Nullable unify fix

src/core/tUnification.ml

@@ -796,6 +796,11 @@ let rec unify (uctx : unification_context) a b =
 	| TEnum (ea,tl1) , TEnum (eb,tl2) ->
 		if ea != eb then error [cannot_unify a b];
 		unify_type_params uctx a b tl1 tl2
+	| TAbstract ({a_path=[],"Null"},[t1]), TAbstract ({a_path=[],"Null"},[t2]) ->
+		(* Unify the wrapped types directly so a monomorph on either side binds to the
+		   unwrapped type instead of capturing the whole Null<...> - see #11077 *)
+		begin try unify uctx t1 t2
+		with Unify_error l -> error (cannot_unify a b :: l) end
 	| TAbstract ({a_path=[],"Null"},[t]),_ ->
 		begin try unify uctx t b
 		with Unify_error l -> error (cannot_unify a b :: l) end

tests/nullsafety/src/cases/TestTypeParameters.hx

@@ -183,3 +183,18 @@ class TestDeepTypeParams {
 		});
 	}
 }
+
+// Inlining a generic function returning Null<T> must not bind the inline
+// type-param monomorph to Null<Int> instead of Int.
+@:nullSafety
+class TestInlineGenericReturn {
+	static function main() {
+		final arr = [1, 2, 3];
+		iter(arr, item -> true);
+	}
+
+	static inline function iter<T>(it:Array<T>, callback:(item:T) -> Bool):Null<T> {
+		for (v in it) callback(v);
+		return null;
+	}
+}