Code that won't ever be executed should not give rise to constraints

In the code

a x = let b = show x in 5, 

ghci derives:

a :: (Show a, Num t) => a -> t, 

which is correct, but I think the rules should be slightly altered, since in reality giving something to a which is not an instance of Show wouldn't be a problem, since it's not used in the first place.

One important application of this is testing a general data structure for one specific element type with assertions in the the general code and having a switch to turn generic and non-generic mode on and off. Unfortunately, this approach doesn't work today, since all kinds of constraints that are essentially part of the debugging process get added. Since deciding whether a certain variable is used is very easy in this setting(for example CPP macros that expand into "if debugging", where debugging will have a constant value. (I don't expect you to solve the Halting Problem).