Stricter check for discriminant properties in type guards

Author: ahejlsberg

src/compiler/checker.ts

@@ -4416,10 +4416,19 @@ namespace ts {
             }
             const propTypes: Type[] = [];
             const declarations: Declaration[] = [];
+            let commonType: Type = undefined;
+            let hasCommonType = true;
             for (const prop of props) {
                 if (prop.declarations) {
                     addRange(declarations, prop.declarations);
                 }
+                const type = getTypeOfSymbol(prop);
+                if (!commonType) {
+                    commonType = type;
+                }
+                else if (type !== commonType) {
+                    hasCommonType = false;
+                }
                 propTypes.push(getTypeOfSymbol(prop));
             }
             const result = <TransientSymbol>createSymbol(
@@ -4429,6 +4438,7 @@ namespace ts {
                 commonFlags,
                 name);
             result.containingType = containingType;
+            result.hasCommonType = hasCommonType;
             result.declarations = declarations;
             result.isReadonly = isReadonly;
             result.type = containingType.flags & TypeFlags.Union ? getUnionType(propTypes) : getIntersectionType(propTypes);
@@ -7793,8 +7803,39 @@ namespace ts {
             return false;
         }
 
-        function rootContainsMatchingReference(source: Node, target: Node) {
-            return target.kind === SyntaxKind.PropertyAccessExpression && containsMatchingReference(source, (<PropertyAccessExpression>target).expression);
+        // Return true if target is a property access xxx.yyy, source is a property access xxx.zzz, the declared
+        // type of xxx is a union type, and yyy is a property that is possibly a discriminant. We consider a property
+        // a possible discriminant if its type differs in the constituents of containing union type, and if every
+        // choice is a unit type or a union of unit types.
+        function containsMatchingReferenceDiscriminant(source: Node, target: Node) {
+            return target.kind === SyntaxKind.PropertyAccessExpression &&
+                containsMatchingReference(source, (<PropertyAccessExpression>target).expression) &&
+                isDiscriminantProperty(getDeclaredTypeOfReference((<PropertyAccessExpression>target).expression), (<PropertyAccessExpression>target).name.text);
+        }
+
+        function getDeclaredTypeOfReference(expr: Node): Type {
+            if (expr.kind === SyntaxKind.Identifier) {
+                return getTypeOfSymbol(getResolvedSymbol(<Identifier>expr));
+            }
+            if (expr.kind === SyntaxKind.PropertyAccessExpression) {
+                const type = getDeclaredTypeOfReference((<PropertyAccessExpression>expr).expression);
+                return type && getTypeOfPropertyOfType(type, (<PropertyAccessExpression>expr).name.text);
+            }
+            return undefined;
+        }
+
+        function isDiscriminantProperty(type: Type, name: string) {
+            if (type && type.flags & TypeFlags.Union) {
+                const prop = getPropertyOfType(type, name);
+                if (prop && prop.flags & SymbolFlags.SyntheticProperty) {
+                    if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
+                        (<TransientSymbol>prop).isDiscriminantProperty = !(<TransientSymbol>prop).hasCommonType &&
+                            isUnitUnionType(getTypeOfSymbol(prop));
+                    }
+                    return (<TransientSymbol>prop).isDiscriminantProperty;
+                }
+            }
+            return false;
         }
 
         function isOrContainsMatchingReference(source: Node, target: Node) {
@@ -8222,7 +8263,7 @@ namespace ts {
                 if (isMatchingReference(reference, expr)) {
                     type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                 }
-                else if (isMatchingPropertyAccess(expr)) {
+                else if (isMatchingReferenceDiscriminant(expr)) {
                     type = narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                 }
                 return createFlowType(type, isIncomplete(flowType));
@@ -8300,10 +8341,11 @@ namespace ts {
                 return cache[key] = getUnionType(antecedentTypes);
             }
 
-            function isMatchingPropertyAccess(expr: Expression) {
+            function isMatchingReferenceDiscriminant(expr: Expression) {
                 return expr.kind === SyntaxKind.PropertyAccessExpression &&
+                    declaredType.flags & TypeFlags.Union &&
                     isMatchingReference(reference, (<PropertyAccessExpression>expr).expression) &&
-                    (declaredType.flags & TypeFlags.Union) !== 0;
+                    isDiscriminantProperty(declaredType, (<PropertyAccessExpression>expr).name.text);
             }
 
             function narrowTypeByDiscriminant(type: Type, propAccess: PropertyAccessExpression, narrowType: (t: Type) => Type): Type {
@@ -8317,10 +8359,10 @@ namespace ts {
                 if (isMatchingReference(reference, expr)) {
                     return getTypeWithFacts(type, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy);
                 }
-                if (isMatchingPropertyAccess(expr)) {
+                if (isMatchingReferenceDiscriminant(expr)) {
                     return narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
                 }
-                if (rootContainsMatchingReference(reference, expr)) {
+                if (containsMatchingReferenceDiscriminant(reference, expr)) {
                     return declaredType;
                 }
                 return type;
@@ -8349,13 +8391,13 @@ namespace ts {
                         if (isMatchingReference(reference, right)) {
                             return narrowTypeByEquality(type, operator, left, assumeTrue);
                         }
-                        if (isMatchingPropertyAccess(left)) {
+                        if (isMatchingReferenceDiscriminant(left)) {
                             return narrowTypeByDiscriminant(type, <PropertyAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
                         }
-                        if (isMatchingPropertyAccess(right)) {
+                        if (isMatchingReferenceDiscriminant(right)) {
                             return narrowTypeByDiscriminant(type, <PropertyAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
                         }
-                        if (rootContainsMatchingReference(reference, left) || rootContainsMatchingReference(reference, right)) {
+                        if (containsMatchingReferenceDiscriminant(reference, left) || containsMatchingReferenceDiscriminant(reference, right)) {
                             return declaredType;
                         }
                         break;

src/compiler/types.ts

@@ -2160,6 +2160,8 @@ namespace ts {
         mapper?: TypeMapper;                // Type mapper for instantiation alias
         referenced?: boolean;               // True if alias symbol has been referenced as a value
         containingType?: UnionOrIntersectionType; // Containing union or intersection type for synthetic property
+        hasCommonType?: boolean;            // True if constituents of synthetic property all have same type
+        isDiscriminantProperty?: boolean;   // True if discriminant synthetic property
         resolvedExports?: SymbolTable;      // Resolved exports of module
         exportsChecked?: boolean;           // True if exports of external module have been checked
         isDeclarationWithCollidingName?: boolean;    // True if symbol is block scoped redeclaration