Two files:

test.d:
import std.file;
import jsonx;

void main() {
    jsonDecode("bla");
}

jsonx.d:
import std.conv;
import std.variant;
T jsonDecode(T = Variant, R)(R input) {
    auto val = jsonDecode_impl!T(input);
    return val;
}

/* Decode anything -> Variant */
Variant jsonDecode_impl(T : Variant, R)(ref R input) {
    Variant v;

        v = jsonDecode_impl!real(input);
    return v;
}

/* Decode JSON number -> D number */
T jsonDecode_impl(T, R)(ref R input) {
    try        return parse!T(input);
catch(ConvException e)         throw new JsonException("ConvException: " ~ e.msg);
}

class JsonException : Exception {
    this(string s) {
        super(s);
    }
}

dmd test.d json.x:
D:\DMD\dmd2\windows\bin\..\..\src\phobos\std\conv.d(2089): Error: function std.conv.parse!(real,string).parse compiler error, parameter 'p', bugzilla 2962?
Assertion failure: '0' on line 709 in file 'glue.c'

I get the same ICE and error using both d-json from http://github.com/gianm/d-json.git and Robert Jacques's new json library. Combine this with std.json's runtime access violations on seemingly correct code and I would almost label this as a blocker.