import std.traits: CommonType;

nothrow CommonType!(T1, T2) ifThrown(E : Throwable = Exception, T1, T2)(lazy scope T1 expression, /*lazy*/ scope T2 expression2)

{

    static assert(!is(typeof(return) == void),

        "The error handler's return value("

        ~ T2.stringof ~

        ") does not have a common type with the expression("

        ~ T1.stringof ~

        ")."

    );

    try

    {

        return expression();

    }

    catch (E)

    {

        return expression2;

    }

}

​

int throwIfNot42(int x) {

  import std.exception : enforce;

  enforce(x == 42, "some error");

  return x;

}

​

nothrow string foo(int x, string def) {

  import std.format: format;

  return format("%d", throwIfNot42(x)).ifThrown(def);

}

​

void main() {

  import std.stdio: writeln;

  writeln(foo(42, "error"));

  writeln(foo(43, "error"));

}