getIndexedAccessType for assignment target uses intersections

Author: ahejlsberg

src/compiler/checker.ts

@@ -9875,7 +9875,7 @@ namespace ts {
             return false;
         }
 
-        function getPropertyTypeForIndexType(objectType: Type, indexType: Type, accessNode: ElementAccessExpression | IndexedAccessTypeNode | PropertyName | BindingName | SyntheticExpression | undefined, cacheSymbol: boolean, missingType: Type) {
+        function getPropertyTypeForIndexType(originalObjectType: Type, objectType: Type, indexType: Type, accessNode: ElementAccessExpression | IndexedAccessTypeNode | PropertyName | BindingName | SyntheticExpression | undefined, cacheSymbol: boolean, missingType: Type) {
             const accessExpression = accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode : undefined;
             const propName = isTypeUsableAsPropertyName(indexType) ?
                 getPropertyNameFromType(indexType) :
@@ -9898,7 +9898,6 @@ namespace ts {
                             getNodeLinks(accessNode!).resolvedSymbol = prop;
                         }
                     }
-
                     const propType = getTypeOfSymbol(prop);
                     return accessExpression && getAssignmentTargetKind(accessExpression) !== AssignmentKind.Definite ?
                         getFlowTypeOfReference(accessExpression, propType) :
@@ -9922,7 +9921,8 @@ namespace ts {
                 if (objectType.flags & (TypeFlags.Any | TypeFlags.Never)) {
                     return objectType;
                 }
-                const indexInfo = isTypeAssignableToKind(indexType, TypeFlags.NumberLike) && getIndexInfoOfType(objectType, IndexKind.Number) ||
+                const indexInfo = accessExpression && isAssignmentTarget(accessExpression) && isGenericObjectType(originalObjectType) ? undefined :
+                    isTypeAssignableToKind(indexType, TypeFlags.NumberLike) && getIndexInfoOfType(objectType, IndexKind.Number) ||
                     getIndexInfoOfType(objectType, IndexKind.String) ||
                     undefined;
                 if (indexInfo) {
@@ -10105,7 +10105,7 @@ namespace ts {
                 const propTypes: Type[] = [];
                 let wasMissingProp = false;
                 for (const t of (<UnionType>indexType).types) {
-                    const propType = getPropertyTypeForIndexType(apparentObjectType, t, accessNode, /*cacheSymbol*/ false, missingType);
+                    const propType = getPropertyTypeForIndexType(objectType, apparentObjectType, t, accessNode, /*cacheSymbol*/ false, missingType);
                     if (propType === missingType) {
                         if (!accessNode) {
                             // If there's no error node, we can immeditely stop, since error reporting is off
@@ -10121,9 +10121,9 @@ namespace ts {
                 if (wasMissingProp) {
                     return missingType;
                 }
-                return getUnionType(propTypes);
+                return !accessNode || getAssignmentTargetKind(accessNode) === AssignmentKind.None ? getUnionType(propTypes) : getIntersectionType(propTypes);
             }
-            return getPropertyTypeForIndexType(apparentObjectType, indexType, accessNode, /*cacheSymbol*/ true, missingType);
+            return getPropertyTypeForIndexType(objectType, apparentObjectType, indexType, accessNode, /*cacheSymbol*/ true, missingType);
         }
 
         function getTypeFromIndexedAccessTypeNode(node: IndexedAccessTypeNode) {
@@ -19570,7 +19570,7 @@ namespace ts {
                 markAliasReferenced(parentSymbol, node);
             }
             if (!prop) {
-                const indexInfo = getIndexInfoOfType(apparentType, IndexKind.String);
+                const indexInfo = assignmentKind === AssignmentKind.None || !isGenericObjectType(leftType) ? getIndexInfoOfType(apparentType, IndexKind.String) : undefined;
                 if (!(indexInfo && indexInfo.type)) {
                     if (isJSLiteralType(leftType)) {
                         return anyType;