Defer indexed access type resolution

src/compiler/checker.ts

@@ -2583,10 +2583,8 @@ namespace ts {
                 }
 
                 function createTypeNodeFromObjectType(type: ObjectType): TypeNode {
-                    if (type.objectFlags & ObjectFlags.Mapped) {
-                        if (getConstraintTypeFromMappedType(<MappedType>type).flags & (TypeFlags.TypeParameter | TypeFlags.Index)) {
-                            return createMappedTypeNodeFromType(<MappedType>type);
-                        }
+                    if (isGenericMappedType(type)) {
+                        return createMappedTypeNodeFromType(<MappedType>type);
                     }
 
                     const resolved = resolveStructuredTypeMembers(type);
@@ -3489,11 +3487,9 @@ namespace ts {
                 }
 
                 function writeLiteralType(type: ObjectType, flags: TypeFormatFlags) {
-                    if (type.objectFlags & ObjectFlags.Mapped) {
-                        if (getConstraintTypeFromMappedType(<MappedType>type).flags & (TypeFlags.TypeParameter | TypeFlags.Index)) {
-                            writeMappedType(<MappedType>type);
-                            return;
-                        }
+                    if (isGenericMappedType(type)) {
+                        writeMappedType(<MappedType>type);
+                        return;
                     }
 
                     const resolved = resolveStructuredTypeMembers(type);
@@ -5792,8 +5788,7 @@ namespace ts {
         }
 
         function isGenericMappedType(type: Type) {
-            return getObjectFlags(type) & ObjectFlags.Mapped &&
-                maybeTypeOfKind(getConstraintTypeFromMappedType(<MappedType>type), TypeFlags.TypeVariable | TypeFlags.Index);
+            return getObjectFlags(type) & ObjectFlags.Mapped && isGenericIndexType(getConstraintTypeFromMappedType(<MappedType>type));
         }
 
         function resolveStructuredTypeMembers(type: StructuredType): ResolvedType {
@@ -5905,6 +5900,10 @@ namespace ts {
         }
 
         function getConstraintOfIndexedAccess(type: IndexedAccessType) {
+            const transformed = getTransformedIndexedAccessType(type);
+            if (transformed) {
+                return transformed;
+            }
             const baseObjectType = getBaseConstraintOfType(type.objectType);
             const baseIndexType = getBaseConstraintOfType(type.indexType);
             return baseObjectType || baseIndexType ? getIndexedAccessType(baseObjectType || type.objectType, baseIndexType || type.indexType) : undefined;
@@ -5976,11 +5975,18 @@ namespace ts {
                     return stringType;
                 }
                 if (t.flags & TypeFlags.IndexedAccess) {
+                    const transformed = getTransformedIndexedAccessType(<IndexedAccessType>t);
+                    if (transformed) {
+                        return getBaseConstraint(transformed);
+                    }
                     const baseObjectType = getBaseConstraint((<IndexedAccessType>t).objectType);
                     const baseIndexType = getBaseConstraint((<IndexedAccessType>t).indexType);
                     const baseIndexedAccess = baseObjectType && baseIndexType ? getIndexedAccessType(baseObjectType, baseIndexType) : undefined;
                     return baseIndexedAccess && baseIndexedAccess !== unknownType ? getBaseConstraint(baseIndexedAccess) : undefined;
                 }
+                if (isGenericMappedType(t)) {
+                    return emptyObjectType;
+                }
                 return t;
             }
         }
@@ -7602,26 +7608,73 @@ namespace ts {
             return instantiateType(getTemplateTypeFromMappedType(type), templateMapper);
         }
 
-        function getIndexedAccessType(objectType: Type, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode) {
-            // If the index type is generic, if the object type is generic and doesn't originate in an expression,
-            // or if the object type is a mapped type with a generic constraint, we are performing a higher-order
-            // index access where we cannot meaningfully access the properties of the object type. Note that for a
-            // generic T and a non-generic K, we eagerly resolve T[K] if it originates in an expression. This is to
-            // preserve backwards compatibility. For example, an element access 'this["foo"]' has always been resolved
-            // eagerly using the constraint type of 'this' at the given location.
-            if (maybeTypeOfKind(indexType, TypeFlags.TypeVariable | TypeFlags.Index) ||
-                maybeTypeOfKind(objectType, TypeFlags.TypeVariable) && !(accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression) ||
-                isGenericMappedType(objectType)) {
+        function isGenericObjectType(type: Type): boolean {
+            return type.flags & TypeFlags.TypeVariable ? true :
+                getObjectFlags(type) & ObjectFlags.Mapped ? isGenericIndexType(getConstraintTypeFromMappedType(<MappedType>type)) :
+                type.flags & TypeFlags.UnionOrIntersection ? forEach((<UnionOrIntersectionType>type).types, isGenericObjectType) :
+                false;
+        }
+
+        function isGenericIndexType(type: Type): boolean {
+            return type.flags & (TypeFlags.TypeVariable | TypeFlags.Index) ? true :
+                type.flags & TypeFlags.UnionOrIntersection ? forEach((<UnionOrIntersectionType>type).types, isGenericIndexType) :
+                false;
+        }
+
+        // Return true if the given type is a non-generic object type with a string index signature and no
+        // other members.
+        function isStringIndexOnlyType(type: Type) {
+            if (type.flags & TypeFlags.Object && !isGenericMappedType(type)) {
+                const t = resolveStructuredTypeMembers(<ObjectType>type);
+                return t.properties.length === 0 &&
+                    t.callSignatures.length === 0 && t.constructSignatures.length === 0 &&
+                    t.stringIndexInfo && !t.numberIndexInfo;
+            }
+            return false;
+        }
+
+        // Given an indexed access type T[K], if T is an intersection containing one or more generic types and one or
+        // more object types with only a string index signature, e.g. '(U & V & { [x: string]: D })[K]', return a
+        // transformed type of the form '(U & V)[K] | D'. This allows us to properly reason about higher order indexed
+        // access types with default property values as expressed by D.
+        function getTransformedIndexedAccessType(type: IndexedAccessType): Type {
+            const objectType = type.objectType;
+            if (objectType.flags & TypeFlags.Intersection && isGenericObjectType(objectType) && some((<IntersectionType>objectType).types, isStringIndexOnlyType)) {
+                const regularTypes: Type[] = [];
+                const stringIndexTypes: Type[] = [];
+                for (const t of (<IntersectionType>objectType).types) {
+                    if (isStringIndexOnlyType(t)) {
+                        stringIndexTypes.push(getIndexTypeOfType(t, IndexKind.String));
+                    }
+                    else {
+                        regularTypes.push(t);
+                    }
+                }
+                return getUnionType([
+                    getIndexedAccessType(getIntersectionType(regularTypes), type.indexType),
+                    getIntersectionType(stringIndexTypes)
+                ]);
+            }
+            return undefined;
+        }
+
+        function getIndexedAccessType(objectType: Type, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode): Type {
+            // If the object type is a mapped type { [P in K]: E }, where K is generic, we instantiate E using a mapper
+            // that substitutes the index type for P. For example, for an index access { [P in K]: Box<T[P]> }[X], we
+            // construct the type Box<T[X]>.
+            if (isGenericMappedType(objectType)) {
+                return getIndexedAccessForMappedType(<MappedType>objectType, indexType, accessNode);
+            }
+            // Otherwise, if the index type is generic, or if the object type is generic and doesn't originate in an
+            // expression, we are performing a higher-order index access where we cannot meaningfully access the properties
+            // of the object type. Note that for a generic T and a non-generic K, we eagerly resolve T[K] if it originates
+            // in an expression. This is to preserve backwards compatibility. For example, an element access 'this["foo"]'
+            // has always been resolved eagerly using the constraint type of 'this' at the given location.
+            if (isGenericIndexType(indexType) || !(accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression) && isGenericObjectType(objectType)) {
                 if (objectType.flags & TypeFlags.Any) {
                     return objectType;
                 }
-                // If the object type is a mapped type { [P in K]: E }, we instantiate E using a mapper that substitutes
-                // the index type for P. For example, for an index access { [P in K]: Box<T[P]> }[X], we construct the
-                // type Box<T[X]>.
-                if (isGenericMappedType(objectType)) {
-                    return getIndexedAccessForMappedType(<MappedType>objectType, indexType, accessNode);
-                }
-                // Otherwise we defer the operation by creating an indexed access type.
+                // Defer the operation by creating an indexed access type.
                 const id = objectType.id + "," + indexType.id;
                 let type = indexedAccessTypes.get(id);
                 if (!type) {
@@ -18657,6 +18710,8 @@ namespace ts {
         }
 
         function checkIndexedAccessType(node: IndexedAccessTypeNode) {
+            checkSourceElement(node.objectType);
+            checkSourceElement(node.indexType);
             checkIndexedAccessIndexType(getTypeFromIndexedAccessTypeNode(node), node);
         }
 

tests/baselines/reference/anyIndexedAccessArrayNoException.errors.txt

@@ -1,8 +1,11 @@
+tests/cases/compiler/anyIndexedAccessArrayNoException.ts(1,12): error TS1122: A tuple type element list cannot be empty.
 tests/cases/compiler/anyIndexedAccessArrayNoException.ts(1,12): error TS2538: Type '[]' cannot be used as an index type.
 
 
-==== tests/cases/compiler/anyIndexedAccessArrayNoException.ts (1 errors) ====
+==== tests/cases/compiler/anyIndexedAccessArrayNoException.ts (2 errors) ====
     var x: any[[]];
                ~~
+!!! error TS1122: A tuple type element list cannot be empty.
+               ~~
 !!! error TS2538: Type '[]' cannot be used as an index type.
 

tests/baselines/reference/deferredLookupTypeResolution.js

@@ -0,0 +1,64 @@
+//// [deferredLookupTypeResolution.ts]
+// Repro from #17456
+
+type StringContains<S extends string, L extends string> = (
+    { [K in S]:      'true' } &
+    { [key: string]: 'false' }
+  )[L]
+
+type ObjectHasKey<O, L extends string> = StringContains<keyof O, L>
+
+type First<T> = ObjectHasKey<T, '0'>;  // Should be deferred
+
+type T1 = ObjectHasKey<{ a: string }, 'a'>;  // 'true'
+type T2 = ObjectHasKey<{ a: string }, 'b'>;  // 'false'
+
+// Verify that mapped type isn't eagerly resolved in type-to-string operation
+
+declare function f1<A extends string, B extends string>(a: A, b: B): { [P in A | B]: any };
+
+function f2<A extends string>(a: A) {
+    return f1(a, 'x');
+}
+
+function f3(x: 'a' | 'b') {
+    return f2(x);
+}
+
+
+//// [deferredLookupTypeResolution.js]
+"use strict";
+// Repro from #17456
+function f2(a) {
+    return f1(a, 'x');
+}
+function f3(x) {
+    return f2(x);
+}
+
+
+//// [deferredLookupTypeResolution.d.ts]
+declare type StringContains<S extends string, L extends string> = ({
+    [K in S]: 'true';
+} & {
+    [key: string]: 'false';
+})[L];
+declare type ObjectHasKey<O, L extends string> = StringContains<keyof O, L>;
+declare type First<T> = ObjectHasKey<T, '0'>;
+declare type T1 = ObjectHasKey<{
+    a: string;
+}, 'a'>;
+declare type T2 = ObjectHasKey<{
+    a: string;
+}, 'b'>;
+declare function f1<A extends string, B extends string>(a: A, b: B): {
+    [P in A | B]: any;
+};
+declare function f2<A extends string>(a: A): {
+    [P in A | "x"]: any;
+};
+declare function f3(x: 'a' | 'b'): {
+    a: any;
+    b: any;
+    x: any;
+};

tests/baselines/reference/deferredLookupTypeResolution.symbols

@@ -0,0 +1,76 @@
+=== tests/cases/compiler/deferredLookupTypeResolution.ts ===
+// Repro from #17456
+
+type StringContains<S extends string, L extends string> = (
+>StringContains : Symbol(StringContains, Decl(deferredLookupTypeResolution.ts, 0, 0))
+>S : Symbol(S, Decl(deferredLookupTypeResolution.ts, 2, 20))
+>L : Symbol(L, Decl(deferredLookupTypeResolution.ts, 2, 37))
+
+    { [K in S]:      'true' } &
+>K : Symbol(K, Decl(deferredLookupTypeResolution.ts, 3, 7))
+>S : Symbol(S, Decl(deferredLookupTypeResolution.ts, 2, 20))
+
+    { [key: string]: 'false' }
+>key : Symbol(key, Decl(deferredLookupTypeResolution.ts, 4, 7))
+
+  )[L]
+>L : Symbol(L, Decl(deferredLookupTypeResolution.ts, 2, 37))
+
+type ObjectHasKey<O, L extends string> = StringContains<keyof O, L>
+>ObjectHasKey : Symbol(ObjectHasKey, Decl(deferredLookupTypeResolution.ts, 5, 6))
+>O : Symbol(O, Decl(deferredLookupTypeResolution.ts, 7, 18))
+>L : Symbol(L, Decl(deferredLookupTypeResolution.ts, 7, 20))
+>StringContains : Symbol(StringContains, Decl(deferredLookupTypeResolution.ts, 0, 0))
+>O : Symbol(O, Decl(deferredLookupTypeResolution.ts, 7, 18))
+>L : Symbol(L, Decl(deferredLookupTypeResolution.ts, 7, 20))
+
+type First<T> = ObjectHasKey<T, '0'>;  // Should be deferred
+>First : Symbol(First, Decl(deferredLookupTypeResolution.ts, 7, 67))
+>T : Symbol(T, Decl(deferredLookupTypeResolution.ts, 9, 11))
+>ObjectHasKey : Symbol(ObjectHasKey, Decl(deferredLookupTypeResolution.ts, 5, 6))
+>T : Symbol(T, Decl(deferredLookupTypeResolution.ts, 9, 11))
+
+type T1 = ObjectHasKey<{ a: string }, 'a'>;  // 'true'
+>T1 : Symbol(T1, Decl(deferredLookupTypeResolution.ts, 9, 37))
+>ObjectHasKey : Symbol(ObjectHasKey, Decl(deferredLookupTypeResolution.ts, 5, 6))
+>a : Symbol(a, Decl(deferredLookupTypeResolution.ts, 11, 24))
+
+type T2 = ObjectHasKey<{ a: string }, 'b'>;  // 'false'
+>T2 : Symbol(T2, Decl(deferredLookupTypeResolution.ts, 11, 43))
+>ObjectHasKey : Symbol(ObjectHasKey, Decl(deferredLookupTypeResolution.ts, 5, 6))
+>a : Symbol(a, Decl(deferredLookupTypeResolution.ts, 12, 24))
+
+// Verify that mapped type isn't eagerly resolved in type-to-string operation
+
+declare function f1<A extends string, B extends string>(a: A, b: B): { [P in A | B]: any };
+>f1 : Symbol(f1, Decl(deferredLookupTypeResolution.ts, 12, 43))
+>A : Symbol(A, Decl(deferredLookupTypeResolution.ts, 16, 20))
+>B : Symbol(B, Decl(deferredLookupTypeResolution.ts, 16, 37))
+>a : Symbol(a, Decl(deferredLookupTypeResolution.ts, 16, 56))
+>A : Symbol(A, Decl(deferredLookupTypeResolution.ts, 16, 20))
+>b : Symbol(b, Decl(deferredLookupTypeResolution.ts, 16, 61))
+>B : Symbol(B, Decl(deferredLookupTypeResolution.ts, 16, 37))
+>P : Symbol(P, Decl(deferredLookupTypeResolution.ts, 16, 72))
+>A : Symbol(A, Decl(deferredLookupTypeResolution.ts, 16, 20))
+>B : Symbol(B, Decl(deferredLookupTypeResolution.ts, 16, 37))
+
+function f2<A extends string>(a: A) {
+>f2 : Symbol(f2, Decl(deferredLookupTypeResolution.ts, 16, 91))
+>A : Symbol(A, Decl(deferredLookupTypeResolution.ts, 18, 12))
+>a : Symbol(a, Decl(deferredLookupTypeResolution.ts, 18, 30))
+>A : Symbol(A, Decl(deferredLookupTypeResolution.ts, 18, 12))
+
+    return f1(a, 'x');
+>f1 : Symbol(f1, Decl(deferredLookupTypeResolution.ts, 12, 43))
+>a : Symbol(a, Decl(deferredLookupTypeResolution.ts, 18, 30))
+}
+
+function f3(x: 'a' | 'b') {
+>f3 : Symbol(f3, Decl(deferredLookupTypeResolution.ts, 20, 1))
+>x : Symbol(x, Decl(deferredLookupTypeResolution.ts, 22, 12))
+
+    return f2(x);
+>f2 : Symbol(f2, Decl(deferredLookupTypeResolution.ts, 16, 91))
+>x : Symbol(x, Decl(deferredLookupTypeResolution.ts, 22, 12))
+}
+