Add test with notes

Author: andrewbranch

tests/cases/compiler/checkOrderDependenceGenericAssignability.ts

@@ -0,0 +1,46 @@
+/*
+
+Debugging notes: variance measurement for `Parent` is getting set to
+`VarianceFlags.Independent`, implying that its type parameter is never
+witnessed at all. It arrived at this conclusion by checking the assignability
+of `Parent` instantiated with marker types. It first checks assignability in
+both directions with instantiations with super/sub-related marker types, and
+assignability appears to return true in both directions; however, it actually
+is returning `Ternary.Unknown`, due to being unable to answer questions about
+the assignability of the types' `parent` and `child` properties without knowing
+their variances. After (incorrectly) concluding that `Parent` is bivariant on `A`,
+it checks another set of instantiations with markers that are unrelated to each
+other. That too comes back as `Ternary.Unknown` but is interpreted as true, so
+the variance gets updated to `Independent`, since instantiating `Parent` with
+all kinds of different markers with different assignability to each other
+apparently had no effect on the instantiations' assignability to each other.
+
+I'm not sure if any of those comparisons ever actually looked at `a` and `b`,
+which should provide some non-recursive concrete variance information. I'm also
+not sure if `outofbandVarianceMarkerHandler` should have been called at some point,
+but it was not.
+
+*/
+
+interface Parent<A> {
+  child: Child<A> | null;
+  parent: Parent<A> | null;
+}
+
+interface Child<A, B = unknown> extends Parent<A> {
+  readonly a: A;
+  // This field isn't necessary to the repro, but the
+  // type parameter is, so including it
+  readonly b: B;
+}
+
+function fn<A>(inp: Child<A>) {
+  // This assignability check defeats the later one
+  const a: Child<unknown> = inp;
+}
+
+// Allowed initialization of pu
+const pu: Parent<unknown> = { child: { a: 0, b: 0, child: null, parent: null }, parent: null };
+
+// Should error
+const notString: Parent<string> = pu;