Type inference on registers with can contain multiple types

My architecture has an FPU, but uses integer registers to store
floating-point values. So each register can store either an int or an
IEEE float. I define a register class like this:

def GR32 : RegisterClass<"MyArch", [i32, f32], 32,
        (sequence "R%u", 0, 32)>;

So far so good. However, when I write a rule to store a register:

def STORE32r : S32<
        (outs), (ins GR32:$rS, GR32:$rD),
        "st {$rS, ($rD)}",
        [(store GR32:$rS, (iPTR GR32:$rD))]>;

...then I get the dreaded 'cannot infer all types in pattern' error.
This is presumably because tablegen can't tell whether the input is an
i32 or a f32.

FWIW, I ran into a similar issue and my solution was to use a Pat:

def : Pat<(store f32:$srcDest, ADDRri:$addr), (SW f32:$srcDest, ADDRri:$addr)>;

(SW is my integer store instruction)