Aha, thanks Csaba. So I’m not losing my marbles. The AST has a type
signature of the “initial” but implements the “lowered”. So with
-ddump-stg we can observe it:

The version claimed in the type signature (returning a tuple):

Foreign.it :: Foreign.C.Types.CDouble
[GblId] =
    [] \u []
        case ds_r1HA of {
          GHC.Types.D# ds2_s1HW [Occ=Once] ->
              case __pkg_ccall_GC main [ds2_s1HW GHC.Prim.realWorld#] of {
                (#,#) _ [Occ=Dead] ds4_s1I0 [Occ=Once] -> GHC.Types.D# [ds4_s1I0];
              };
        };

The final “lowered” version:

Foreign.it :: Foreign.C.Types.CDouble
[GblId] =
    [] \u []
        case ds_r1HA of {
          GHC.Types.D# ds2_s1HW [Occ=Once] ->
              case __pkg_ccall_GC main [ds2_s1HW GHC.Prim.realWorld#] of {
                Unit# ds4_s1I0 [Occ=Once] -> GHC.Types.D# [ds4_s1I0];
              };
        };

Cheers!

Chris