Improve typing of && operator with --strictNullChecks

Author: ahejlsberg

src/compiler/checker.ts

@@ -2844,7 +2844,7 @@ namespace ts {
             }
             // In strict null checking mode, if a default value of a non-undefined type is specified, remove
             // undefined from the final type.
-            if (strictNullChecks && declaration.initializer && !(getNullableKind(checkExpressionCached(declaration.initializer)) & TypeFlags.Undefined)) {
+            if (strictNullChecks && declaration.initializer && !(getCombinedTypeFlags(checkExpressionCached(declaration.initializer)) & TypeFlags.Undefined)) {
                 type = getTypeWithFacts(type, TypeFacts.NEUndefined);
             }
             return type;
@@ -2887,7 +2887,7 @@ namespace ts {
         }
 
         function addOptionality(type: Type, optional: boolean): Type {
-            return strictNullChecks && optional ? addNullableKind(type, TypeFlags.Undefined) : type;
+            return strictNullChecks && optional ? addTypeKind(type, TypeFlags.Undefined) : type;
         }
 
         // Return the inferred type for a variable, parameter, or property declaration
@@ -3222,7 +3222,7 @@ namespace ts {
             if (!links.type) {
                 const type = createObjectType(TypeFlags.Anonymous, symbol);
                 links.type = strictNullChecks && symbol.flags & SymbolFlags.Optional ?
-                    addNullableKind(type, TypeFlags.Undefined) : type;
+                    addTypeKind(type, TypeFlags.Undefined) : type;
             }
             return links.type;
         }
@@ -6746,7 +6746,7 @@ namespace ts {
                 return getUnionType(types);
             }
             const supertype = forEach(primaryTypes, t => isSupertypeOfEach(t, primaryTypes) ? t : undefined);
-            return supertype && addNullableKind(supertype, getCombinedFlagsOfTypes(types) & TypeFlags.Nullable);
+            return supertype && addTypeKind(supertype, getCombinedFlagsOfTypes(types) & TypeFlags.Nullable);
         }
 
         function reportNoCommonSupertypeError(types: Type[], errorLocation: Node, errorMessageChainHead: DiagnosticMessageChain): void {
@@ -6817,28 +6817,22 @@ namespace ts {
             return !!(type.flags & TypeFlags.Tuple);
         }
 
-        function getNullableKind(type: Type): TypeFlags {
-            let flags = type.flags;
-            if (flags & TypeFlags.Union) {
-                for (const t of (type as UnionType).types) {
-                    flags |= t.flags;
-                }
-            }
-            return flags & TypeFlags.Nullable;
+        function getCombinedTypeFlags(type: Type): TypeFlags {
+            return type.flags & TypeFlags.Union ? getCombinedFlagsOfTypes((<UnionType>type).types) : type.flags;
         }
 
-        function addNullableKind(type: Type, kind: TypeFlags): Type {
-            if ((getNullableKind(type) & kind) !== kind) {
-                const types = [type];
-                if (kind & TypeFlags.Undefined) {
-                    types.push(undefinedType);
-                }
-                if (kind & TypeFlags.Null) {
-                    types.push(nullType);
-                }
-                type = getUnionType(types);
+        function addTypeKind(type: Type, kind: TypeFlags) {
+            if ((getCombinedTypeFlags(type) & kind) === kind) {
+                return type;
             }
-            return type;
+            const types = [type];
+            if (kind & TypeFlags.String) types.push(stringType);
+            if (kind & TypeFlags.Number) types.push(numberType);
+            if (kind & TypeFlags.Boolean) types.push(booleanType);
+            if (kind & TypeFlags.Void) types.push(voidType);
+            if (kind & TypeFlags.Undefined) types.push(undefinedType);
+            if (kind & TypeFlags.Null) types.push(nullType);
+            return getUnionType(types);
         }
 
         function getNonNullableType(type: Type): Type {
@@ -7667,7 +7661,7 @@ namespace ts {
             if (!reference.flowNode || assumeInitialized && !(declaredType.flags & TypeFlags.Narrowable)) {
                 return declaredType;
             }
-            const initialType = assumeInitialized ? declaredType : addNullableKind(declaredType, TypeFlags.Undefined);
+            const initialType = assumeInitialized ? declaredType : addTypeKind(declaredType, TypeFlags.Undefined);
             const visitedFlowStart = visitedFlowCount;
             const result = getTypeAtFlowNode(reference.flowNode);
             visitedFlowCount = visitedFlowStart;
@@ -8163,7 +8157,7 @@ namespace ts {
                 getRootDeclaration(declaration).kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
                 !isDeclarationIncludedInFlow(node, declaration, includeOuterFunctions);
             const flowType = getFlowTypeOfReference(node, type, assumeInitialized, includeOuterFunctions);
-            if (!assumeInitialized && !(getNullableKind(type) & TypeFlags.Undefined) && getNullableKind(flowType) & TypeFlags.Undefined) {
+            if (!assumeInitialized && !(getCombinedTypeFlags(type) & TypeFlags.Undefined) && getCombinedTypeFlags(flowType) & TypeFlags.Undefined) {
                 error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                 // Return the declared type to reduce follow-on errors
                 return type;
@@ -9945,7 +9939,7 @@ namespace ts {
         function checkNonNullExpression(node: Expression | QualifiedName) {
             const type = checkExpression(node);
             if (strictNullChecks) {
-                const kind = getNullableKind(type);
+                const kind = getCombinedTypeFlags(type) & TypeFlags.Nullable;
                 if (kind) {
                     error(node, kind & TypeFlags.Undefined ? kind & TypeFlags.Null ?
                         Diagnostics.Object_is_possibly_null_or_undefined :
@@ -11485,7 +11479,7 @@ namespace ts {
             if (strictNullChecks) {
                 const declaration = symbol.valueDeclaration;
                 if (declaration && (<VariableLikeDeclaration>declaration).initializer) {
-                    return addNullableKind(type, TypeFlags.Undefined);
+                    return addTypeKind(type, TypeFlags.Undefined);
                 }
             }
             return type;
@@ -12411,7 +12405,7 @@ namespace ts {
                 case SyntaxKind.InKeyword:
                     return checkInExpression(left, right, leftType, rightType);
                 case SyntaxKind.AmpersandAmpersandToken:
-                    return strictNullChecks ? addNullableKind(rightType, getNullableKind(leftType)) : rightType;
+                    return strictNullChecks ? addTypeKind(rightType, getCombinedTypeFlags(leftType) & TypeFlags.Falsy) : rightType;
                 case SyntaxKind.BarBarToken:
                     return getUnionType([getNonNullableType(leftType), rightType]);
                 case SyntaxKind.EqualsToken:

src/compiler/types.ts

@@ -2209,6 +2209,7 @@ namespace ts {
 
         /* @internal */
         Nullable = Undefined | Null,
+        Falsy = String | Number | Boolean | Void | Undefined | Null,
         /* @internal */
         Intrinsic = Any | String | Number | Boolean | ESSymbol | Void | Undefined | Null | Never,
         /* @internal */