make sure monomorphs are bound to Dynamic in `build_instance` (closes #4666)

codegen.ml

@@ -660,7 +660,9 @@ let build_instance ctx mtype p =
 			let r = exc_protect ctx (fun r ->
 				let t = mk_mono() in
 				r := (fun() -> t);
-				unify_raise ctx (f()) t p;
+				let tf = (f()) in
+				unify_raise ctx tf t p;
+				link_dynamic t tf;
 				t
 			) s in
 			delay ctx PForce (fun() -> ignore ((!r)()));

tests/unit/src/unit/issues/Issue4666.hx

@@ -0,0 +1,11 @@
+package unit.issues;
+
+@:genericBuild(unit.issues.misc.Issue4666Macro.getType())
+private class C { }
+
+class Issue4666 extends Test {
+	public function test() {
+		var c:C;
+		unit.TestType.typedAs(c, (null:Dynamic));
+	}
+}

tests/unit/src/unit/issues/misc/Issue4666Macro.hx

@@ -0,0 +1,7 @@
+package unit.issues.misc;
+
+class Issue4666Macro {
+	static public function getType() {
+		return macro : Dynamic;
+	}
+}

type.ml

@@ -1235,6 +1235,11 @@ let rec link e a b =
 		true
 	end
 
+let link_dynamic a b = match follow a,follow b with
+	| TMono r,TDynamic _ -> r := Some b
+	| TDynamic _,TMono r -> r := Some a
+	| _ -> ()
+
 let rec fast_eq a b =
 	if a == b then
 		true