Note [Instantiating stupid theta]

Consider a data type with a "stupid theta" (see
Note [The stupid context]):

  data Ord a => T a = MkT (Maybe a)

We want to generate an Ord constraint for every use of MkT; but
we also want to allow visible type application, such as

   MkT @Int

To achieve this, the wrapper for a data (or newtype) constructor
with a datatype context contains a lambda which drops the dictionary
argments corresponding to the datatype context:

   /\a \(_d:Ord a). MkT @a

Notice that the wrapper discards the dictionary argument d.
We don't need it; it was only there to generate a Wanted constraint.
(That is why it is stupid.)

This all happens in GHC.Types.Id.Make.mkDataConRep.

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\subsection{Data constructors}
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