Parse unchecked gracefully in operators

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7094,4 +7094,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureRelaxedShiftOperator" xml:space="preserve">
     <value>relaxed shift operator</value>
   </data>
+  <data name="ERR_MisplacedUnchecked" xml:space="preserve">
+    <value>Unexpected keyword 'unchecked'</value>
+  </data>
 </root>

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

@@ -2068,6 +2068,7 @@ internal enum ErrorCode
 
         ERR_CannotBeConvertedToUTF8 = 9026,
         ERR_ExpressionTreeContainsUTF8StringLiterals = 9027,
+        ERR_MisplacedUnchecked = 9028,
 
         #endregion
 

src/Compilers/CSharp/Portable/Parser/DocumentationCommentParser.cs

@@ -988,7 +988,7 @@ private OperatorMemberCrefSyntax ParseOperatorMemberCref()
         {
             Debug.Assert(CurrentToken.Kind == SyntaxKind.OperatorKeyword);
             SyntaxToken operatorKeyword = EatToken();
-            SyntaxToken checkedKeyword = TryEatCheckedKeyword(isConversion: false);
+            SyntaxToken checkedKeyword = TryEatCheckedKeyword(isConversion: false, ref operatorKeyword);
 
             SyntaxToken operatorToken;
 
@@ -1113,9 +1113,9 @@ CurrentToken.Kind is (SyntaxKind.GreaterThanToken or SyntaxKind.GreaterThanEqual
             return SyntaxFactory.OperatorMemberCref(operatorKeyword, checkedKeyword, operatorToken, parameters);
         }
 
-        private SyntaxToken TryEatCheckedKeyword(bool isConversion)
+        private SyntaxToken TryEatCheckedKeyword(bool isConversion, ref SyntaxToken operatorKeyword)
         {
-            SyntaxToken checkedKeyword = TryEatToken(SyntaxKind.CheckedKeyword); // https://github.com/dotnet/roslyn/issues/60394 : consider gracefully recovering from erroneous use of 'unchecked' at this location 
+            SyntaxToken checkedKeyword = tryEatCheckedOrHandleUnchecked(ref operatorKeyword);
 
             if (checkedKeyword is not null &&
                 (isConversion || SyntaxFacts.IsAnyOverloadableOperator(CurrentToken.Kind)))
@@ -1124,6 +1124,19 @@ private SyntaxToken TryEatCheckedKeyword(bool isConversion)
             }
 
             return checkedKeyword;
+
+            SyntaxToken tryEatCheckedOrHandleUnchecked(ref SyntaxToken operatorKeyword)
+            {
+                if (CurrentToken.Kind == SyntaxKind.UncheckedKeyword)
+                {
+                    // if we encounter `operator unchecked`, we place the `unchecked` as skipped trivia on `operator`
+                    var misplacedToken = this.AddError(this.EatToken(), ErrorCode.ERR_MisplacedUnchecked);
+                    operatorKeyword = AddTrailingSkippedSyntax(operatorKeyword, misplacedToken);
+                    return null;
+                }
+
+                return TryEatToken(SyntaxKind.CheckedKeyword);
+            }
         }
 
         /// <summary>
@@ -1136,7 +1149,7 @@ private ConversionOperatorMemberCrefSyntax ParseConversionOperatorMemberCref()
             SyntaxToken implicitOrExplicit = EatToken();
 
             SyntaxToken operatorKeyword = EatToken(SyntaxKind.OperatorKeyword);
-            SyntaxToken checkedKeyword = TryEatCheckedKeyword(isConversion: true);
+            SyntaxToken checkedKeyword = TryEatCheckedKeyword(isConversion: true, ref operatorKeyword);
 
             TypeSyntax type = ParseCrefType(typeArgumentsMustBeIdentifiers: false);
 

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -3348,7 +3348,7 @@ private ConversionOperatorDeclarationSyntax TryParseConversionOperatorDeclaratio
                     {
                         possibleConversion = false;
                     }
-                    else if (this.PeekToken(1).Kind == SyntaxKind.CheckedKeyword) // https://github.com/dotnet/roslyn/issues/60394 : consider gracefully recovering from erroneous use of 'unchecked' at this location 
+                    else if (this.PeekToken(1).Kind is SyntaxKind.CheckedKeyword or SyntaxKind.UncheckedKeyword)
                     {
                         possibleConversion = !SyntaxFacts.IsAnyOverloadableOperator(this.PeekToken(2).Kind);
                     }
@@ -3392,7 +3392,7 @@ private ConversionOperatorDeclarationSyntax TryParseConversionOperatorDeclaratio
                 }
 
                 opKeyword = this.EatToken(SyntaxKind.OperatorKeyword);
-                var checkedKeyword = this.TryEatToken(SyntaxKind.CheckedKeyword); // https://github.com/dotnet/roslyn/issues/60394 : consider gracefully recovering from erroneous use of 'unchecked' at this location 
+                var checkedKeyword = TryEatCheckedOrHandleUnchecked(ref opKeyword);
 
                 this.Release(ref point);
                 point = GetResetPoint();
@@ -3510,14 +3510,27 @@ ExplicitInterfaceSpecifierSyntax tryParseExplicitInterfaceSpecifier()
             }
         }
 
+        private SyntaxToken TryEatCheckedOrHandleUnchecked(ref SyntaxToken operatorKeyword)
+        {
+            if (CurrentToken.Kind == SyntaxKind.UncheckedKeyword)
+            {
+                // if we encounter `operator unchecked`, we place the `unchecked` as skipped trivia on `operator`
+                var misplacedToken = this.AddError(this.EatToken(), ErrorCode.ERR_MisplacedUnchecked);
+                operatorKeyword = AddTrailingSkippedSyntax(operatorKeyword, misplacedToken);
+                return null;
+            }
+
+            return TryEatToken(SyntaxKind.CheckedKeyword);
+        }
+
         private OperatorDeclarationSyntax ParseOperatorDeclaration(
             SyntaxList<AttributeListSyntax> attributes,
             SyntaxListBuilder modifiers,
             TypeSyntax type,
             ExplicitInterfaceSpecifierSyntax explicitInterfaceOpt)
         {
             var opKeyword = this.EatToken(SyntaxKind.OperatorKeyword);
-            var checkedKeyword = this.TryEatToken(SyntaxKind.CheckedKeyword); // https://github.com/dotnet/roslyn/issues/60394 : consider gracefully recovering from erroneous use of 'unchecked' at this location 
+            var checkedKeyword = TryEatCheckedOrHandleUnchecked(ref opKeyword);
             SyntaxToken opToken;
             int opTokenErrorOffset;
             int opTokenErrorWidth;

src/Compilers/CSharp/Portable/Parser/SyntaxParser.cs

@@ -1086,7 +1086,6 @@ protected TNode CheckFeatureAvailability<TNode>(TNode node, MessageID feature, b
             where TNode : GreenNode
         {
             LanguageVersion availableVersion = this.Options.LanguageVersion;
-            LanguageVersion requiredVersion = feature.RequiredVersion();
 
             // There are special error codes for some features, so handle those separately.
             switch (feature)