Add regression test

tests/baselines/reference/mappedTypeIndexedAccess.errors.txt

@@ -0,0 +1,49 @@
+tests/cases/compiler/mappedTypeIndexedAccess.ts(18,5): error TS2322: Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; } | { key: "bar"; value: number; }'.
+  Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; }'.
+    Types of property 'value' are incompatible.
+      Type 'number' is not assignable to type 'string'.
+tests/cases/compiler/mappedTypeIndexedAccess.ts(24,5): error TS2322: Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; } | { key: "bar"; value: number; }'.
+  Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; }'.
+    Types of property 'value' are incompatible.
+      Type 'number' is not assignable to type 'string'.
+
+
+==== tests/cases/compiler/mappedTypeIndexedAccess.ts (2 errors) ====
+    // Repro from #15756
+    
+    type Pairs<T> = {
+        [TKey in keyof T]: {
+            key: TKey;
+            value: T[TKey];
+        };
+    };
+    
+    type Pair<T> = Pairs<T>[keyof T];
+    
+    type FooBar = {
+        foo: string;
+        bar: number;
+    };
+    
+    // Error expected here
+    let pair1: Pair<FooBar> = {
+        ~~~~~
+!!! error TS2322: Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; } | { key: "bar"; value: number; }'.
+!!! error TS2322:   Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; }'.
+!!! error TS2322:     Types of property 'value' are incompatible.
+!!! error TS2322:       Type 'number' is not assignable to type 'string'.
+        key: "foo",
+        value: 3
+    };
+    
+    // Error expected here
+    let pair2: Pairs<FooBar>[keyof FooBar] = {
+        ~~~~~
+!!! error TS2322: Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; } | { key: "bar"; value: number; }'.
+!!! error TS2322:   Type '{ key: "foo"; value: number; }' is not assignable to type '{ key: "foo"; value: string; }'.
+!!! error TS2322:     Types of property 'value' are incompatible.
+!!! error TS2322:       Type 'number' is not assignable to type 'string'.
+        key: "foo",
+        value: 3
+    };
+    

tests/baselines/reference/mappedTypeIndexedAccess.js

@@ -0,0 +1,43 @@
+//// [mappedTypeIndexedAccess.ts]
+// Repro from #15756
+
+type Pairs<T> = {
+    [TKey in keyof T]: {
+        key: TKey;
+        value: T[TKey];
+    };
+};
+
+type Pair<T> = Pairs<T>[keyof T];
+
+type FooBar = {
+    foo: string;
+    bar: number;
+};
+
+// Error expected here
+let pair1: Pair<FooBar> = {
+    key: "foo",
+    value: 3
+};
+
+// Error expected here
+let pair2: Pairs<FooBar>[keyof FooBar] = {
+    key: "foo",
+    value: 3
+};
+
+
+//// [mappedTypeIndexedAccess.js]
+"use strict";
+// Repro from #15756
+// Error expected here
+var pair1 = {
+    key: "foo",
+    value: 3
+};
+// Error expected here
+var pair2 = {
+    key: "foo",
+    value: 3
+};

tests/cases/compiler/mappedTypeIndexedAccess.ts

@@ -0,0 +1,29 @@
+// @strict: true
+
+// Repro from #15756
+
+type Pairs<T> = {
+    [TKey in keyof T]: {
+        key: TKey;
+        value: T[TKey];
+    };
+};
+
+type Pair<T> = Pairs<T>[keyof T];
+
+type FooBar = {
+    foo: string;
+    bar: number;
+};
+
+// Error expected here
+let pair1: Pair<FooBar> = {
+    key: "foo",
+    value: 3
+};
+
+// Error expected here
+let pair2: Pairs<FooBar>[keyof FooBar] = {
+    key: "foo",
+    value: 3
+};