Using Debian7, gcc-4.8, gdc-4.8 from the `testing main` debian source.

        import std.stdio;

        class Bar(T) if (is(T == int))
        {
                this()
                {
                        writefln(T.stringof);
                }
        }

        class Foo(T) : Bar!(int) if (is(T == string)) // <-- template constraint causes error.
        {
                this()
                {
                        super();
                        writefln(T.stringof);
                }
        }

        void main()
        {
                auto baz = new Foo!(string)();
        }

$ gdc test.d -o test

test.d:11: Error: members expected
test.d:11: Error: { } expected following aggregate declaration
test.d:11: Error: Declaration expected, not 'if'
test.d:16: Error: function declaration without return type. (Note that constructors are always named 'this')
test.d:16: Error: no identifier for declarator writefln(T.stringof)
test.d:17: Error: unrecognized declaration

Am Thu, 10 Oct 2013 16:46:18 +0200
schrieb "Gary Willoughby" <dev@nomad.so>:

>          class Foo(T) : Bar!(int) if (is(T == string)) // <--
> template constraint causes error.

This syntax was added in a more recent frontend release. You should do this instead for older frontend versions:
-------
class Foo(T) if (is(T == string)) : Bar!(int)
-------