No longer issue pedantic warning about pre-c++2b compat

We were accidentally issuing "overloaded 'operator[]' with more than
one parameter is a C++2b extension" with -pedantic because it was an
ExtWarn diagnostic rather than a Warning. This corrects the diagnostic
category and adds some test coverage.

Fixes #61582

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -778,6 +778,10 @@ Bug Fixes to Attribute Support
 
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
+- No longer issue a pre-C++2b compatibility warning in ``-pedantic`` mode
+  regading overloaded `operator[]` with more than one parmeter. (`#61582
+  <https://github.com/llvm/llvm-project/issues/61582>`_)
+
 - Address the thread identification problems in coroutines.
   (`#47177 <https://github.com/llvm/llvm-project/issues/47177>`_,
   `#47179 <https://github.com/llvm/llvm-project/issues/47179>`_)

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9138,8 +9138,9 @@ def err_operator_overload_static : Error<
 def err_operator_overload_default_arg : Error<
   "parameter of overloaded %0 cannot have a default argument">;
 
-def ext_subscript_overload : ExtWarn<
-  "overloaded %0 with %select{no|a defaulted|more than one}1 parameter is a C++2b extension">, InGroup<CXXPre2bCompat>, DefaultIgnore;
+def ext_subscript_overload : Warning<
+  "overloaded %0 with %select{no|a defaulted|more than one}1 parameter is a "
+  "C++2b extension">, InGroup<CXXPre2bCompat>, DefaultIgnore;
 def error_subscript_overload : Error<
   "overloaded %0 cannot have %select{no|a defaulted|more than one}1 parameter before C++2b">;
 

clang/test/SemaCXX/cxx2b-overloaded-operator-pedantic.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -verify -std=c++2b -pedantic %s
+// RUN: %clang_cc1 -verify=compat -std=c++2b -Wpre-c++2b-compat %s
+
+// expected-no-diagnostics
+
+struct GH61582 {
+  // We accidentally would issue this diagnostic in pedantic mode; show that we
+  // only issue it when enabling the compat warnings now.
+  void operator[](int, int); // compat-warning {{overloaded 'operator[]' with more than one parameter is a C++2b extension}}
+};
+