The function:

   fn!([1:2, 3:4])();

is mangled as:

   _D8demangle21__T2fnVHiiA2i1i2i3i4Z2fnFZv

The pertinent part being the way associative arrays are mangled:

   HiiA2i1i2i3i4

The type is "Hii", so an AA of int->int.  But then the value is "A2i1i2i3i4", which taken by itself suggests an array literal, not an AA literal.  This is the only instance that I've encountered where the determined type has to be communicated to the value parser so it can demangle things correctly (since it would be otherwise treated as a plain old array literal and represented as "[1,2]").  It would be better if the representation for this AA were:

   HiiH2i1i2i3i4