cgen: fix assigning option of struct fntype field (#17224)

vlib/v/gen/c/assign.v

@@ -162,6 +162,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 			}
 			// TODO: no buffer fiddling
 			ast.AnonFn {
+				if !(var_type.has_flag(.option) || var_type.has_flag(.result)) {
 					if blank_assign {
 						g.write('{')
 					}
@@ -193,6 +194,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 					}
 					continue
 				}
+			}
 			else {}
 		}
 		unwrapped_val_type := g.unwrap_generic(val_type)

vlib/v/tests/assign_option_of_struct_fntype_field_test.v

@@ -0,0 +1,16 @@
+struct Foo {
+mut:
+	bar ?fn (string) string
+}
+
+fn test_assign_option_of_struct_fntype_field() {
+	mut foo := Foo{}
+	foo.bar = fn (s string) string {
+		return s
+	}
+	if ff := foo.bar {
+		ret := ff('hello')
+		println(ret)
+		assert ret == 'hello'
+	}
+}