Type inference with GADTs - a0 is untouchable

Lets say I have this program

{-# LANGUAGE GADTs #-}data My a where  A :: Int  -> My Int  B :: Char -> My Charmain :: IO ()main = do  let x = undefined :: My a  case x of    A v -> print v  -- print x

compiles fine.

But when I comment in the print x, I get:

gadt.hs: line 13, column 12:  Couldn't match type ‘a0’ with ‘()’‘a0’ is untouchable      inside the constraints (a1 ~ GHC.Types.Int)      bound by a pattern with constructor                 Main.A :: GHC.Types.Int -> Main.My GHC.Types.Int,               in a case alternative      at /home/niklas/src/hs/gadt-binary.hs:13:5-7  Expected type: GHC.Types.IO a0    Actual type: GHC.Types.IO ()  In the expression: System.IO.print v  In a case alternative: Main.A v -> System.IO.print v

Why do I get this error in line 13 (A v -> print v) instead of only in the print x line?

I thought the first occurrence should determine the type.

Please enlighten me :)