fwiw, i tried to work around this by changing the second conv function to:
T conv(T, U)(in Match!(U) arg) { return T(arg._len); }
and then got a bunch of errors like:
spiritd\impl\parse.d(36): Error: variable m cannot be read at compile time
Which looks like bug 3809.
If i change parseImpl to initialize 't' manually rather than using the struct
initializer, then the test app compiles and appears to work ok.
