[gcc r15-9789] libstdc++: Fix flat_map::operator[] for const lvalue keys [PR120432]

[Patrick Palka via Gcc-cvs]

https://gcc.gnu.org/g:e75e42f315e1e8bb4befee8ed242bd241c182091

commit r15-9789-ge75e42f315e1e8bb4befee8ed242bd241c182091
Author: Patrick Palka <ppa...@redhat.com>
Date:   Fri Jun 6 09:34:17 2025 -0400

    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.
    
    Rather than correcting the forwarded type this patch just makes the
    non-template overloads call try_emplace directly instead.  That way we
    can remove the non-standard same_as constraint on the generic overload
    and match the spec more closely.
    
            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.
    
    Reviewed-by: Tomasz Kamiński <tkami...@redhat.com>
    Reviewed-by: Jonathan Wakely <jwak...@redhat.com>
    (cherry picked from commit 91ed3248ce26aaaee4d7471aa4edbc07b3f1a90e)

Diff:
---
 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 4bd4963c2ad7..de006ad1c533 100644
--- a/libstdc++-v3/include/std/flat_map
+++ b/libstdc++-v3/include/std/flat_map
@@ -1148,14 +1148,14 @@ _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 9d99796878e1..d7f27a05a878 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
@@ -252,6 +252,15 @@ 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()
 {
@@ -265,4 +274,5 @@ main()
   test05();
   test06();
   test07();
+  test08();
 }