[PATCH] libstdc++: Fix flat_map::operator[] for const lvalue keys [PR120432]

[Jonathan Wakely]

On 28/05/25 09:00 -0400, Patrick Palka wrote:
>On Wed, 28 May 2025, Tomasz Kaminski wrote:
>
>>
>>
>> On Tue, May 27, 2025 at 7:08 PM Patrick Palka <ppalka@redhat.com> wrote:
>>       Tested on x86_64-pc-linux-gnu, does this look OK for trunk/15?
>>
>>       The 'volatile' issue from that PR Will be fixed in a separate patch as
>>       operator[] isn't the only operation that's affected.
>>
>>       -- >8 --
>>
>>       The const lvalue operator[] overload wasn't properly forwarding the key
>>       type to the generic overload.
>>
>>               PR libstdc++/120432
>>
>>       libstdc++-v3/ChangeLog:
>>
>>               * include/std/flat_map (_Flat_map_base::operator[]): Correct
>>               forwarding from the const lvalue key overload.
>>               * testsuite/23_containers/flat_map/1.cc (test08): New test.
>>               * testsuite/23_containers/flat_multimap/1.cc (test08): New test.
>>       ---
>>        libstdc++-v3/include/std/flat_map                      |  2 +-
>>        libstdc++-v3/testsuite/23_containers/flat_map/1.cc     | 10 ++++++++++
>>        .../testsuite/23_containers/flat_multimap/1.cc         | 10 ++++++++++
>>        3 files changed, 21 insertions(+), 1 deletion(-)
>>
>>       diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
>>       index 6593988d213c..4d9ced1e8191 100644
>>       --- a/libstdc++-v3/include/std/flat_map
>>       +++ b/libstdc++-v3/include/std/flat_map
>>       @@ -1142,7 +1142,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>              // element access
>>              mapped_type&
>>              operator[](const key_type& __x)
>>       -      { return operator[]<const key_type>(__x); }
>>       +      { return operator[]<const key_type&>(__x); }
>>
>> Given that the operator[] that we are forading to is implemented as:
>>         { return try_emplace(std::forward<_Key2>(__x)).first->second; }
>> I would just call try_emplace directly:
>
>Good point, the implementation is a simple one-liner either way, and it
>addresses the volatile key issue.  Like so?

OK for trunk (and it applies to gcc-15 too, right? approved there too)

>-- >8 --
>
>Subject: [PATCH] libstdc++: Fix flat_map::operator[] for const lvalue keys
> [PR120432]
>
>The const lvalue operator[] overload wasn't properly forwarding the key
>type to the generic overload, causing a hard error for const keys.
>
>This patch fixes this by making the non-template overloads call
>try_emplace directly instead, which means we can remove the non-standard
>same_as constraint on the generic overload.
>
>	PR libstdc++/120432
>
>libstdc++-v3/ChangeLog:
>
>	* include/std/flat_map (flat_map::operator[]): Make the
>	non-template overloads call try_emplace directly.  Remove
>	non-standard same_as constraint on the template overload.
>	* testsuite/23_containers/flat_map/1.cc (test08): New test.
>---
> libstdc++-v3/include/std/flat_map                  |  6 +++---
> libstdc++-v3/testsuite/23_containers/flat_map/1.cc | 10 ++++++++++
> 2 files changed, 13 insertions(+), 3 deletions(-)
>
>diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
>index 6593988d213c..5f9a2eda1939 100644
>--- a/libstdc++-v3/include/std/flat_map
>+++ b/libstdc++-v3/include/std/flat_map
>@@ -1142,14 +1142,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       // element access
>       mapped_type&
>       operator[](const key_type& __x)
>-      { return operator[]<const key_type>(__x); }
>+      { return try_emplace(__x).first->second; }
>
>       mapped_type&
>       operator[](key_type&& __x)
>-      { return operator[]<key_type>(std::move(__x)); }
>+      { return try_emplace(std::move(__x)).first->second; }
>
>       template<typename _Key2>
>-	requires same_as<remove_cvref_t<_Key2>, _Key> || __transparent_comparator<_Compare>
>+	requires __transparent_comparator<_Compare>
> 	mapped_type&
> 	operator[](_Key2&& __x)
> 	{ return try_emplace(std::forward<_Key2>(__x)).first->second; }
>diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
>index a9690208b09f..2af516410279 100644
>--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
>+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
>@@ -253,6 +253,15 @@ test07()
>   VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4}}) );
> }
>
>+void
>+test08()
>+{
>+  // PR libstdc++/120432 - flat_map operator[] is broken for const lvalue keys
>+  std::flat_map<int, int> m;
>+  const int k = 42;
>+  m[k] = 0;
>+}
>+
> int
> main()
> {
>@@ -266,4 +275,5 @@ main()
>   test05();
>   test06();
>   test07();
>+  test08();
> }
>-- 
>2.49.0.654.g845c48a16a