checker: handle void in struct field init (fix #13944) (#14876)

vlib/v/checker/struct.v

@@ -377,6 +377,9 @@ pub fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 				expected_type = field_info.typ
 				c.expected_type = expected_type
 				expr_type = c.expr(field.expr)
+				if expr_type == ast.void_type {
+					c.error('`$field.expr` (no value) used as value', field.pos)
+				}
 				if !field_info.typ.has_flag(.optional) {
 					expr_type = c.check_expr_opt_call(field.expr, expr_type)
 				}

vlib/v/checker/tests/struct_field_init_with_void_expr_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_field_init_with_void_expr_err.vv:19:3: error: `app.test_func()` (no value) used as value
+   17 |         name: 'add'
+   18 |         description: 'Add something.'
+   19 |         execute: app.test_func()
+      |         ~~~~~~~~~~~~~~~~~~~~~~~~
+   20 |     }
+   21 |

vlib/v/checker/tests/struct_field_init_with_void_expr_err.vv

@@ -0,0 +1,25 @@
+import cli { Command }
+import os
+
+struct App {}
+
+fn (a App) test_func() {}
+
+fn main() {
+	mut cmd := Command{
+		name: 'cli'
+		version: '0.0.1'
+	}
+
+	app := App{}
+
+	mut add := Command{
+		name: 'add'
+		description: 'Add something.'
+		execute: app.test_func()
+	}
+
+	cmd.add_command(add)
+	cmd.setup()
+	cmd.parse(os.args)
+}