Cherry-pick PR microsoft#39216 into release-4.0

Component commits:
6298d84 Leverage syntax cursor as part of reparse

Author: rbuckton

src/compiler/binder.ts

@@ -2109,20 +2109,39 @@ namespace ts {
         }
 
         // The binder visits every node in the syntax tree so it is a convenient place to perform a single localized
-        // check for reserved words used as identifiers in strict mode code.
-        function checkStrictModeIdentifier(node: Identifier) {
-            if (inStrictMode &&
-                node.originalKeywordKind! >= SyntaxKind.FirstFutureReservedWord &&
-                node.originalKeywordKind! <= SyntaxKind.LastFutureReservedWord &&
-                !isIdentifierName(node) &&
+        // check for reserved words used as identifiers in strict mode code, as well as `yield` or `await` in
+        // [Yield] or [Await] contexts, respectively.
+        function checkContextualIdentifier(node: Identifier) {
+            // Report error only if there are no parse errors in file
+            if (!file.parseDiagnostics.length &&
                 !(node.flags & NodeFlags.Ambient) &&
-                !(node.flags & NodeFlags.JSDoc)) {
+                !(node.flags & NodeFlags.JSDoc) &&
+                !isIdentifierName(node)) {
 
-                // Report error only if there are no parse errors in file
-                if (!file.parseDiagnostics.length) {
+                // strict mode identifiers
+                if (inStrictMode &&
+                    node.originalKeywordKind! >= SyntaxKind.FirstFutureReservedWord &&
+                    node.originalKeywordKind! <= SyntaxKind.LastFutureReservedWord) {
                     file.bindDiagnostics.push(createDiagnosticForNode(node,
                         getStrictModeIdentifierMessage(node), declarationNameToString(node)));
                 }
+                else if (node.originalKeywordKind === SyntaxKind.AwaitKeyword) {
+                    if (isExternalModule(file) && isInTopLevelContext(node)) {
+                        file.bindDiagnostics.push(createDiagnosticForNode(node,
+                            Diagnostics.Identifier_expected_0_is_a_reserved_word_at_the_top_level_of_a_module,
+                            declarationNameToString(node)));
+                    }
+                    else if (node.flags & NodeFlags.AwaitContext) {
+                        file.bindDiagnostics.push(createDiagnosticForNode(node,
+                            Diagnostics.Identifier_expected_0_is_a_reserved_word_that_cannot_be_used_here,
+                            declarationNameToString(node)));
+                    }
+                }
+                else if (node.originalKeywordKind === SyntaxKind.YieldKeyword && node.flags & NodeFlags.YieldContext) {
+                    file.bindDiagnostics.push(createDiagnosticForNode(node,
+                        Diagnostics.Identifier_expected_0_is_a_reserved_word_that_cannot_be_used_here,
+                        declarationNameToString(node)));
+                }
             }
         }
 
@@ -2423,7 +2442,7 @@ namespace ts {
                     if (currentFlow && (isExpression(node) || parent.kind === SyntaxKind.ShorthandPropertyAssignment)) {
                         node.flowNode = currentFlow;
                     }
-                    return checkStrictModeIdentifier(<Identifier>node);
+                    return checkContextualIdentifier(<Identifier>node);
                 case SyntaxKind.SuperKeyword:
                     node.flowNode = currentFlow;
                     break;

src/compiler/checker.ts

@@ -28557,11 +28557,6 @@ namespace ts {
             return undefinedWideningType;
         }
 
-        function isInTopLevelContext(node: Node) {
-            const container = getThisContainer(node, /*includeArrowFunctions*/ true);
-            return isSourceFile(container);
-        }
-
         function checkAwaitExpression(node: AwaitExpression): Type {
             // Grammar checking
             if (produceDiagnostics) {
@@ -35168,7 +35163,6 @@ namespace ts {
         }
 
         function checkImportBinding(node: ImportEqualsDeclaration | ImportClause | NamespaceImport | ImportSpecifier) {
-            checkGrammarAwaitIdentifier(node.name);
             checkCollisionWithRequireExportsInGeneratedCode(node, node.name!);
             checkCollisionWithGlobalPromiseInGeneratedCode(node, node.name!);
             checkAliasSymbol(node);
@@ -37934,31 +37928,19 @@ namespace ts {
             return false;
         }
 
-        function checkGrammarAwaitIdentifier(name: Identifier | undefined): boolean {
-            if (name && isIdentifier(name) && name.originalKeywordKind === SyntaxKind.AwaitKeyword && isInTopLevelContext(name.parent)) {
-                const file = getSourceFileOfNode(name);
-                if (!file.isDeclarationFile && isExternalModule(file)) {
-                    return grammarErrorOnNode(name, Diagnostics.Identifier_expected_0_is_a_reserved_word_at_the_top_level_of_a_module, idText(name));
-                }
-            }
-            return false;
-        }
-
         function checkGrammarFunctionLikeDeclaration(node: FunctionLikeDeclaration | MethodSignature): boolean {
             // Prevent cascading error by short-circuit
             const file = getSourceFileOfNode(node);
             return checkGrammarDecoratorsAndModifiers(node) ||
                 checkGrammarTypeParameterList(node.typeParameters, file) ||
-                (isFunctionDeclaration(node) && checkGrammarAwaitIdentifier(node.name)) ||
                 checkGrammarParameterList(node.parameters) ||
                 checkGrammarArrowFunction(node, file) ||
                 (isFunctionLikeDeclaration(node) && checkGrammarForUseStrictSimpleParameterList(node));
         }
 
         function checkGrammarClassLikeDeclaration(node: ClassLikeDeclaration): boolean {
             const file = getSourceFileOfNode(node);
-            return (isClassDeclaration(node) && checkGrammarAwaitIdentifier(node.name)) ||
-                checkGrammarClassDeclarationHeritageClauses(node) ||
+            return checkGrammarClassDeclarationHeritageClauses(node) ||
                 checkGrammarTypeParameterList(node.typeParameters, file);
         }
 
@@ -38601,10 +38583,6 @@ namespace ts {
                 }
             }
 
-            if (isIdentifier(node.name) && checkGrammarAwaitIdentifier(node.name)) {
-                return true;
-            }
-
             if (node.dotDotDotToken && node.initializer) {
                 // Error on equals token which immediately precedes the initializer
                 return grammarErrorAtPos(node, node.initializer.pos - 1, 1, Diagnostics.A_rest_element_cannot_have_an_initializer);
@@ -38668,9 +38646,6 @@ namespace ts {
                     }
                 }
             }
-            if (isIdentifier(node.name) && checkGrammarAwaitIdentifier(node.name)) {
-                return true;
-            }
 
             if (node.exclamationToken && (node.parent.parent.kind !== SyntaxKind.VariableStatement || !node.type || node.initializer || node.flags & NodeFlags.Ambient)) {
                 return grammarErrorOnNode(node.exclamationToken, Diagnostics.Definite_assignment_assertions_can_only_be_used_along_with_a_type_annotation);