fix

vlib/v/checker/tests/wrong_ref_type_as_concrete_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/wrong_ref_type_as_concrete_err.vv:9:39: error: cannot use &Type as generic concrete type, declare the function parameter as &T instead
+    7 | 
+    8 | fn main() {
+    9 |     mut dl := datatypes.DoublyLinkedList[&User]{}
+      |                                          ^
+   10 |     println(dl)
+   11 | }

vlib/v/checker/tests/wrong_ref_type_as_concrete_err.vv

@@ -0,0 +1,11 @@
+import datatypes
+
+[heap]
+struct User {
+	name string
+}
+
+fn main() {
+	mut dl := datatypes.DoublyLinkedList[&User]{}
+	println(dl)
+}

vlib/v/parser/parse_type.v

@@ -741,6 +741,10 @@ fn (mut p Parser) parse_generic_inst_type(name string) ast.Type {
 	mut is_instance := true
 	for p.tok.kind != .eof {
 		mut type_pos := p.tok.pos()
+		if p.tok.kind == .amp {
+			p.error_with_pos('cannot use &Type as generic concrete type, declare the function parameter as &T instead',
+				type_pos)
+		}
 		gt := p.parse_type()
 		type_pos = type_pos.extend(p.prev_tok.pos())
 		if gt.has_flag(.generic) {