checker: check struct field with default expression (#15151)

vlib/v/checker/struct.v

@@ -320,7 +320,7 @@ pub fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 	}
 	if type_sym.name.len == 1 && !isnil(c.table.cur_fn) && c.table.cur_fn.generic_names.len == 0 {
 		c.error('unknown struct `$type_sym.name`', node.pos)
-		return 0
+		return ast.void_type
 	}
 	match type_sym.kind {
 		.placeholder {

vlib/v/checker/tests/struct_field_with_default_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_field_with_default_err.vv:2:22: error: unknown struct `T`
+    1 | struct Dummy<T> {
+    2 |     arbitrary_field T = T{}
+      |                         ~~~
+    3 | }
+    4 |

vlib/v/checker/tests/struct_field_with_default_err.vv

@@ -0,0 +1,5 @@
+struct Dummy<T> {
+	arbitrary_field T = T{}
+}
+
+fn main() {}