From 91ed3248ce26aaaee4d7471aa4edbc07b3f1a90e Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Fri, 6 Jun 2025 09:34:17 -0400
Subject: [PATCH] libstdc++: Fix flat_map::operator[] for const lvalue keys
 [PR120432]
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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 <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
---
 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 4bd4963c2ad..de006ad1c53 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 1b593135f22..01278d7dc33 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
@@ -254,6 +254,15 @@ 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()
 {
@@ -267,4 +276,5 @@ main()
   test05();
   test06();
   test07();
+  test08();
 }
-- 
2.47.2