Improved error messages for property declarations

src/compiler/checker.ts

@@ -15409,6 +15409,11 @@ namespace ts {
             let flags = 0;
             for (const modifier of node.modifiers) {
                 switch (modifier.kind) {
+                    case SyntaxKind.ConstKeyword:
+                        if (node.kind !== SyntaxKind.EnumDeclaration && node.parent.kind === SyntaxKind.ClassDeclaration) {
+                            return grammarErrorOnNode(node, Diagnostics.A_class_member_cannot_have_the_0_keyword, tokenToString(SyntaxKind.ConstKeyword));
+                        }
+                        break;
                     case SyntaxKind.PublicKeyword:
                     case SyntaxKind.ProtectedKeyword:
                     case SyntaxKind.PrivateKeyword:

src/compiler/diagnosticMessages.json

@@ -791,7 +791,10 @@
         "category": "Error",
         "code": 1247
     },
-
+    "A class member cannot have the '{0}' keyword.": {
+        "category": "Error",
+        "code": 1248
+    },
     "'with' statements are not allowed in an async function block.": {
         "category": "Error",
         "code": 1300

src/compiler/parser.ts

@@ -1134,6 +1134,14 @@ namespace ts {
             return token === t && tryParse(nextTokenCanFollowModifier);
         }
 
+        function nextTokenIsOnSameLineAndCanFollowModifier() {
+            nextToken();
+            if (scanner.hasPrecedingLineBreak()) {
+                return false;
+            }
+            return canFollowModifier();
+        }
+
         function nextTokenCanFollowModifier() {
             if (token === SyntaxKind.ConstKeyword) {
                 // 'const' is only a modifier if followed by 'enum'.
@@ -1154,11 +1162,7 @@ namespace ts {
                 return canFollowModifier();
             }
 
-            nextToken();
-            if (scanner.hasPrecedingLineBreak()) {
-                return false;
-            }
-            return canFollowModifier();
+            return nextTokenIsOnSameLineAndCanFollowModifier();
         }
 
         function parseAnyContextualModifier(): boolean {
@@ -4923,15 +4927,31 @@ namespace ts {
             return decorators;
         }
 
-        function parseModifiers(): ModifiersArray {
+        /*
+         * There are situations in which a modifier like 'const' will appear unexpectedly, such as on a class member.
+         * In those situations, if we are entirely sure that 'const' is not valid on its own (such as when ASI takes effect
+         * and turns it into a standalone declaration), then it is better to parse it and report an error later.
+         *
+         * In such situations, 'permitInvalidConstAsModifier' should be set to true.
+         */
+        function parseModifiers(permitInvalidConstAsModifier?: boolean): ModifiersArray {
             let flags = 0;
             let modifiers: ModifiersArray;
             while (true) {
                 const modifierStart = scanner.getStartPos();
                 const modifierKind = token;
 
-                if (!parseAnyContextualModifier()) {
-                    break;
+                if (token === SyntaxKind.ConstKeyword && permitInvalidConstAsModifier) {
+                    // We need to ensure that any subsequent modifiers appear on the same line
+                    // so that when 'const' is a standalone declaration, we don't issue an error.                
+                    if (!tryParse(nextTokenIsOnSameLineAndCanFollowModifier)) {
+                        break;
+                    }
+                }
+                else {
+                    if (!parseAnyContextualModifier()) {
+                        break;
+                    }
                 }
 
                 if (!modifiers) {
@@ -4976,7 +4996,7 @@ namespace ts {
 
             const fullStart = getNodePos();
             const decorators = parseDecorators();
-            const modifiers = parseModifiers();
+            const modifiers = parseModifiers(/*permitInvalidConstAsModifier*/ true);
 
             const accessor = tryParseAccessorDeclaration(fullStart, decorators, modifiers);
             if (accessor) {