cgen: fix auto_str for fn type (#17988)

vlib/v/gen/c/auto_str_methods.v

@@ -190,6 +190,8 @@ fn (mut g Gen) gen_str_for_option(typ ast.Type, styp string, str_fn_name string)
 		g.auto_str_funcs.writeln('\t\tres = ${str_intp_sq(tmp_res)};')
 	} else if should_use_indent_func(sym.kind) && !sym_has_str_method {
 		g.auto_str_funcs.writeln('\t\tres = indent_${parent_str_fn_name}(*(${sym.cname}*)it.data, indent_count);')
+	} else if sym.kind == .function {
+		g.auto_str_funcs.writeln('\t\tres = ${parent_str_fn_name}();')
 	} else {
 		g.auto_str_funcs.writeln('\t\tres = ${parent_str_fn_name}(*(${sym.cname}*)it.data);')
 	}

vlib/v/gen/c/str.v

@@ -162,7 +162,8 @@ fn (mut g Gen) gen_expr_to_string(expr ast.Expr, etype ast.Type) {
 				g.write('*')
 			}
 			g.expr_with_cast(expr, typ, typ)
-		} else {
+		} else if typ.has_flag(.option) {
+			// only Option fn receive argument
 			g.expr_with_cast(expr, typ, typ)
 		}
 		g.write(')')

vlib/v/tests/print_fn_test.v

@@ -0,0 +1,15 @@
+struct Foo {
+mut:
+	f ?fn (int)
+}
+
+fn test_print_fn() {
+	a := fn (a string) int {
+		return 1
+	}
+	println(a)
+
+	foo := Foo{}
+	println(foo.f)
+	assert foo.f == none
+}