cgen: fix lambda initialization on option struct field (fix #19474) (#…

…19995)

vlib/v/gen/c/struct.v

@@ -616,6 +616,8 @@ fn (mut g Gen) struct_init_field(sfield ast.StructInitField, language ast.Langua
 			if (sfield.expected_type.has_flag(.option) && !sfield.typ.has_flag(.option))
 				|| (sfield.expected_type.has_flag(.result) && !sfield.typ.has_flag(.result)) {
 				g.expr_with_opt(sfield.expr, sfield.typ, sfield.expected_type)
+			} else if sfield.expr is ast.LambdaExpr && sfield.expected_type.has_flag(.option) {
+				g.expr_opt_with_cast(sfield.expr, sfield.typ, sfield.expected_type)
 			} else {
 				g.left_is_opt = true
 				g.expr_with_cast(sfield.expr, sfield.typ, sfield.expected_type)

vlib/v/tests/anon_fn_option_test.v

@@ -0,0 +1,26 @@
+struct Foo {
+	bar ?fn ()
+}
+
+fn test_main() {
+	foo1 := Foo{
+		bar: || println('foo1')
+	}
+	foo2 := Foo{
+		bar: fn () {
+			println('foo2')
+		}
+	}
+	if bar_fn := foo1.bar {
+		bar_fn()
+		assert true
+	} else {
+		assert false
+	}
+	if bar_fn := foo2.bar {
+		bar_fn()
+		assert true
+	} else {
+		assert false
+	}
+}