[clang] [clang] Handle templated operators with reversed arguments (PR #69595)

[Ilya Biryukov via cfe-commits]

================
@@ -37,6 +37,25 @@ These changes are ones which we think may surprise users when upgrading to
 Clang |release| because of the opportunity they pose for disruption to existing
 code bases.
 
+- Fix a bug in reversed argument for templated operators.
+  This breaks code in C++20 which was previously accepted in C++17. Eg:
+
+  .. code-block:: cpp
+
+    struct P {};
+    template<class S> bool operator==(const P&, const S&);
+
+    struct A : public P {};
+    struct B : public P {};
+
+    bool ambiguous(A a, B b) { return a == b; } // This equality is now ambiguous in C++20.
+
+    template<class S> bool operator!=(const P&, const S&);
+    bool fine(A a, B b) { return a == b; } // Ok. Found a matching operator!=.
+
+  To reduce widespread breakages, as an extension, clang would accept this with a
+  ``-Wambiguous-reversed-operator`` warning.
----------------
ilya-biryukov wrote:

NIT: it reads as if it is a new warning, I suggest saying "Clang accepts this code with an existing warning -W...."

https://github.com/llvm/llvm-project/pull/69595