Infer between generic mapped types before inferring from apparent type (#56640)

src/compiler/checker.ts

@@ -25544,6 +25544,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             }
             else {
                 source = getReducedType(source);
+                if (isGenericMappedType(source) && isGenericMappedType(target)) {
+                    invokeOnce(source, target, inferFromGenericMappedTypes);
+                }
                 if (!(priority & InferencePriority.NoConstraints && source.flags & (TypeFlags.Intersection | TypeFlags.Instantiable))) {
                     const apparentSource = getApparentType(source);
                     // getApparentType can return _any_ type, since an indexed access or conditional may simplify to any other type.
@@ -25581,6 +25584,19 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             priority = savePriority;
         }
 
+        // Ensure an inference action is performed only once for the given source and target types.
+        // This includes two things:
+        // Avoiding inferring between the same pair of source and target types,
+        // and avoiding circularly inferring between source and target types.
+        // For an example of the last, consider if we are inferring between source type
+        // `type Deep<T> = { next: Deep<Deep<T>> }` and target type `type Loop<U> = { next: Loop<U> }`.
+        // We would then infer between the types of the `next` property: `Deep<Deep<T>>` = `{ next: Deep<Deep<Deep<T>>> }` and `Loop<U>` = `{ next: Loop<U> }`.
+        // We will then infer again between the types of the `next` property:
+        // `Deep<Deep<Deep<T>>>` and `Loop<U>`, and so on, such that we would be forever inferring
+        // between instantiations of the same types `Deep` and `Loop`.
+        // In particular, we would be inferring from increasingly deep instantiations of `Deep` to `Loop`,
+        // such that we would go on inferring forever, even though we would never infer
+        // between the same pair of types.
         function invokeOnce<Source extends Type, Target extends Type>(source: Source, target: Target, action: (source: Source, target: Target) => void) {
             const key = source.id + "," + target.id;
             const status = visited && visited.get(key);
@@ -25888,6 +25904,16 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             }
         }
 
+        function inferFromGenericMappedTypes(source: MappedType, target: MappedType) {
+            // The source and target types are generic types { [P in S]: X } and { [P in T]: Y }, so we infer
+            // from S to T and from X to Y.
+            inferFromTypes(getConstraintTypeFromMappedType(source), getConstraintTypeFromMappedType(target));
+            inferFromTypes(getTemplateTypeFromMappedType(source), getTemplateTypeFromMappedType(target));
+            const sourceNameType = getNameTypeFromMappedType(source);
+            const targetNameType = getNameTypeFromMappedType(target);
+            if (sourceNameType && targetNameType) inferFromTypes(sourceNameType, targetNameType);
+        }
+
         function inferFromObjectTypes(source: Type, target: Type) {
             if (
                 getObjectFlags(source) & ObjectFlags.Reference && getObjectFlags(target) & ObjectFlags.Reference && (
@@ -25899,13 +25925,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 return;
             }
             if (isGenericMappedType(source) && isGenericMappedType(target)) {
-                // The source and target types are generic types { [P in S]: X } and { [P in T]: Y }, so we infer
-                // from S to T and from X to Y.
-                inferFromTypes(getConstraintTypeFromMappedType(source), getConstraintTypeFromMappedType(target));
-                inferFromTypes(getTemplateTypeFromMappedType(source), getTemplateTypeFromMappedType(target));
-                const sourceNameType = getNameTypeFromMappedType(source);
-                const targetNameType = getNameTypeFromMappedType(target);
-                if (sourceNameType && targetNameType) inferFromTypes(sourceNameType, targetNameType);
+                inferFromGenericMappedTypes(source, target);
             }
             if (getObjectFlags(target) & ObjectFlags.Mapped && !(target as MappedType).declaration.nameType) {
                 const constraintType = getConstraintTypeFromMappedType(target as MappedType);

tests/baselines/reference/mappedTypeInferenceFromApparentType.errors.txt

@@ -0,0 +1,26 @@
+mappedTypeInferenceFromApparentType.ts(10,1): error TS2322: Type 'foo' is not assignable to type 'bar'.
+  Types of parameters 'target' and 'source' are incompatible.
+    Type '{ [K in keyof U]: Obj[K]; }' is not assignable to type '{ [K in keyof U]: U[K]; }'.
+      Type 'Obj[K]' is not assignable to type 'U[K]'.
+        Type 'Obj' is not assignable to type 'U'.
+          'U' could be instantiated with an arbitrary type which could be unrelated to 'Obj'.
+
+
+==== mappedTypeInferenceFromApparentType.ts (1 errors) ====
+    type Obj = {
+        [s: string]: number;
+    };
+    
+    type foo = <T>(target: { [K in keyof T]: T[K] }) => void;
+    type bar = <U extends string[]>(source: { [K in keyof U]: Obj[K] }) => void;
+    
+    declare let f: foo;
+    declare let b: bar;
+    b = f;
+    ~
+!!! error TS2322: Type 'foo' is not assignable to type 'bar'.
+!!! error TS2322:   Types of parameters 'target' and 'source' are incompatible.
+!!! error TS2322:     Type '{ [K in keyof U]: Obj[K]; }' is not assignable to type '{ [K in keyof U]: U[K]; }'.
+!!! error TS2322:       Type 'Obj[K]' is not assignable to type 'U[K]'.
+!!! error TS2322:         Type 'Obj' is not assignable to type 'U'.
+!!! error TS2322:           'U' could be instantiated with an arbitrary type which could be unrelated to 'Obj'.

tests/baselines/reference/mappedTypeInferenceFromApparentType.symbols

@@ -0,0 +1,41 @@
+//// [tests/cases/compiler/mappedTypeInferenceFromApparentType.ts] ////
+
+=== mappedTypeInferenceFromApparentType.ts ===
+type Obj = {
+>Obj : Symbol(Obj, Decl(mappedTypeInferenceFromApparentType.ts, 0, 0))
+
+    [s: string]: number;
+>s : Symbol(s, Decl(mappedTypeInferenceFromApparentType.ts, 1, 5))
+
+};
+
+type foo = <T>(target: { [K in keyof T]: T[K] }) => void;
+>foo : Symbol(foo, Decl(mappedTypeInferenceFromApparentType.ts, 2, 2))
+>T : Symbol(T, Decl(mappedTypeInferenceFromApparentType.ts, 4, 12))
+>target : Symbol(target, Decl(mappedTypeInferenceFromApparentType.ts, 4, 15))
+>K : Symbol(K, Decl(mappedTypeInferenceFromApparentType.ts, 4, 26))
+>T : Symbol(T, Decl(mappedTypeInferenceFromApparentType.ts, 4, 12))
+>T : Symbol(T, Decl(mappedTypeInferenceFromApparentType.ts, 4, 12))
+>K : Symbol(K, Decl(mappedTypeInferenceFromApparentType.ts, 4, 26))
+
+type bar = <U extends string[]>(source: { [K in keyof U]: Obj[K] }) => void;
+>bar : Symbol(bar, Decl(mappedTypeInferenceFromApparentType.ts, 4, 57))
+>U : Symbol(U, Decl(mappedTypeInferenceFromApparentType.ts, 5, 12))
+>source : Symbol(source, Decl(mappedTypeInferenceFromApparentType.ts, 5, 32))
+>K : Symbol(K, Decl(mappedTypeInferenceFromApparentType.ts, 5, 43))
+>U : Symbol(U, Decl(mappedTypeInferenceFromApparentType.ts, 5, 12))
+>Obj : Symbol(Obj, Decl(mappedTypeInferenceFromApparentType.ts, 0, 0))
+>K : Symbol(K, Decl(mappedTypeInferenceFromApparentType.ts, 5, 43))
+
+declare let f: foo;
+>f : Symbol(f, Decl(mappedTypeInferenceFromApparentType.ts, 7, 11))
+>foo : Symbol(foo, Decl(mappedTypeInferenceFromApparentType.ts, 2, 2))
+
+declare let b: bar;
+>b : Symbol(b, Decl(mappedTypeInferenceFromApparentType.ts, 8, 11))
+>bar : Symbol(bar, Decl(mappedTypeInferenceFromApparentType.ts, 4, 57))
+
+b = f;
+>b : Symbol(b, Decl(mappedTypeInferenceFromApparentType.ts, 8, 11))
+>f : Symbol(f, Decl(mappedTypeInferenceFromApparentType.ts, 7, 11))
+

tests/baselines/reference/mappedTypeInferenceFromApparentType.types

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/mappedTypeInferenceFromApparentType.ts] ////
+
+=== mappedTypeInferenceFromApparentType.ts ===
+type Obj = {
+>Obj : { [s: string]: number; }
+
+    [s: string]: number;
+>s : string
+
+};
+
+type foo = <T>(target: { [K in keyof T]: T[K] }) => void;
+>foo : <T>(target: { [K in keyof T]: T[K]; }) => void
+>target : { [K in keyof T]: T[K]; }
+
+type bar = <U extends string[]>(source: { [K in keyof U]: Obj[K] }) => void;
+>bar : <U extends string[]>(source: { [K in keyof U]: Obj[K]; }) => void
+>source : { [K in keyof U]: Obj[K]; }
+
+declare let f: foo;
+>f : foo
+
+declare let b: bar;
+>b : bar
+
+b = f;
+>b = f : foo
+>b : bar
+>f : foo
+

tests/baselines/reference/mappedTypeInferenceToMappedType.symbols

@@ -0,0 +1,42 @@
+//// [tests/cases/compiler/mappedTypeInferenceToMappedType.ts] ////
+
+=== mappedTypeInferenceToMappedType.ts ===
+// #56133
+
+declare class Base<T> {
+>Base : Symbol(Base, Decl(mappedTypeInferenceToMappedType.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeInferenceToMappedType.ts, 2, 19))
+
+    someProp: T;
+>someProp : Symbol(Base.someProp, Decl(mappedTypeInferenceToMappedType.ts, 2, 23))
+>T : Symbol(T, Decl(mappedTypeInferenceToMappedType.ts, 2, 19))
+
+    method<U extends unknown[]>(x: { [K in keyof U]: U[K] }): Base<U>;
+>method : Symbol(Base.method, Decl(mappedTypeInferenceToMappedType.ts, 3, 16))
+>U : Symbol(U, Decl(mappedTypeInferenceToMappedType.ts, 4, 11))
+>x : Symbol(x, Decl(mappedTypeInferenceToMappedType.ts, 4, 32))
+>K : Symbol(K, Decl(mappedTypeInferenceToMappedType.ts, 4, 38))
+>U : Symbol(U, Decl(mappedTypeInferenceToMappedType.ts, 4, 11))
+>U : Symbol(U, Decl(mappedTypeInferenceToMappedType.ts, 4, 11))
+>K : Symbol(K, Decl(mappedTypeInferenceToMappedType.ts, 4, 38))
+>Base : Symbol(Base, Decl(mappedTypeInferenceToMappedType.ts, 0, 0))
+>U : Symbol(U, Decl(mappedTypeInferenceToMappedType.ts, 4, 11))
+}
+
+declare class Derived<T> extends Base<T> {
+>Derived : Symbol(Derived, Decl(mappedTypeInferenceToMappedType.ts, 5, 1))
+>T : Symbol(T, Decl(mappedTypeInferenceToMappedType.ts, 7, 22))
+>Base : Symbol(Base, Decl(mappedTypeInferenceToMappedType.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeInferenceToMappedType.ts, 7, 22))
+
+    method<V extends unknown[]>(x: { [K in keyof V]: V[K] }): Base<V>;
+>method : Symbol(Derived.method, Decl(mappedTypeInferenceToMappedType.ts, 7, 42))
+>V : Symbol(V, Decl(mappedTypeInferenceToMappedType.ts, 8, 11))
+>x : Symbol(x, Decl(mappedTypeInferenceToMappedType.ts, 8, 32))
+>K : Symbol(K, Decl(mappedTypeInferenceToMappedType.ts, 8, 38))
+>V : Symbol(V, Decl(mappedTypeInferenceToMappedType.ts, 8, 11))
+>V : Symbol(V, Decl(mappedTypeInferenceToMappedType.ts, 8, 11))
+>K : Symbol(K, Decl(mappedTypeInferenceToMappedType.ts, 8, 38))
+>Base : Symbol(Base, Decl(mappedTypeInferenceToMappedType.ts, 0, 0))
+>V : Symbol(V, Decl(mappedTypeInferenceToMappedType.ts, 8, 11))
+}

tests/baselines/reference/mappedTypeInferenceToMappedType.types

@@ -0,0 +1,24 @@
+//// [tests/cases/compiler/mappedTypeInferenceToMappedType.ts] ////
+
+=== mappedTypeInferenceToMappedType.ts ===
+// #56133
+
+declare class Base<T> {
+>Base : Base<T>
+
+    someProp: T;
+>someProp : T
+
+    method<U extends unknown[]>(x: { [K in keyof U]: U[K] }): Base<U>;
+>method : <U extends unknown[]>(x: { [K in keyof U]: U[K]; }) => Base<U>
+>x : { [K in keyof U]: U[K]; }
+}
+
+declare class Derived<T> extends Base<T> {
+>Derived : Derived<T>
+>Base : Base<T>
+
+    method<V extends unknown[]>(x: { [K in keyof V]: V[K] }): Base<V>;
+>method : <V extends unknown[]>(x: { [K in keyof V]: V[K]; }) => Base<V>
+>x : { [K in keyof V]: V[K]; }
+}

tests/cases/compiler/mappedTypeInferenceFromApparentType.ts

@@ -0,0 +1,13 @@
+// @strict: true
+// @noEmit: true
+
+type Obj = {
+    [s: string]: number;
+};
+
+type foo = <T>(target: { [K in keyof T]: T[K] }) => void;
+type bar = <U extends string[]>(source: { [K in keyof U]: Obj[K] }) => void;
+
+declare let f: foo;
+declare let b: bar;
+b = f;

tests/cases/compiler/mappedTypeInferenceToMappedType.ts

@@ -0,0 +1,13 @@
+// @strict: true
+// @noEmit: true
+
+// #56133
+
+declare class Base<T> {
+    someProp: T;
+    method<U extends unknown[]>(x: { [K in keyof U]: U[K] }): Base<U>;
+}
+
+declare class Derived<T> extends Base<T> {
+    method<V extends unknown[]>(x: { [K in keyof V]: V[K] }): Base<V>;
+}