Update diagnostic usage for langversion in C# (#17231)

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -1232,7 +1232,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to Invalid option &apos;{0}&apos; for /langversion; must be ISO-1, ISO-2, Default or an integer in range 1 to 6..
+        ///   Looks up a localized string similar to Invalid option &apos;{0}&apos; for /langversion; must be ISO-1, ISO-2, Default or an integer in range 1 to 7..
         /// </summary>
         internal static string ERR_BadCompatMode {
             get {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -2687,7 +2687,7 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
     <value>This warning occurs if the assembly attributes AssemblyKeyFileAttribute or AssemblyKeyNameAttribute found in source conflict with the /keyfile or /keycontainer command line option or key file name or key container specified in the Project Properties.</value>
   </data>
   <data name="ERR_BadCompatMode" xml:space="preserve">
-    <value>Invalid option '{0}' for /langversion; must be ISO-1, ISO-2, Default or an integer in range 1 to 6.</value>
+    <value>Invalid option '{0}' for /langversion; must be ISO-1, ISO-2, Default or an integer in range 1 to 7.</value>
   </data>
   <data name="ERR_DelegateOnConditional" xml:space="preserve">
     <value>Cannot create delegate with '{0}' because it or a method it overrides has a Conditional attribute</value>

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -1322,6 +1322,14 @@ public void LangVersion()
             Assert.Equal(defaultVersion, parsedArgs.ParseOptions.LanguageVersion);
         }
 
+        [Fact]
+        public void RememberToUpdateDiagnosticsWhenUpdatingLangVersion()
+        {
+            // When new language versions are added, this test will fail. Remember to update the diagnostics message (ERR_BadCompatMode).
+            Assert.Equal(LanguageVersion.CSharp7, LanguageVersion.Latest.MapSpecifiedToEffectiveVersion());
+            Assert.Equal(LanguageVersion.CSharp7, LanguageVersion.Default.MapSpecifiedToEffectiveVersion());
+        }
+
         [Fact]
         [WorkItem(546961, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/546961")]
         public void Define()