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diff --git a/‎src/compiler/checker.ts
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+130-31Lines changed: 130 additions & 31 deletions b/‎src/compiler/checker.ts
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+130-31Lines changed: 130 additions & 31 deletions
diff --git a/‎src/compiler/types.ts
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+9Lines changed: 9 additions & 0 deletions b/‎src/compiler/types.ts
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+9Lines changed: 9 additions & 0 deletions
diff --git a/‎tests/baselines/reference/api/tsserverlibrary.d.ts
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+1Lines changed: 1 addition & 0 deletions b/‎tests/baselines/reference/api/tsserverlibrary.d.ts
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+1Lines changed: 1 addition & 0 deletions
diff --git a/‎tests/baselines/reference/api/typescript.d.ts
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+1Lines changed: 1 addition & 0 deletions b/‎tests/baselines/reference/api/typescript.d.ts
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+1Lines changed: 1 addition & 0 deletions
diff --git a/‎tests/baselines/reference/asyncFunctionDeclaration10_es2017.errors.txt
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+6-2Lines changed: 6 additions & 2 deletions b/‎tests/baselines/reference/asyncFunctionDeclaration10_es2017.errors.txt
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+6-2Lines changed: 6 additions & 2 deletions
@@ -260,6 +260,7 @@ namespace ts {
         const literalTypes = createMap<LiteralType>();
         const indexedAccessTypes = createMap<IndexedAccessType>();
         const evolvingArrayTypes: EvolvingArrayType[] = [];
+        const undefinedProperties = createMap<Symbol>() as UnderscoreEscapedMap<Symbol>;
 
         const unknownSymbol = createSymbol(SymbolFlags.Property, "unknown" as __String);
         const resolvingSymbol = createSymbol(0, InternalSymbolName.Resolving);
@@ -7984,7 +7985,7 @@ namespace ts {
             const spread = createAnonymousType(undefined, members, emptyArray, emptyArray, stringIndexInfo, numberIndexInfo);
             spread.flags |= propagatedFlags;
             spread.flags |= TypeFlags.FreshLiteral | TypeFlags.ContainsObjectLiteral;
-            (spread as ObjectType).objectFlags |= ObjectFlags.ObjectLiteral;
+            (spread as ObjectType).objectFlags |= (ObjectFlags.ObjectLiteral | ObjectFlags.ContainsSpread);
             spread.symbol = symbol;
             return spread;
         }
@@ -9026,7 +9027,7 @@ namespace ts {
 
                 if (isSimpleTypeRelatedTo(source, target, relation, reportErrors ? reportError : undefined)) return Ternary.True;
 
-                if (getObjectFlags(source) & ObjectFlags.ObjectLiteral && source.flags & TypeFlags.FreshLiteral) {
+                if (isObjectLiteralType(source) && source.flags & TypeFlags.FreshLiteral) {
                     if (hasExcessProperties(<FreshObjectLiteralType>source, target, reportErrors)) {
                         if (reportErrors) {
                             reportRelationError(headMessage, source, target);
@@ -9596,14 +9597,27 @@ namespace ts {
                 if (relation === identityRelation) {
                     return propertiesIdenticalTo(source, target);
                 }
-                const requireOptionalProperties = relation === subtypeRelation && !(getObjectFlags(source) & ObjectFlags.ObjectLiteral);
+                const requireOptionalProperties = relation === subtypeRelation && !isObjectLiteralType(source);
                 const unmatchedProperty = getUnmatchedProperty(source, target, requireOptionalProperties);
                 if (unmatchedProperty) {
                     if (reportErrors) {
                         reportError(Diagnostics.Property_0_is_missing_in_type_1, symbolToString(unmatchedProperty), typeToString(source));
                     }
                     return Ternary.False;
                 }
+                if (isObjectLiteralType(target)) {
+                    for (const sourceProp of getPropertiesOfType(source)) {
+                        if (!getPropertyOfObjectType(target, sourceProp.escapedName)) {
+                            const sourceType = getTypeOfSymbol(sourceProp);
+                            if (!(sourceType === undefinedType || sourceType === undefinedWideningType)) {
+                                if (reportErrors) {
+                                    reportError(Diagnostics.Property_0_does_not_exist_on_type_1, symbolToString(sourceProp), typeToString(target));
+                                }
+                                return Ternary.False;
+                            }
+                        }
+                    }
+                }
                 let result = Ternary.True;
                 const properties = getPropertiesOfObjectType(target);
                 for (const targetProp of properties) {
@@ -10425,7 +10439,7 @@ namespace ts {
          * Leave signatures alone since they are not subject to the check.
          */
         function getRegularTypeOfObjectLiteral(type: Type): Type {
-            if (!(getObjectFlags(type) & ObjectFlags.ObjectLiteral && type.flags & TypeFlags.FreshLiteral)) {
+            if (!(isObjectLiteralType(type) && type.flags & TypeFlags.FreshLiteral)) {
                 return type;
             }
             const regularType = (<FreshObjectLiteralType>type).regularType;
@@ -10447,18 +10461,74 @@ namespace ts {
             return regularNew;
         }
 
-        function getWidenedProperty(prop: Symbol): Symbol {
+        function createWideningContext(parent: WideningContext, propertyName: __String, siblings: Type[]): WideningContext {
+            return { parent, propertyName, siblings, resolvedPropertyNames: undefined };
+        }
+
+        function getSiblingsOfContext(context: WideningContext): Type[] {
+            if (!context.siblings) {
+                const siblings: Type[] = [];
+                for (const type of getSiblingsOfContext(context.parent)) {
+                    if (isObjectLiteralType(type)) {
+                        const prop = getPropertyOfObjectType(type, context.propertyName);
+                        if (prop) {
+                            forEachType(getTypeOfSymbol(prop), t => {
+                                siblings.push(t);
+                            });
+                        }
+                    }
+                }
+                context.siblings = siblings;
+            }
+            return context.siblings;
+        }
+
+        function getPropertyNamesOfContext(context: WideningContext): __String[] {
+            if (!context.resolvedPropertyNames) {
+                const names = createMap<boolean>() as UnderscoreEscapedMap<boolean>;
+                for (const t of getSiblingsOfContext(context)) {
+                    if (isObjectLiteralType(t) && !(getObjectFlags(t) & ObjectFlags.ContainsSpread)) {
+                        for (const prop of getPropertiesOfType(t)) {
+                            names.set(prop.escapedName, true);
+                        }
+                    }
+                }
+                context.resolvedPropertyNames = arrayFrom(names.keys());
+            }
+            return context.resolvedPropertyNames;
+        }
+
+        function getWidenedProperty(prop: Symbol, context: WideningContext): Symbol {
             const original = getTypeOfSymbol(prop);
-            const widened = getWidenedType(original);
+            const propContext = context && createWideningContext(context, prop.escapedName, /*siblings*/ undefined);
+            const widened = getWidenedTypeWithContext(original, propContext);
             return widened === original ? prop : createSymbolWithType(prop, widened);
         }
 
-        function getWidenedTypeOfObjectLiteral(type: Type): Type {
+        function getUndefinedProperty(name: __String) {
+            const cached = undefinedProperties.get(name);
+            if (cached) {
+                return cached;
+            }
+            const result = createSymbol(SymbolFlags.Property | SymbolFlags.Optional, name);
+            result.type = undefinedType;
+            undefinedProperties.set(name, result);
+            return result;
+        }
+
+        function getWidenedTypeOfObjectLiteral(type: Type, context: WideningContext): Type {
             const members = createSymbolTable();
             for (const prop of getPropertiesOfObjectType(type)) {
                 // Since get accessors already widen their return value there is no need to
                 // widen accessor based properties here.
-                members.set(prop.escapedName, prop.flags & SymbolFlags.Property ? getWidenedProperty(prop) : prop);
+                members.set(prop.escapedName, prop.flags & SymbolFlags.Property ? getWidenedProperty(prop, context) : prop);
+            }
+            if (context) {
+                for (const name of getPropertyNamesOfContext(context)) {
+                    if (!members.has(name)) {
+                        members.set(name, getUndefinedProperty(name));
+                    }
+                }
             }
             const stringIndexInfo = getIndexInfoOfType(type, IndexKind.String);
             const numberIndexInfo = getIndexInfoOfType(type, IndexKind.Number);
@@ -10467,20 +10537,25 @@ namespace ts {
                 numberIndexInfo && createIndexInfo(getWidenedType(numberIndexInfo.type), numberIndexInfo.isReadonly));
         }
 
-        function getWidenedConstituentType(type: Type): Type {
-            return type.flags & TypeFlags.Nullable ? type : getWidenedType(type);
+        function getWidenedType(type: Type) {
+            return getWidenedTypeWithContext(type, /*context*/ undefined);
         }
 
-        function getWidenedType(type: Type): Type {
+        function getWidenedTypeWithContext(type: Type, context: WideningContext): Type {
             if (type.flags & TypeFlags.RequiresWidening) {
                 if (type.flags & TypeFlags.Nullable) {
                     return anyType;
                 }
-                if (getObjectFlags(type) & ObjectFlags.ObjectLiteral) {
-                    return getWidenedTypeOfObjectLiteral(type);
+                if (isObjectLiteralType(type)) {
+                    return getWidenedTypeOfObjectLiteral(type, context);
                 }
                 if (type.flags & TypeFlags.Union) {
-                    return getUnionType(sameMap((<UnionType>type).types, getWidenedConstituentType));
+                    const unionContext = context || createWideningContext(/*parent*/ undefined, /*propertyName*/ undefined, (<UnionType>type).types);
+                    const widenedTypes = sameMap((<UnionType>type).types, t => t.flags & TypeFlags.Nullable ? t : getWidenedTypeWithContext(t, unionContext));
+                    // Widening an empty object literal transitions from a highly restrictive type to
+                    // a highly inclusive one. For that reason we perform subtype reduction here if the
+                    // union includes empty object types (e.g. reducing {} | string to just {}).
+                    return getUnionType(widenedTypes, some(widenedTypes, isEmptyObjectType));
                 }
                 if (isArrayType(type) || isTupleType(type)) {
                     return createTypeReference((<TypeReference>type).target, sameMap((<TypeReference>type).typeArguments, getWidenedType));
@@ -10502,28 +10577,35 @@ namespace ts {
          */
         function reportWideningErrorsInType(type: Type): boolean {
             let errorReported = false;
-            if (type.flags & TypeFlags.Union) {
-                for (const t of (<UnionType>type).types) {
-                    if (reportWideningErrorsInType(t)) {
+            if (type.flags & TypeFlags.ContainsWideningType) {
+                if (type.flags & TypeFlags.Union) {
+                    if (some((<UnionType>type).types, isEmptyObjectType)) {
                         errorReported = true;
                     }
+                    else {
+                        for (const t of (<UnionType>type).types) {
+                            if (reportWideningErrorsInType(t)) {
+                                errorReported = true;
+                            }
+                        }
+                    }
                 }
-            }
-            if (isArrayType(type) || isTupleType(type)) {
-                for (const t of (<TypeReference>type).typeArguments) {
-                    if (reportWideningErrorsInType(t)) {
-                        errorReported = true;
+                if (isArrayType(type) || isTupleType(type)) {
+                    for (const t of (<TypeReference>type).typeArguments) {
+                        if (reportWideningErrorsInType(t)) {
+                            errorReported = true;
+                        }
                     }
                 }
-            }
-            if (getObjectFlags(type) & ObjectFlags.ObjectLiteral) {
-                for (const p of getPropertiesOfObjectType(type)) {
-                    const t = getTypeOfSymbol(p);
-                    if (t.flags & TypeFlags.ContainsWideningType) {
-                        if (!reportWideningErrorsInType(t)) {
-                            error(p.valueDeclaration, Diagnostics.Object_literal_s_property_0_implicitly_has_an_1_type, symbolName(p), typeToString(getWidenedType(t)));
+                if (isObjectLiteralType(type)) {
+                    for (const p of getPropertiesOfObjectType(type)) {
+                        const t = getTypeOfSymbol(p);
+                        if (t.flags & TypeFlags.ContainsWideningType) {
+                            if (!reportWideningErrorsInType(t)) {
+                                error(p.valueDeclaration, Diagnostics.Object_literal_s_property_0_implicitly_has_an_1_type, symbolName(p), typeToString(getWidenedType(t)));
+                            }
+                            errorReported = true;
                         }
-                        errorReported = true;
                     }
                 }
             }
@@ -11029,11 +11111,28 @@ namespace ts {
             return constraint && maybeTypeOfKind(constraint, TypeFlags.Primitive | TypeFlags.Index);
         }
 
+        function isObjectLiteralType(type: Type) {
+            return !!(getObjectFlags(type) & ObjectFlags.ObjectLiteral);
+        }
+
+        function widenObjectLiteralCandidates(candidates: Type[]): Type[] {
+            if (candidates.length > 1) {
+                const objectLiterals = filter(candidates, isObjectLiteralType);
+                if (objectLiterals.length) {
+                    const objectLiteralsType = getWidenedType(getUnionType(objectLiterals, /*subtypeReduction*/ true));
+                    return concatenate(filter(candidates, t => !isObjectLiteralType(t)), [objectLiteralsType]);
+                }
+            }
+            return candidates;
+        }
+
         function getInferredType(context: InferenceContext, index: number): Type {
             const inference = context.inferences[index];
             let inferredType = inference.inferredType;
             if (!inferredType) {
                 if (inference.candidates) {
+                    // Extract all object literal types and replace them with a single widened and normalized type.
+                    const candidates = widenObjectLiteralCandidates(inference.candidates);
                     // We widen inferred literal types if
                     // all inferences were made to top-level ocurrences of the type parameter, and
                     // the type parameter has no constraint or its constraint includes no primitive or literal types, and
@@ -11042,7 +11141,7 @@ namespace ts {
                     const widenLiteralTypes = inference.topLevel &&
                         !hasPrimitiveConstraint(inference.typeParameter) &&
                         (inference.isFixed || !isTypeParameterAtTopLevel(getReturnTypeOfSignature(signature), inference.typeParameter));
-                    const baseCandidates = widenLiteralTypes ? sameMap(inference.candidates, getWidenedLiteralType) : inference.candidates;
+                    const baseCandidates = widenLiteralTypes ? sameMap(candidates, getWidenedLiteralType) : candidates;
                     // If all inferences were made from contravariant positions, infer a common subtype. Otherwise, if
                     // union types were requested or if all inferences were made from the return type position, infer a
                     // union type. Otherwise, infer a common supertype.
 
@@ -3327,6 +3327,7 @@ namespace ts {
         ObjectLiteral    = 1 << 7,  // Originates in an object literal
         EvolvingArray    = 1 << 8,  // Evolving array type
         ObjectLiteralPatternWithComputedProperties = 1 << 9,  // Object literal pattern with computed properties
+        ContainsSpread   = 1 << 10, // Object literal contains spread operation
         ClassOrInterface = Class | Interface
     }
 
@@ -3609,6 +3610,14 @@ namespace ts {
         compareTypes: TypeComparer;         // Type comparer function
     }
 
+    /* @internal */
+    export interface WideningContext {
+        parent?: WideningContext;            // Parent context
+        propertyName?: __String;             // Name of property in parent
+        siblings?: Type[];                   // Types of siblings
+        resolvedPropertyNames?: __String[];  // Property names occurring in sibling object literals
+    }
+
     /* @internal */
     export const enum SpecialPropertyAssignmentKind {
         None,
 
@@ -2040,6 +2040,7 @@ declare namespace ts {
         ObjectLiteral = 128,
         EvolvingArray = 256,
         ObjectLiteralPatternWithComputedProperties = 512,
+        ContainsSpread = 1024,
         ClassOrInterface = 3,
     }
     interface ObjectType extends Type {
 
@@ -2040,6 +2040,7 @@ declare namespace ts {
         ObjectLiteral = 128,
         EvolvingArray = 256,
         ObjectLiteralPatternWithComputedProperties = 512,
+        ContainsSpread = 1024,
         ClassOrInterface = 3,
     }
     interface ObjectType extends Type {
 
@@ -4,11 +4,12 @@ tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclarati
 tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts(1,33): error TS2304: Cannot find name 'await'.
 tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts(1,38): error TS1005: ';' expected.
 tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts(1,39): error TS1128: Declaration or statement expected.
+tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts(1,41): error TS2365: Operator '>' cannot be applied to types 'boolean' and '{}'.
 tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts(1,49): error TS2532: Object is possibly 'undefined'.
 tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts(1,53): error TS1109: Expression expected.
 
 
-==== tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts (8 errors) ====
+==== tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclaration10_es2017.ts (9 errors) ====
     async function foo(a = await => await): Promise<void> {
                        ~~~~~~~~~
 !!! error TS2371: A parameter initializer is only allowed in a function or constructor implementation.
@@ -22,8 +23,11 @@ tests/cases/conformance/async/es2017/functionDeclarations/asyncFunctionDeclarati
 !!! error TS1005: ';' expected.
                                           ~
 !!! error TS1128: Declaration or statement expected.
+                                            ~~~~~~~~~~~~~~~
                                                     ~~~~
 !!! error TS2532: Object is possibly 'undefined'.
                                                         ~
 !!! error TS1109: Expression expected.
-    }
+    }
+    ~
+!!! error TS2365: Operator '>' cannot be applied to types 'boolean' and '{}'.
Original file line number	Diff line number	Diff line change
`@@ -2040,6 +2040,7 @@ declare namespace ts {`
`2040`	`2040`	`ObjectLiteral = 128,`
`2041`	`2041`	`EvolvingArray = 256,`
`2042`	`2042`	`ObjectLiteralPatternWithComputedProperties = 512,`
	`2043`	`+ ContainsSpread = 1024,`
`2043`	`2044`	`ClassOrInterface = 3,`
`2044`	`2045`	`}`
`2045`	`2046`	`interface ObjectType extends Type {`