Add event-specific diagnostic to fix incorrect 'required' suggestion in CS8618 for events

Signed-off-by: lakshay122007 <[email protected]>

Author: lakshay122007

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5639,6 +5639,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_UninitializedNonNullableField" xml:space="preserve">
     <value>Non-nullable {0} '{1}' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the {0} as nullable.</value>
   </data>
+  <data name="ERR_NonNullableEventNotInitialized" xml:space="preserve">
+    <value>Non-nullable event '{0}' must contain a non-null value when exiting constructor. Consider declaring the event as nullable or initializing it.</value>
+  </data>
   <data name="WRN_UninitializedNonNullableField_Title" xml:space="preserve">
     <value>Non-nullable field must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring as nullable.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1741,6 +1741,8 @@ internal enum ErrorCode
 
         ERR_DuplicateNullSuppression = 8715,
         ERR_DefaultLiteralNoTargetType = 8716,
+        ERR_NonNullableEventNotInitialized = 8717,
+
 
         ERR_ReAbstractionInNoPIAType = 8750,
 

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -822,9 +822,21 @@ void checkMemberStateOnConstructorExit(MethodSymbol constructor, Symbol member,
                     : NullableFlowState.MaybeNull;
                 if (memberState >= badState) // is 'memberState' as bad as or worse than 'badState'?
                 {
-                    var errorCode = usesFieldKeyword ? ErrorCode.WRN_UninitializedNonNullableBackingField : ErrorCode.WRN_UninitializedNonNullableField;
-                    var info = new CSDiagnosticInfo(errorCode, new object[] { symbol.Kind.Localize(), symbol.Name }, ImmutableArray<Symbol>.Empty, additionalLocations: symbol.Locations);
-                    Diagnostics.Add(info, exitLocation ?? symbol.GetFirstLocationOrNone());
+                    var errorCode =
+                        symbol.Kind == SymbolKind.Event
+                            ? ErrorCode.ERR_NonNullableEventNotInitialized
+                            : (usesFieldKeyword
+                                ? ErrorCode.WRN_UninitializedNonNullableBackingField
+                                : ErrorCode.WRN_UninitializedNonNullableField);
+
+                var info = new CSDiagnosticInfo(
+                    errorCode,
+                    new object[] { symbol.Kind.Localize(), symbol.Name },
+                    ImmutableArray<Symbol>.Empty,
+                    additionalLocations);
+
+                Diagnostics.Add(info, exitLocation ?? symbol.GetFirstLocationOrNone());
+
                 }
             }