No unsound assignments to T[K] when T and K are both generic

ahejlsberg · ahejlsberg · commit c0c215fb6685 · 2018-10-01T16:20:15.000-07:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -11876,8 +11876,9 @@ namespace ts {
                     }
                 }
                 else if (target.flags & TypeFlags.IndexedAccess) {
-                    // A type S is related to a type T[K] if S is related to C, where C is the base constraint of T[K]
-                    if (relation !== identityRelation) {
+                    // A type S is related to a type T[K], where T and K aren't both type variables, if S is related to C,
+                    // where C is the base constraint of T[K]
+                    if (relation !== identityRelation && !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {
                         const constraint = getBaseConstraintOfType(target);
                         if (constraint && constraint !== target) {
                             if (result = isRelatedTo(source, constraint, reportErrors)) {

Original file line number	Diff line number	Diff line change
`@@ -11876,8 +11876,9 @@ namespace ts {`
`11876`	`11876`	`}`
`11877`	`11877`	`}`
`11878`	`11878`	`else if (target.flags & TypeFlags.IndexedAccess) {`
`11879`		`- // A type S is related to a type T[K] if S is related to C, where C is the base constraint of T[K]`
`11880`		`- if (relation !== identityRelation) {`
	`11879`	`+ // A type S is related to a type T[K], where T and K aren't both type variables, if S is related to C,`
	`11880`	`+ // where C is the base constraint of T[K]`
	`11881`	`+ if (relation !== identityRelation && !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {`
`11881`	`11882`	`const constraint = getBaseConstraintOfType(target);`
`11882`	`11883`	`if (constraint && constraint !== target) {`
`11883`	`11884`	`if (result = isRelatedTo(source, constraint, reportErrors)) {`