Remove reportErrors check in relateVariances

src/compiler/checker.ts

@@ -23344,6 +23344,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 tracing?.pop();
             }
 
+            if (entry !== undefined) {
+                // If the previous entry and the result disagree, then something has gone wrong.
+                const entrySucceeded = !!(entry & RelationComparisonResult.Succeeded);
+                const resultSucceeded = result !== Ternary.False;
+                Debug.assertEqual(entrySucceeded, resultSucceeded, "Cached relationship does not match recalculated result");
+            }
+
             if (outofbandVarianceMarkerHandler) {
                 outofbandVarianceMarkerHandler = originalHandler;
             }
@@ -24074,9 +24081,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     // types are invariant, if any of the type parameters are invariant we reset the reported
                     // errors and instead force a structural comparison (which will include elaborations that
                     // reveal the reason).
-                    // We can switch on `reportErrors` here, since varianceCheckFailed guarantees we return `False`,
                     // we can return `False` early here to skip calculating the structural error message we don't need.
-                    if (varianceCheckFailed && !(reportErrors && some(variances, v => (v & VarianceFlags.VarianceMask) === VarianceFlags.Invariant))) {
+                    if (varianceCheckFailed && !some(variances, v => (v & VarianceFlags.VarianceMask) === VarianceFlags.Invariant)) {
                         return Ternary.False;
                     }
                     // We remember the original error information so we can restore it in case the structural

tests/baselines/reference/conditionalTypeDoesntSpinForever.types

@@ -1,8 +1,9 @@
 //// [tests/cases/compiler/conditionalTypeDoesntSpinForever.ts] ////
 
 === Performance Stats ===
-Type Count: 1,000
-Instantiation count: 2,500
+Assignability cache: 1,000
+Type Count: 2,500
+Instantiation count: 5,000
 
 === conditionalTypeDoesntSpinForever.ts ===
 // A *self-contained* demonstration of the problem follows...

tests/baselines/reference/issue55217.js

@@ -0,0 +1,45 @@
+//// [tests/cases/compiler/issue55217.ts] ////
+
+//// [types.ts]
+export type ProductName = 'a' | 'b'
+
+export type SubproductNameForProductName<P extends ProductName> = P extends unknown
+  ? keyof EntitiesByProductName[P]
+  : never
+
+type EntitiesByProductName = {
+  a: { a1: { value: 'a-a1' } }
+  b: { b1: { value: 'b-b1' } }
+}
+
+export type DiscriminatedUnion<
+  P extends ProductName = ProductName,
+  E extends SubproductNameForProductName<P> = SubproductNameForProductName<P>,
+> = P extends ProductName
+    ? E extends SubproductNameForProductName<P>
+    // ? E extends unknown // With unknown, the exception doesn't happen. 
+      ? EntitiesByProductName[P][E]
+      : never
+    : never
+
+//// [app.ts]
+import { SubproductNameForProductName, DiscriminatedUnion, ProductName } from './types'
+
+export const bug = <P extends ProductName>() => {
+  const subproducts: DiscriminatedUnion<P, SubproductNameForProductName<P>>[] = []
+  subproducts.map((_: DiscriminatedUnion) => null)
+}
+
+
+//// [types.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+//// [app.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.bug = void 0;
+var bug = function () {
+    var subproducts = [];
+    subproducts.map(function (_) { return null; });
+};
+exports.bug = bug;

tests/baselines/reference/issue55217.symbols

@@ -0,0 +1,91 @@
+//// [tests/cases/compiler/issue55217.ts] ////
+
+=== types.ts ===
+export type ProductName = 'a' | 'b'
+>ProductName : Symbol(ProductName, Decl(types.ts, 0, 0))
+
+export type SubproductNameForProductName<P extends ProductName> = P extends unknown
+>SubproductNameForProductName : Symbol(SubproductNameForProductName, Decl(types.ts, 0, 35))
+>P : Symbol(P, Decl(types.ts, 2, 41))
+>ProductName : Symbol(ProductName, Decl(types.ts, 0, 0))
+>P : Symbol(P, Decl(types.ts, 2, 41))
+
+  ? keyof EntitiesByProductName[P]
+>EntitiesByProductName : Symbol(EntitiesByProductName, Decl(types.ts, 4, 9))
+>P : Symbol(P, Decl(types.ts, 2, 41))
+
+  : never
+
+type EntitiesByProductName = {
+>EntitiesByProductName : Symbol(EntitiesByProductName, Decl(types.ts, 4, 9))
+
+  a: { a1: { value: 'a-a1' } }
+>a : Symbol(a, Decl(types.ts, 6, 30))
+>a1 : Symbol(a1, Decl(types.ts, 7, 6))
+>value : Symbol(value, Decl(types.ts, 7, 12))
+
+  b: { b1: { value: 'b-b1' } }
+>b : Symbol(b, Decl(types.ts, 7, 30))
+>b1 : Symbol(b1, Decl(types.ts, 8, 6))
+>value : Symbol(value, Decl(types.ts, 8, 12))
+}
+
+export type DiscriminatedUnion<
+>DiscriminatedUnion : Symbol(DiscriminatedUnion, Decl(types.ts, 9, 1))
+
+  P extends ProductName = ProductName,
+>P : Symbol(P, Decl(types.ts, 11, 31))
+>ProductName : Symbol(ProductName, Decl(types.ts, 0, 0))
+>ProductName : Symbol(ProductName, Decl(types.ts, 0, 0))
+
+  E extends SubproductNameForProductName<P> = SubproductNameForProductName<P>,
+>E : Symbol(E, Decl(types.ts, 12, 38))
+>SubproductNameForProductName : Symbol(SubproductNameForProductName, Decl(types.ts, 0, 35))
+>P : Symbol(P, Decl(types.ts, 11, 31))
+>SubproductNameForProductName : Symbol(SubproductNameForProductName, Decl(types.ts, 0, 35))
+>P : Symbol(P, Decl(types.ts, 11, 31))
+
+> = P extends ProductName
+>P : Symbol(P, Decl(types.ts, 11, 31))
+>ProductName : Symbol(ProductName, Decl(types.ts, 0, 0))
+
+    ? E extends SubproductNameForProductName<P>
+>E : Symbol(E, Decl(types.ts, 12, 38))
+>SubproductNameForProductName : Symbol(SubproductNameForProductName, Decl(types.ts, 0, 35))
+>P : Symbol(P, Decl(types.ts, 11, 31))
+
+    // ? E extends unknown // With unknown, the exception doesn't happen. 
+      ? EntitiesByProductName[P][E]
+>EntitiesByProductName : Symbol(EntitiesByProductName, Decl(types.ts, 4, 9))
+>P : Symbol(P, Decl(types.ts, 11, 31))
+>E : Symbol(E, Decl(types.ts, 12, 38))
+
+      : never
+    : never
+
+=== app.ts ===
+import { SubproductNameForProductName, DiscriminatedUnion, ProductName } from './types'
+>SubproductNameForProductName : Symbol(SubproductNameForProductName, Decl(app.ts, 0, 8))
+>DiscriminatedUnion : Symbol(DiscriminatedUnion, Decl(app.ts, 0, 38))
+>ProductName : Symbol(ProductName, Decl(app.ts, 0, 58))
+
+export const bug = <P extends ProductName>() => {
+>bug : Symbol(bug, Decl(app.ts, 2, 12))
+>P : Symbol(P, Decl(app.ts, 2, 20))
+>ProductName : Symbol(ProductName, Decl(app.ts, 0, 58))
+
+  const subproducts: DiscriminatedUnion<P, SubproductNameForProductName<P>>[] = []
+>subproducts : Symbol(subproducts, Decl(app.ts, 3, 7))
+>DiscriminatedUnion : Symbol(DiscriminatedUnion, Decl(app.ts, 0, 38))
+>P : Symbol(P, Decl(app.ts, 2, 20))
+>SubproductNameForProductName : Symbol(SubproductNameForProductName, Decl(app.ts, 0, 8))
+>P : Symbol(P, Decl(app.ts, 2, 20))
+
+  subproducts.map((_: DiscriminatedUnion) => null)
+>subproducts.map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>subproducts : Symbol(subproducts, Decl(app.ts, 3, 7))
+>map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>_ : Symbol(_, Decl(app.ts, 4, 19))
+>DiscriminatedUnion : Symbol(DiscriminatedUnion, Decl(app.ts, 0, 38))
+}
+

tests/baselines/reference/issue55217.types

@@ -0,0 +1,84 @@
+//// [tests/cases/compiler/issue55217.ts] ////
+
+=== types.ts ===
+export type ProductName = 'a' | 'b'
+>ProductName : ProductName
+>            : ^^^^^^^^^^^
+
+export type SubproductNameForProductName<P extends ProductName> = P extends unknown
+>SubproductNameForProductName : SubproductNameForProductName<P>
+>                             : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  ? keyof EntitiesByProductName[P]
+  : never
+
+type EntitiesByProductName = {
+>EntitiesByProductName : EntitiesByProductName
+>                      : ^^^^^^^^^^^^^^^^^^^^^
+
+  a: { a1: { value: 'a-a1' } }
+>a : { a1: { value: "a-a1"; }; }
+>  : ^^^^^^                  ^^^
+>a1 : { value: "a-a1"; }
+>   : ^^^^^^^^^      ^^^
+>value : "a-a1"
+>      : ^^^^^^
+
+  b: { b1: { value: 'b-b1' } }
+>b : { b1: { value: "b-b1"; }; }
+>  : ^^^^^^                  ^^^
+>b1 : { value: "b-b1"; }
+>   : ^^^^^^^^^      ^^^
+>value : "b-b1"
+>      : ^^^^^^
+}
+
+export type DiscriminatedUnion<
+>DiscriminatedUnion : DiscriminatedUnion<P, E>
+>                   : ^^^^^^^^^^^^^^^^^^^^^^^^
+
+  P extends ProductName = ProductName,
+  E extends SubproductNameForProductName<P> = SubproductNameForProductName<P>,
+> = P extends ProductName
+    ? E extends SubproductNameForProductName<P>
+    // ? E extends unknown // With unknown, the exception doesn't happen. 
+      ? EntitiesByProductName[P][E]
+      : never
+    : never
+
+=== app.ts ===
+import { SubproductNameForProductName, DiscriminatedUnion, ProductName } from './types'
+>SubproductNameForProductName : any
+>                             : ^^^
+>DiscriminatedUnion : any
+>                   : ^^^
+>ProductName : any
+>            : ^^^
+
+export const bug = <P extends ProductName>() => {
+>bug : <P extends ProductName>() => void
+>    : ^ ^^^^^^^^^           ^^^^^^^^^^^
+><P extends ProductName>() => {  const subproducts: DiscriminatedUnion<P, SubproductNameForProductName<P>>[] = []  subproducts.map((_: DiscriminatedUnion) => null)} : <P extends ProductName>() => void
+>                                                                                                                                                                    : ^ ^^^^^^^^^           ^^^^^^^^^^^
+
+  const subproducts: DiscriminatedUnion<P, SubproductNameForProductName<P>>[] = []
+>subproducts : DiscriminatedUnion<P, SubproductNameForProductName<P>>[]
+>            : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>[] : never[]
+>   : ^^^^^^^
+
+  subproducts.map((_: DiscriminatedUnion) => null)
+>subproducts.map((_: DiscriminatedUnion) => null) : null[]
+>                                                 : ^^^^^^
+>subproducts.map : <U>(callbackfn: (value: DiscriminatedUnion<P, SubproductNameForProductName<P>>, index: number, array: DiscriminatedUnion<P, SubproductNameForProductName<P>>[]) => U, thisArg?: any) => U[]
+>                : ^ ^^          ^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^       ^^^   ^^^^^^^^
+>subproducts : DiscriminatedUnion<P, SubproductNameForProductName<P>>[]
+>            : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>map : <U>(callbackfn: (value: DiscriminatedUnion<P, SubproductNameForProductName<P>>, index: number, array: DiscriminatedUnion<P, SubproductNameForProductName<P>>[]) => U, thisArg?: any) => U[]
+>    : ^ ^^          ^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^       ^^^   ^^^^^^^^
+>(_: DiscriminatedUnion) => null : (_: DiscriminatedUnion) => null
+>                                : ^ ^^                  ^^^^^^^^^
+>_ : DiscriminatedUnion
+>  : ^^^^^^^^^^^^^^^^^^
+}
+

tests/cases/compiler/issue55217.ts

@@ -0,0 +1,31 @@
+// @strict: true
+
+// @filename: types.ts
+export type ProductName = 'a' | 'b'
+
+export type SubproductNameForProductName<P extends ProductName> = P extends unknown
+  ? keyof EntitiesByProductName[P]
+  : never
+
+type EntitiesByProductName = {
+  a: { a1: { value: 'a-a1' } }
+  b: { b1: { value: 'b-b1' } }
+}
+
+export type DiscriminatedUnion<
+  P extends ProductName = ProductName,
+  E extends SubproductNameForProductName<P> = SubproductNameForProductName<P>,
+> = P extends ProductName
+    ? E extends SubproductNameForProductName<P>
+    // ? E extends unknown // With unknown, the exception doesn't happen. 
+      ? EntitiesByProductName[P][E]
+      : never
+    : never
+
+// @filename: app.ts
+import { SubproductNameForProductName, DiscriminatedUnion, ProductName } from './types'
+
+export const bug = <P extends ProductName>() => {
+  const subproducts: DiscriminatedUnion<P, SubproductNameForProductName<P>>[] = []
+  subproducts.map((_: DiscriminatedUnion) => null)
+}