Conditional types

src/compiler/binder.ts

@@ -3440,6 +3440,7 @@ namespace ts {
             case SyntaxKind.TupleType:
             case SyntaxKind.UnionType:
             case SyntaxKind.IntersectionType:
+            case SyntaxKind.ConditionalType:
             case SyntaxKind.ParenthesizedType:
             case SyntaxKind.InterfaceDeclaration:
             case SyntaxKind.TypeAliasDeclaration:

src/compiler/core.ts

@@ -426,23 +426,21 @@ namespace ts {
     export function sameMap<T>(array: T[], f: (x: T, i: number) => T): T[];
     export function sameMap<T>(array: ReadonlyArray<T>, f: (x: T, i: number) => T): ReadonlyArray<T>;
     export function sameMap<T>(array: T[], f: (x: T, i: number) => T): T[] {
-        let result: T[];
         if (array) {
             for (let i = 0; i < array.length; i++) {
-                if (result) {
-                    result.push(f(array[i], i));
-                }
-                else {
-                    const item = array[i];
-                    const mapped = f(item, i);
-                    if (item !== mapped) {
-                        result = array.slice(0, i);
-                        result.push(mapped);
+                const item = array[i];
+                const mapped = f(item, i);
+                if (item !== mapped) {
+                    const result = array.slice(0, i);
+                    result.push(mapped);
+                    for (i++; i < array.length; i++) {
+                        result.push(f(array[i], i));
                     }
+                    return result;
                 }
             }
         }
-        return result || array;
+        return array;
     }
 
     /**

src/compiler/declarationEmitter.ts

@@ -450,6 +450,8 @@ namespace ts {
                     return emitUnionType(<UnionTypeNode>type);
                 case SyntaxKind.IntersectionType:
                     return emitIntersectionType(<IntersectionTypeNode>type);
+                case SyntaxKind.ConditionalType:
+                    return emitConditionalType(<ConditionalTypeNode>type);
                 case SyntaxKind.ParenthesizedType:
                     return emitParenType(<ParenthesizedTypeNode>type);
                 case SyntaxKind.TypeOperator:
@@ -545,6 +547,16 @@ namespace ts {
                 emitSeparatedList(type.types, " & ", emitType);
             }
 
+            function emitConditionalType(node: ConditionalTypeNode) {
+                emitType(node.checkType);
+                write(" extends ");
+                emitType(node.extendsType);
+                write(" ? ");
+                emitType(node.trueType);
+                write(" : ");
+                emitType(node.falseType);
+            }
+
             function emitParenType(type: ParenthesizedTypeNode) {
                 write("(");
                 emitType(type.type);

src/compiler/emitter.ts

@@ -600,6 +600,8 @@ namespace ts {
                     return emitUnionType(<UnionTypeNode>node);
                 case SyntaxKind.IntersectionType:
                     return emitIntersectionType(<IntersectionTypeNode>node);
+                case SyntaxKind.ConditionalType:
+                    return emitConditionalType(<ConditionalTypeNode>node);
                 case SyntaxKind.ParenthesizedType:
                     return emitParenthesizedType(<ParenthesizedTypeNode>node);
                 case SyntaxKind.ExpressionWithTypeArguments:
@@ -1190,6 +1192,16 @@ namespace ts {
             emitList(node, node.types, ListFormat.IntersectionTypeConstituents);
         }
 
+        function emitConditionalType(node: ConditionalTypeNode) {
+            emit(node.checkType);
+            write(" extends ");
+            emit(node.extendsType);
+            write(" ? ");
+            emit(node.trueType);
+            write(" : ");
+            emit(node.falseType);
+        }
+
         function emitParenthesizedType(node: ParenthesizedTypeNode) {
             writePunctuation("(");
             emit(node.type);

src/compiler/factory.ts

@@ -729,6 +729,24 @@ namespace ts {
             : node;
     }
 
+    export function createConditionalTypeNode(checkType: TypeNode, extendsType: TypeNode, trueType: TypeNode, falseType: TypeNode) {
+        const node = createSynthesizedNode(SyntaxKind.ConditionalType) as ConditionalTypeNode;
+        node.checkType = parenthesizeConditionalTypeMember(checkType);
+        node.extendsType = parenthesizeConditionalTypeMember(extendsType);
+        node.trueType = trueType;
+        node.falseType = falseType;
+        return node;
+    }
+
+    export function updateConditionalTypeNode(node: ConditionalTypeNode, checkType: TypeNode, extendsType: TypeNode, trueType: TypeNode, falseType: TypeNode) {
+        return node.checkType !== checkType
+            || node.extendsType !== extendsType
+            || node.trueType !== trueType
+            || node.falseType !== falseType
+            ? updateNode(createConditionalTypeNode(checkType, extendsType, trueType, falseType), node)
+            : node;
+    }
+
     export function createParenthesizedType(type: TypeNode) {
         const node = <ParenthesizedTypeNode>createSynthesizedNode(SyntaxKind.ParenthesizedType);
         node.type = type;
@@ -4092,6 +4110,10 @@ namespace ts {
         return expression;
     }
 
+    export function parenthesizeConditionalTypeMember(member: TypeNode) {
+        return member.kind === SyntaxKind.ConditionalType ? createParenthesizedType(member) : member;
+    }
+
     export function parenthesizeElementTypeMember(member: TypeNode) {
         switch (member.kind) {
             case SyntaxKind.UnionType:
@@ -4100,7 +4122,7 @@ namespace ts {
             case SyntaxKind.ConstructorType:
                 return createParenthesizedType(member);
         }
-        return member;
+        return parenthesizeConditionalTypeMember(member);
     }
 
     export function parenthesizeArrayTypeMember(member: TypeNode) {

src/compiler/parser.ts

@@ -175,6 +175,11 @@ namespace ts {
             case SyntaxKind.UnionType:
             case SyntaxKind.IntersectionType:
                 return visitNodes(cbNode, cbNodes, (<UnionOrIntersectionTypeNode>node).types);
+            case SyntaxKind.ConditionalType:
+                return visitNode(cbNode, (<ConditionalTypeNode>node).checkType) ||
+                    visitNode(cbNode, (<ConditionalTypeNode>node).extendsType) ||
+                    visitNode(cbNode, (<ConditionalTypeNode>node).trueType) ||
+                    visitNode(cbNode, (<ConditionalTypeNode>node).falseType);
             case SyntaxKind.ParenthesizedType:
             case SyntaxKind.TypeOperator:
                 return visitNode(cbNode, (<ParenthesizedTypeNode | TypeOperatorNode>node).type);
@@ -1494,6 +1499,11 @@ namespace ts {
             return isStartOfExpression();
         }
 
+        function nextTokenIsStartOfType() {
+            nextToken();
+            return isStartOfType();
+        }
+
         // True if positioned at a list terminator
         function isListTerminator(kind: ParsingContext): boolean {
             if (token() === SyntaxKind.EndOfFileToken) {
@@ -2789,6 +2799,10 @@ namespace ts {
                         type = createJSDocPostfixType(SyntaxKind.JSDocNonNullableType, type);
                         break;
                     case SyntaxKind.QuestionToken:
+                        // If not in JSDoc and next token is start of a type we have a conditional type
+                        if (!(contextFlags & NodeFlags.JSDoc) && lookAhead(nextTokenIsStartOfType)) {
+                            return type;
+                        }
                         type = createJSDocPostfixType(SyntaxKind.JSDocNullableType, type);
                         break;
                     case SyntaxKind.OpenBracketToken:
@@ -2950,14 +2964,26 @@ namespace ts {
             return doOutsideOfContext(NodeFlags.TypeExcludesFlags, parseTypeWorker);
         }
 
-        function parseTypeWorker(): TypeNode {
+        function parseTypeWorker(noConditionalTypes?: boolean): TypeNode {
             if (isStartOfFunctionType()) {
                 return parseFunctionOrConstructorType(SyntaxKind.FunctionType);
             }
             if (token() === SyntaxKind.NewKeyword) {
                 return parseFunctionOrConstructorType(SyntaxKind.ConstructorType);
             }
-            return parseUnionTypeOrHigher();
+            const type = parseUnionTypeOrHigher();
+            if (!noConditionalTypes && parseOptional(SyntaxKind.ExtendsKeyword)) {
+                const node = <ConditionalTypeNode>createNode(SyntaxKind.ConditionalType, type.pos);
+                node.checkType = type;
+                // The type following 'extends' is not permitted to be another conditional type
+                node.extendsType = parseTypeWorker(/*noConditionalTypes*/ true);
+                parseExpected(SyntaxKind.QuestionToken);
+                node.trueType = parseTypeWorker();
+                parseExpected(SyntaxKind.ColonToken);
+                node.falseType = parseTypeWorker();
+                return finishNode(node);
+            }
+            return type;
         }
 
         function parseTypeAnnotation(): TypeNode {

src/compiler/transformers/ts.ts

@@ -385,6 +385,7 @@ namespace ts {
                 case SyntaxKind.TypeReference:
                 case SyntaxKind.UnionType:
                 case SyntaxKind.IntersectionType:
+                case SyntaxKind.ConditionalType:
                 case SyntaxKind.ParenthesizedType:
                 case SyntaxKind.ThisType:
                 case SyntaxKind.TypeOperator:

src/compiler/types.ts

@@ -265,6 +265,7 @@ namespace ts {
         TupleType,
         UnionType,
         IntersectionType,
+        ConditionalType,
         ParenthesizedType,
         ThisType,
         TypeOperator,
@@ -1116,6 +1117,14 @@ namespace ts {
         types: NodeArray<TypeNode>;
     }
 
+    export interface ConditionalTypeNode extends TypeNode {
+        kind: SyntaxKind.ConditionalType;
+        checkType: TypeNode;
+        extendsType: TypeNode;
+        trueType: TypeNode;
+        falseType: TypeNode;
+    }
+
     export interface ParenthesizedTypeNode extends TypeNode {
         kind: SyntaxKind.ParenthesizedType;
         type: TypeNode;
@@ -3478,18 +3487,19 @@ namespace ts {
         Intersection            = 1 << 18,  // Intersection (T & U)
         Index                   = 1 << 19,  // keyof T
         IndexedAccess           = 1 << 20,  // T[K]
+        Conditional             = 1 << 21,  // T extends U ? X : Y
+        Substitution            = 1 << 22,  // Type parameter substitution
         /* @internal */
-        FreshLiteral            = 1 << 21,  // Fresh literal or unique type
+        FreshLiteral            = 1 << 23,  // Fresh literal or unique type
         /* @internal */
-        ContainsWideningType    = 1 << 22,  // Type is or contains undefined or null widening type
+        ContainsWideningType    = 1 << 24,  // Type is or contains undefined or null widening type
         /* @internal */
-        ContainsObjectLiteral   = 1 << 23,  // Type is or contains object literal type
+        ContainsObjectLiteral   = 1 << 25,  // Type is or contains object literal type
         /* @internal */
-        ContainsAnyFunctionType = 1 << 24,  // Type is or contains the anyFunctionType
-        NonPrimitive            = 1 << 25,  // intrinsic object type
+        ContainsAnyFunctionType = 1 << 26,  // Type is or contains the anyFunctionType
+        NonPrimitive            = 1 << 27,  // intrinsic object type
         /* @internal */
-        JsxAttributes           = 1 << 26,  // Jsx attributes type
-        MarkerType              = 1 << 27,  // Marker type used for variance probing
+        GenericMappedType       = 1 << 29,  // Flag used by maybeTypeOfKind
 
         /* @internal */
         Nullable = Undefined | Null,
@@ -3512,17 +3522,21 @@ namespace ts {
         ESSymbolLike = ESSymbol | UniqueESSymbol,
         UnionOrIntersection = Union | Intersection,
         StructuredType = Object | Union | Intersection,
-        StructuredOrTypeVariable = StructuredType | TypeParameter | Index | IndexedAccess,
         TypeVariable = TypeParameter | IndexedAccess,
+        InstantiableNonPrimitive = TypeVariable | Conditional | Substitution,
+        InstantiablePrimitive = Index,
+        Instantiable = InstantiableNonPrimitive | InstantiablePrimitive,
+        StructuredOrInstantiable = StructuredType | Instantiable,
 
         // 'Narrowable' types are types where narrowing actually narrows.
         // This *should* be every type other than null, undefined, void, and never
-        Narrowable = Any | StructuredType | TypeParameter | Index | IndexedAccess | StringLike | NumberLike | BooleanLike | ESSymbol | UniqueESSymbol | NonPrimitive,
+        Narrowable = Any | StructuredOrInstantiable | StringLike | NumberLike | BooleanLike | ESSymbol | UniqueESSymbol | NonPrimitive,
         NotUnionOrUnit = Any | ESSymbol | Object | NonPrimitive,
         /* @internal */
         RequiresWidening = ContainsWideningType | ContainsObjectLiteral,
         /* @internal */
         PropagatingFlags = ContainsWideningType | ContainsObjectLiteral | ContainsAnyFunctionType,
+        /* @internal */
     }
 
     export type DestructuringPattern = BindingPattern | ObjectLiteralExpression | ArrayLiteralExpression;
@@ -3581,7 +3595,9 @@ namespace ts {
         EvolvingArray    = 1 << 8,  // Evolving array type
         ObjectLiteralPatternWithComputedProperties = 1 << 9,  // Object literal pattern with computed properties
         ContainsSpread   = 1 << 10, // Object literal contains spread operation
-        ReverseMapped = 1 << 11,    // Object contains a property from a reverse-mapped type
+        ReverseMapped    = 1 << 11, // Object contains a property from a reverse-mapped type
+        JsxAttributes    = 1 << 12, // Jsx attributes type
+        MarkerType       = 1 << 13, // Marker type used for variance probing
         ClassOrInterface = Class | Interface
     }
 
@@ -3735,15 +3751,15 @@ namespace ts {
         syntheticType?: Type;
     }
 
-    export interface TypeVariable extends Type {
+    export interface InstantiableType extends Type {
         /* @internal */
         resolvedBaseConstraint?: Type;
         /* @internal */
         resolvedIndexType?: IndexType;
     }
 
     // Type parameters (TypeFlags.TypeParameter)
-    export interface TypeParameter extends TypeVariable {
+    export interface TypeParameter extends InstantiableType {
         /** Retrieve using getConstraintFromTypeParameter */
         /* @internal */
         constraint?: Type;        // Constraint
@@ -3761,15 +3777,38 @@ namespace ts {
 
     // Indexed access types (TypeFlags.IndexedAccess)
     // Possible forms are T[xxx], xxx[T], or xxx[keyof T], where T is a type variable
-    export interface IndexedAccessType extends TypeVariable {
+    export interface IndexedAccessType extends InstantiableType {
         objectType: Type;
         indexType: Type;
         constraint?: Type;
     }
 
     // keyof T types (TypeFlags.Index)
-    export interface IndexType extends Type {
-        type: TypeVariable | UnionOrIntersectionType;
+    export interface IndexType extends InstantiableType {
+        type: InstantiableType | UnionOrIntersectionType;
+    }
+
+    // T extends U ? X : Y (TypeFlags.Conditional)
+    export interface ConditionalType extends InstantiableType {
+        checkType: Type;
+        extendsType: Type;
+        trueType: Type;
+        falseType: Type;
+        /* @internal */
+        target?: ConditionalType;
+        /* @internal */
+        mapper?: TypeMapper;
+    }
+
+    // Type parameter substitution (TypeFlags.Substitution)
+    // Substitution types are created for type parameter references that occur in the true branch
+    // of a conditional type. For example, in 'T extends string ? Foo<T> : Bar<T>', the reference to
+    // T in Foo<T> is resolved as a substitution type that substitutes 'string & T' for T. Thus, if
+    // Foo has a 'string' constraint on its type parameter, T will satisfy it. Substitution types
+    // disappear upon instantiation (just like type parameters).
+    export interface SubstitutionType extends InstantiableType {
+        typeParameter: TypeParameter;  // Target type parameter
+        substitute: Type;              // Type to substitute for type parameter
     }
 
     export const enum SignatureKind {

src/compiler/utilities.ts

@@ -4555,6 +4555,10 @@ namespace ts {
         return node.kind === SyntaxKind.IntersectionType;
     }
 
+    export function isConditionalTypeNode(node: Node): node is ConditionalTypeNode {
+        return node.kind === SyntaxKind.ConditionalType;
+    }
+
     export function isParenthesizedTypeNode(node: Node): node is ParenthesizedTypeNode {
         return node.kind === SyntaxKind.ParenthesizedType;
     }

src/compiler/visitor.ts

@@ -387,6 +387,13 @@ namespace ts {
                 return updateIntersectionTypeNode(<IntersectionTypeNode>node,
                     nodesVisitor((<IntersectionTypeNode>node).types, visitor, isTypeNode));
 
+            case SyntaxKind.ConditionalType:
+                return updateConditionalTypeNode(<ConditionalTypeNode>node,
+                    visitNode((<ConditionalTypeNode>node).checkType, visitor, isTypeNode),
+                    visitNode((<ConditionalTypeNode>node).extendsType, visitor, isTypeNode),
+                    visitNode((<ConditionalTypeNode>node).trueType, visitor, isTypeNode),
+                    visitNode((<ConditionalTypeNode>node).falseType, visitor, isTypeNode));
+
             case SyntaxKind.ParenthesizedType:
                 return updateParenthesizedType(<ParenthesizedTypeNode>node,
                     visitNode((<ParenthesizedTypeNode>node).type, visitor, isTypeNode));