From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 6 Mar 2025 21:18:21 +0000 (+0000)
Subject: libstdc++: Make std::erase for linked lists convert to bool
X-Git-Tag: releases/gcc-14.3.0~313
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d22bd12554d5c4214da7072aff4d0babaf82b412;p=thirdparty%2Fgcc.git

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>
(cherry picked from commit e6e7b477bbdbfb3fee6b44087a59f94fd1e2c7a3)
---

diff --git a/libstdc++-v3/include/std/forward_list b/libstdc++-v3/include/std/forward_list
index dfd7d48d121..91dd6745bec 100644
--- a/libstdc++-v3/include/std/forward_list
+++ b/libstdc++-v3/include/std/forward_list
@@ -79,8 +79,9 @@ _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 ff632fc1ab2..d6126e493fa 100644
--- a/libstdc++-v3/include/std/list
+++ b/libstdc++-v3/include/std/list
@@ -103,8 +103,9 @@ _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 271cb3a577e..c5a7013f6b6 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 c5d041526dc..0df3a8772bd 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()
 {