Do not mark indexed access object type comparisons as unreliable.

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -21538,11 +21538,6 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     // Relate components directly before falling back to constraint relationships
                     // A type S[K] is related to a type T[J] if S is related to T and K is related to J.
                     if (result = isRelatedTo((source as IndexedAccessType).objectType, (target as IndexedAccessType).objectType, RecursionFlags.Both, reportErrors)) {
-                        // This does _not_ generalize - specific instantiations of `S[K]` and `T[J]` may be related, even if the indexed accesses generally are not.
-                        // For example, `S = {x: string, a: string}`, `T = {x: string, b: string}`, `K = J = "x"`. `S` and `T` are unrelated, but the result of executing
-                        // `S["x"]` and `T["x"]` _are_. Given that, we have to flag the object type comparison here as "unreliable", since while the generic result can reliably
-                        // be used in the affirmative case, it failing is not an indicator that the structural result will not succeed.
-                        instantiateType((source as IndexedAccessType).objectType, reportUnreliableMapper);
                         result &= isRelatedTo((source as IndexedAccessType).indexType, (target as IndexedAccessType).indexType, RecursionFlags.Both, reportErrors);
                     }
                     if (result) {

tests/baselines/reference/genericIndexedAccessVarianceComparisonResultCorrect.errors.txt

@@ -0,0 +1,35 @@
+genericIndexedAccessVarianceComparisonResultCorrect.ts(25,1): error TS2322: Type 'T<A>' is not assignable to type 'T<B>'.
+  Property 'z' is missing in type 'A' but required in type 'B'.
+
+
+==== genericIndexedAccessVarianceComparisonResultCorrect.ts (1 errors) ====
+    class A {
+        x: string = 'A';
+        y: number = 0;
+    }
+    
+    class B {
+        x: string = 'B';
+        z: boolean = true;
+    }
+    
+    type T<X extends { x: any }> = Pick<X, 'x'>;
+    
+    type C = T<A>;
+    type D = T<B>;
+    
+    type C_extends_D = C extends D ? true : false;                                  // true
+    type PickA_extends_PickB = Pick<A, 'x'> extends Pick<B, 'x'> ? true : false;    // true
+    type TA_extends_TB = T<A> extends T<B> ? true : false;                          // should be true
+    
+    declare let a: T<A>;
+    declare let b: T<B>;
+    declare let c: C;
+    declare let d: D;
+    
+    b = a;      // should be no error
+    ~
+!!! error TS2322: Type 'T<A>' is not assignable to type 'T<B>'.
+!!! error TS2322:   Property 'z' is missing in type 'A' but required in type 'B'.
+!!! related TS2728 genericIndexedAccessVarianceComparisonResultCorrect.ts:8:5: 'z' is declared here.
+    c = d;

tests/baselines/reference/genericIndexedAccessVarianceComparisonResultCorrect.types

@@ -46,7 +46,7 @@ type PickA_extends_PickB = Pick<A, 'x'> extends Pick<B, 'x'> ? true : false;
 >false : false
 
 type TA_extends_TB = T<A> extends T<B> ? true : false;                          // should be true
->TA_extends_TB : true
+>TA_extends_TB : false
 >true : true
 >false : false