checker: check if struct field type isn't type (#15359)

vlib/v/checker/struct.v

@@ -84,6 +84,10 @@ pub fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 					struct_sym.info.is_heap = true
 				}
 			}
+			if sym.kind == .multi_return {
+				c.error('cannot use multi return as field type', field.type_pos)
+			}
+
 			if field.has_default_expr {
 				c.expected_type = field.typ
 				default_expr_type := c.expr(field.default_expr)

vlib/v/checker/tests/struct_multi_return_field_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_multi_return_field_err.vv:2:16: error: cannot use multi return as field type
+    1 | struct Tuple{
+    2 |     data (int, int)
+      |                   ^
+    3 | }
+    4 |

vlib/v/checker/tests/struct_multi_return_field_err.vv

@@ -0,0 +1,8 @@
+struct Tuple{
+	data (int, int)
+}
+
+fn main() {
+	x := Tuple{}
+	println(x)
+}