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diff --git a/‎src/compiler/binder.ts
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+5-2Lines changed: 5 additions & 2 deletions b/‎src/compiler/binder.ts
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+5-2Lines changed: 5 additions & 2 deletions
diff --git a/‎src/compiler/checker.ts
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+30-10Lines changed: 30 additions & 10 deletions b/‎src/compiler/checker.ts
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+30-10Lines changed: 30 additions & 10 deletions
diff --git a/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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+4Lines changed: 4 additions & 0 deletions
diff --git a/‎src/compiler/transformers/es2017.ts
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+40-13Lines changed: 40 additions & 13 deletions b/‎src/compiler/transformers/es2017.ts
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+40-13Lines changed: 40 additions & 13 deletions
diff --git a/‎src/compiler/transformers/es2018.ts
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+14-14Lines changed: 14 additions & 14 deletions b/‎src/compiler/transformers/es2018.ts
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+14-14Lines changed: 14 additions & 14 deletions
diff --git a/‎src/compiler/transformers/module/system.ts
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+4-1Lines changed: 4 additions & 1 deletion b/‎src/compiler/transformers/module/system.ts
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+4-1Lines changed: 4 additions & 1 deletion
diff --git a/‎src/compiler/types.ts
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+8-7Lines changed: 8 additions & 7 deletions b/‎src/compiler/types.ts
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+8-7Lines changed: 8 additions & 7 deletions
diff --git a/‎tests/baselines/reference/awaitInNonAsyncFunction.errors.txt
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+2-2Lines changed: 2 additions & 2 deletions b/‎tests/baselines/reference/awaitInNonAsyncFunction.errors.txt
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+2-2Lines changed: 2 additions & 2 deletions
@@ -3902,10 +3902,13 @@ namespace ts {
 
         switch (kind) {
             case SyntaxKind.AsyncKeyword:
-            case SyntaxKind.AwaitExpression:
-                // async/await is ES2017 syntax, but may be ES2018 syntax (for async generators)
+                // async is ES2017 syntax, but may be ES2018 syntax (for async generators)
                 transformFlags |= TransformFlags.AssertES2018 | TransformFlags.AssertES2017;
                 break;
+            case SyntaxKind.AwaitExpression:
+                // await is ES2017 syntax, but may be ES2018 syntax (for async generators)
+                transformFlags |= TransformFlags.AssertES2018 | TransformFlags.AssertES2017 | TransformFlags.ContainsAwait;
+                break;
 
             case SyntaxKind.TypeAssertionExpression:
             case SyntaxKind.AsExpression:
 
@@ -26437,21 +26437,41 @@ namespace ts {
             return undefinedWideningType;
         }
 
+        function isTopLevelAwait(node: AwaitExpression) {
+            const container = getThisContainer(node, /*includeArrowFunctions*/ true);
+            return isSourceFile(container);
+        }
+
         function checkAwaitExpression(node: AwaitExpression): Type {
             // Grammar checking
             if (produceDiagnostics) {
                 if (!(node.flags & NodeFlags.AwaitContext)) {
-                    // use of 'await' in non-async function
-                    const sourceFile = getSourceFileOfNode(node);
-                    if (!hasParseDiagnostics(sourceFile)) {
-                        const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
-                        const diagnostic = createFileDiagnostic(sourceFile, span.start, span.length, Diagnostics.await_expression_is_only_allowed_within_an_async_function);
-                        const func = getContainingFunction(node);
-                        if (func && func.kind !== SyntaxKind.Constructor && (getFunctionFlags(func) & FunctionFlags.Async) === 0) {
-                            const relatedInfo = createDiagnosticForNode(func, Diagnostics.Did_you_mean_to_mark_this_function_as_async);
-                            addRelatedInfo(diagnostic, relatedInfo);
+                    if (isTopLevelAwait(node)) {
+                        const sourceFile = getSourceFileOfNode(node);
+                        if ((moduleKind !== ModuleKind.ESNext && moduleKind !== ModuleKind.System) ||
+                            languageVersion < ScriptTarget.ES2017 ||
+                            !isEffectiveExternalModule(sourceFile, compilerOptions)) {
+                            if (!hasParseDiagnostics(sourceFile)) {
+                                const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
+                                const diagnostic = createFileDiagnostic(sourceFile, span.start, span.length,
+                                    Diagnostics.await_outside_of_an_async_function_is_only_allowed_at_the_top_level_of_a_module_when_module_is_esnext_or_system_and_target_is_es2017_or_higher);
+                                diagnostics.add(diagnostic);
+                            }
+                        }
+                    }
+                    else {
+                        // use of 'await' in non-async function
+                        const sourceFile = getSourceFileOfNode(node);
+                        if (!hasParseDiagnostics(sourceFile)) {
+                            const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
+                            const diagnostic = createFileDiagnostic(sourceFile, span.start, span.length, Diagnostics.await_expression_is_only_allowed_within_an_async_function);
+                            const func = getContainingFunction(node);
+                            if (func && func.kind !== SyntaxKind.Constructor && (getFunctionFlags(func) & FunctionFlags.Async) === 0) {
+                                const relatedInfo = createDiagnosticForNode(func, Diagnostics.Did_you_mean_to_mark_this_function_as_async);
+                                addRelatedInfo(diagnostic, relatedInfo);
+                            }
+                            diagnostics.add(diagnostic);
                         }
-                        diagnostics.add(diagnostic);
                     }
                 }
 
 
@@ -1059,6 +1059,10 @@
         "category": "Error",
         "code": 1360
     },
+    "'await' outside of an async function is only allowed at the top level of a module when '--module' is 'esnext' or 'system' and '--target' is 'es2017' or higher.": {
+        "category": "Error",
+        "code": 1361
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",
 
@@ -7,6 +7,11 @@ namespace ts {
         AsyncMethodsWithSuper = 1 << 0
     }
 
+    const enum ContextFlags {
+        NonTopLevel = 1 << 0,
+        HasLexicalThis = 1 << 1
+    }
+
     export function transformES2017(context: TransformationContext) {
         const {
             resumeLexicalEnvironment,
@@ -41,7 +46,7 @@ namespace ts {
         /** A set of node IDs for generated super accessors (variable statements). */
         const substitutedSuperAccessors: boolean[] = [];
 
-        let topLevel: boolean;
+        let contextFlags: ContextFlags = 0;
 
         // Save the previous transformation hooks.
         const previousOnEmitNode = context.onEmitNode;
@@ -58,17 +63,35 @@ namespace ts {
                 return node;
             }
 
-            topLevel = isEffectiveStrictModeSourceFile(node, compilerOptions);
+            setContextFlag(ContextFlags.NonTopLevel, false);
+            setContextFlag(ContextFlags.HasLexicalThis, !isEffectiveStrictModeSourceFile(node, compilerOptions));
             const visited = visitEachChild(node, visitor, context);
             addEmitHelpers(visited, context.readEmitHelpers());
             return visited;
         }
 
-        function doOutsideOfTopLevel<T, U>(cb: (value: T) => U, value: T) {
-            if (topLevel) {
-                topLevel = false;
+        function setContextFlag(flag: ContextFlags, val: boolean) {
+            contextFlags = val ? contextFlags | flag : contextFlags & ~flag;
+        }
+
+        function inContext(flags: ContextFlags) {
+            return (contextFlags & flags) !== 0;
+        }
+
+        function inTopLevelContext() {
+            return !inContext(ContextFlags.NonTopLevel);
+        }
+
+        function inHasLexicalThisContext() {
+            return inContext(ContextFlags.HasLexicalThis);
+        }
+
+        function doWithContext<T, U>(flags: ContextFlags, cb: (value: T) => U, value: T) {
+            const contextFlagsToSet = flags & ~contextFlags;
+            if (contextFlagsToSet) {
+                setContextFlag(contextFlagsToSet, /*val*/ true);
                 const result = cb(value);
-                topLevel = true;
+                setContextFlag(contextFlagsToSet, /*val*/ false);
                 return result;
             }
             return cb(value);
@@ -91,16 +114,16 @@ namespace ts {
                     return visitAwaitExpression(<AwaitExpression>node);
 
                 case SyntaxKind.MethodDeclaration:
-                    return doOutsideOfTopLevel(visitMethodDeclaration, <MethodDeclaration>node);
+                    return doWithContext(ContextFlags.NonTopLevel | ContextFlags.HasLexicalThis, visitMethodDeclaration, <MethodDeclaration>node);
 
                 case SyntaxKind.FunctionDeclaration:
-                    return doOutsideOfTopLevel(visitFunctionDeclaration, <FunctionDeclaration>node);
+                    return doWithContext(ContextFlags.NonTopLevel | ContextFlags.HasLexicalThis, visitFunctionDeclaration, <FunctionDeclaration>node);
 
                 case SyntaxKind.FunctionExpression:
-                    return doOutsideOfTopLevel(visitFunctionExpression, <FunctionExpression>node);
+                    return doWithContext(ContextFlags.NonTopLevel | ContextFlags.HasLexicalThis, visitFunctionExpression, <FunctionExpression>node);
 
                 case SyntaxKind.ArrowFunction:
-                    return visitArrowFunction(<ArrowFunction>node);
+                    return doWithContext(ContextFlags.NonTopLevel, visitArrowFunction, <ArrowFunction>node);
 
                 case SyntaxKind.PropertyAccessExpression:
                     if (capturedSuperProperties && isPropertyAccessExpression(node) && node.expression.kind === SyntaxKind.SuperKeyword) {
@@ -119,7 +142,7 @@ namespace ts {
                 case SyntaxKind.Constructor:
                 case SyntaxKind.ClassDeclaration:
                 case SyntaxKind.ClassExpression:
-                    return doOutsideOfTopLevel(visitDefault, node);
+                    return doWithContext(ContextFlags.NonTopLevel | ContextFlags.HasLexicalThis, visitDefault, node);
 
                 default:
                     return visitEachChild(node, visitor, context);
@@ -237,6 +260,10 @@ namespace ts {
          * @param node The node to visit.
          */
         function visitAwaitExpression(node: AwaitExpression): Expression {
+            // do not downlevel a top-level await as it is module syntax...
+            if (inTopLevelContext()) {
+                return visitEachChild(node, visitor, context);
+            }
             return setOriginalNode(
                 setTextRange(
                     createYield(
@@ -457,7 +484,7 @@ namespace ts {
                     createReturn(
                         createAwaiterHelper(
                             context,
-                            !topLevel,
+                            inHasLexicalThisContext(),
                             hasLexicalArguments,
                             promiseConstructor,
                             transformAsyncFunctionBodyWorker(<Block>node.body, statementOffset)
@@ -498,7 +525,7 @@ namespace ts {
             else {
                 const expression = createAwaiterHelper(
                     context,
-                    !topLevel,
+                    inHasLexicalThisContext(),
                     hasLexicalArguments,
                     promiseConstructor,
                     transformAsyncFunctionBodyWorker(node.body!)
 
@@ -26,7 +26,7 @@ namespace ts {
         let enabledSubstitutions: ESNextSubstitutionFlags;
         let enclosingFunctionFlags: FunctionFlags;
         let enclosingSuperContainerFlags: NodeCheckFlags = 0;
-        let topLevel: boolean;
+        let hasLexicalThis: boolean;
 
         /** Keeps track of property names accessed on super (`super.x`) within async functions. */
         let capturedSuperProperties: UnderscoreEscapedMap<true>;
@@ -43,7 +43,7 @@ namespace ts {
             }
 
             exportedVariableStatement = false;
-            topLevel = isEffectiveStrictModeSourceFile(node, compilerOptions);
+            hasLexicalThis = !isEffectiveStrictModeSourceFile(node, compilerOptions);
             const visited = visitEachChild(node, visitor, context);
             addEmitHelpers(visited, context.readEmitHelpers());
             return visited;
@@ -64,11 +64,11 @@ namespace ts {
             return node;
         }
 
-        function doOutsideOfTopLevel<T, U>(cb: (value: T) => U, value: T) {
-            if (topLevel) {
-                topLevel = false;
+        function doWithLexicalThis<T, U>(cb: (value: T) => U, value: T) {
+            if (!hasLexicalThis) {
+                hasLexicalThis = true;
                 const result = cb(value);
-                topLevel = true;
+                hasLexicalThis = false;
                 return result;
             }
             return cb(value);
@@ -108,17 +108,17 @@ namespace ts {
                 case SyntaxKind.VoidExpression:
                     return visitVoidExpression(node as VoidExpression);
                 case SyntaxKind.Constructor:
-                    return doOutsideOfTopLevel(visitConstructorDeclaration, node as ConstructorDeclaration);
+                    return doWithLexicalThis(visitConstructorDeclaration, node as ConstructorDeclaration);
                 case SyntaxKind.MethodDeclaration:
-                    return doOutsideOfTopLevel(visitMethodDeclaration, node as MethodDeclaration);
+                    return doWithLexicalThis(visitMethodDeclaration, node as MethodDeclaration);
                 case SyntaxKind.GetAccessor:
-                    return doOutsideOfTopLevel(visitGetAccessorDeclaration, node as GetAccessorDeclaration);
+                    return doWithLexicalThis(visitGetAccessorDeclaration, node as GetAccessorDeclaration);
                 case SyntaxKind.SetAccessor:
-                    return doOutsideOfTopLevel(visitSetAccessorDeclaration, node as SetAccessorDeclaration);
+                    return doWithLexicalThis(visitSetAccessorDeclaration, node as SetAccessorDeclaration);
                 case SyntaxKind.FunctionDeclaration:
-                    return doOutsideOfTopLevel(visitFunctionDeclaration, node as FunctionDeclaration);
+                    return doWithLexicalThis(visitFunctionDeclaration, node as FunctionDeclaration);
                 case SyntaxKind.FunctionExpression:
-                    return doOutsideOfTopLevel(visitFunctionExpression, node as FunctionExpression);
+                    return doWithLexicalThis(visitFunctionExpression, node as FunctionExpression);
                 case SyntaxKind.ArrowFunction:
                     return visitArrowFunction(node as ArrowFunction);
                 case SyntaxKind.Parameter:
@@ -139,7 +139,7 @@ namespace ts {
                     return visitEachChild(node, visitor, context);
                 case SyntaxKind.ClassDeclaration:
                 case SyntaxKind.ClassExpression:
-                    return doOutsideOfTopLevel(visitDefault, node);
+                    return doWithLexicalThis(visitDefault, node);
                 default:
                     return visitEachChild(node, visitor, context);
             }
@@ -774,7 +774,7 @@ namespace ts {
                             visitLexicalEnvironment(node.body!.statements, visitor, context, statementOffset)
                         )
                     ),
-                    !topLevel
+                    hasLexicalThis
                 )
             );
 
 
@@ -262,13 +262,16 @@ namespace ts {
             insertStatementsAfterStandardPrologue(statements, endLexicalEnvironment());
 
             const exportStarFunction = addExportStarIfNeeded(statements)!; // TODO: GH#18217
+            const modifiers = node.transformFlags & TransformFlags.ContainsAwait ?
+                createModifiersFromModifierFlags(ModifierFlags.Async) :
+                undefined;
             const moduleObject = createObjectLiteral([
                 createPropertyAssignment("setters",
                     createSettersArray(exportStarFunction, dependencyGroups)
                 ),
                 createPropertyAssignment("execute",
                     createFunctionExpression(
-                        /*modifiers*/ undefined,
+                        modifiers,
                         /*asteriskToken*/ undefined,
                         /*name*/ undefined,
                         /*typeParameters*/ undefined,
 
@@ -5608,9 +5608,10 @@ namespace ts {
         ContainsBlockScopedBinding = 1 << 15,
         ContainsBindingPattern = 1 << 16,
         ContainsYield = 1 << 17,
-        ContainsHoistedDeclarationOrCompletion = 1 << 18,
-        ContainsDynamicImport = 1 << 19,
-        ContainsClassFields = 1 << 20,
+        ContainsAwait = 1 << 18,
+        ContainsHoistedDeclarationOrCompletion = 1 << 19,
+        ContainsDynamicImport = 1 << 20,
+        ContainsClassFields = 1 << 21,
 
         // Please leave this as 1 << 29.
         // It is the maximum bit we can set before we outgrow the size of a v8 small integer (SMI) on an x86 system.
@@ -5636,10 +5637,10 @@ namespace ts {
         OuterExpressionExcludes = HasComputedFlags,
         PropertyAccessExcludes = OuterExpressionExcludes,
         NodeExcludes = PropertyAccessExcludes,
-        ArrowFunctionExcludes = NodeExcludes | ContainsTypeScriptClassSyntax | ContainsBlockScopedBinding | ContainsYield | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
-        FunctionExcludes = NodeExcludes | ContainsTypeScriptClassSyntax | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsYield | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
-        ConstructorExcludes = NodeExcludes | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsYield | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
-        MethodOrAccessorExcludes = NodeExcludes | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsYield | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
+        ArrowFunctionExcludes = NodeExcludes | ContainsTypeScriptClassSyntax | ContainsBlockScopedBinding | ContainsYield | ContainsAwait | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
+        FunctionExcludes = NodeExcludes | ContainsTypeScriptClassSyntax | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsYield | ContainsAwait | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
+        ConstructorExcludes = NodeExcludes | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsYield | ContainsAwait | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
+        MethodOrAccessorExcludes = NodeExcludes | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsYield | ContainsAwait | ContainsHoistedDeclarationOrCompletion | ContainsBindingPattern | ContainsObjectRestOrSpread,
         PropertyExcludes = NodeExcludes | ContainsLexicalThis,
         ClassExcludes = NodeExcludes | ContainsTypeScriptClassSyntax | ContainsComputedPropertyName,
         ModuleExcludes = NodeExcludes | ContainsTypeScriptClassSyntax | ContainsLexicalThis | ContainsBlockScopedBinding | ContainsHoistedDeclarationOrCompletion,
 
@@ -13,7 +13,7 @@ tests/cases/compiler/awaitInNonAsyncFunction.ts(31,5): error TS1308: 'await' exp
 tests/cases/compiler/awaitInNonAsyncFunction.ts(34,7): error TS1103: A 'for-await-of' statement is only allowed within an async function or async generator.
 tests/cases/compiler/awaitInNonAsyncFunction.ts(35,5): error TS1308: 'await' expression is only allowed within an async function.
 tests/cases/compiler/awaitInNonAsyncFunction.ts(39,5): error TS1103: A 'for-await-of' statement is only allowed within an async function or async generator.
-tests/cases/compiler/awaitInNonAsyncFunction.ts(40,1): error TS1308: 'await' expression is only allowed within an async function.
+tests/cases/compiler/awaitInNonAsyncFunction.ts(40,1): error TS1361: 'await' outside of an async function is only allowed at the top level of a module when '--module' is 'esnext' or 'system' and '--target' is 'es2017' or higher.
 
 
 ==== tests/cases/compiler/awaitInNonAsyncFunction.ts (16 errors) ====
@@ -100,4 +100,4 @@ tests/cases/compiler/awaitInNonAsyncFunction.ts(40,1): error TS1308: 'await' exp
 !!! error TS1103: A 'for-await-of' statement is only allowed within an async function or async generator.
     await null;
     ~~~~~
-!!! error TS1308: 'await' expression is only allowed within an async function.
+!!! error TS1361: 'await' outside of an async function is only allowed at the top level of a module when '--module' is 'esnext' or 'system' and '--target' is 'es2017' or higher.