[0.73] C++ 20: Fix `Explicit template specialization cannot have a st…

…orage class` (#12328) (#13681)

* C++ 20: Fix `Explicit template specialization cannot have a storage class` (#12328)

* C++ 20: Fix "Explicit template specialization cannot have a storage class"

Fixes "[Explicit template specialization cannot have a storage class](https://stackoverflow.com/questions/29041775/explicit-template-specialization-cannot-have-a-storage-class-member-method-spe)" in C++ 20 by moving a function outside of a struct.

* Change files

* Update change file type

change/react-native-windows-39577653-1e51-4a5f-899b-b34e2da6d70f.json

@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "C++ 20: Fix \"Explicit template specialization cannot have a storage class\"",
+  "packageName": "react-native-windows",
+  "email": "ngerlem@fb.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative.Cxx/NativeModules.h

@@ -356,6 +356,21 @@ struct MethodSignatureMatchResult {
   bool IsSucceeded;
 };
 
+template <class TInputArg, class TOtherInputArg>
+constexpr bool MatchInputArg() noexcept {
+  return std::is_same_v<TInputArg, TOtherInputArg>;
+}
+
+template <>
+constexpr bool MatchInputArg<std::wstring, std::string>() noexcept {
+  return true;
+}
+
+template <>
+constexpr bool MatchInputArg<std::string, std::wstring>() noexcept {
+  return true;
+}
+
 template <class TResult, class TInputArgs, class TOutputCallbacks, class TOutputPromises>
 struct MethodSignature {
   using Result = TResult;
@@ -375,21 +390,6 @@ struct MethodSignature {
     }
   }
 
-  template <class TInputArg, class TOtherInputArg>
-  static constexpr bool MatchInputArg() noexcept {
-    return std::is_same_v<TInputArg, TOtherInputArg>;
-  }
-
-  template <>
-  static constexpr bool MatchInputArg<std::wstring, std::string>() noexcept {
-    return true;
-  }
-
-  template <>
-  static constexpr bool MatchInputArg<std::string, std::wstring>() noexcept {
-    return true;
-  }
-
   template <class TOtherInputArgs, size_t... I>
   static constexpr bool MatchInputArgs(std::index_sequence<I...>) noexcept {
     return (MatchInputArg<std::tuple_element_t<I, InputArgs>, std::tuple_element_t<I, TOtherInputArgs>>() && ...);