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GHC already warns about variable shadowing:

$ cat test.hs
module Test where

timesTwoPlusOne x
    = timesTwo x + 1
  where timesTwo x = x * 2

$ ghc -fwarn-name-shadowing test.hs
...
Warning:
    This binding for `x' shadows the existing binding
      bound at <location>

However the similar warning doesn't happen for type variables.

$ cat T9244.hs
module T9244 where

import Control.Exception

tryMaybe :: IO a -> IO (Maybe a)
tryMaybe action = do
    result <- (try action) :: IO (Either SomeException a)
    return $ case result of
        Left _  -> Nothing
        Right v -> Just v

$ ghc -fwarn-name-shadowing T9244.hs
...
    Couldn't match type `a' with `a1'
      `a' is a rigid type variable bound by
          the type signature for tryMaybe :: IO a -> IO (Maybe a)
          at types.hs:<line>:13
      `a1' is a rigid type variable bound by
           an expression type signature: IO (Either SomeException a1)
           at types.hs:<line>:15
    Expected type: IO a1
      Actual type: IO a
...

Here, I thought that the 'a' in the function's type declaration was the same 'a' in the expression type declaration. However in Haskell 98, they are completely different variables.

Suggestion: if a type variable is renamed by the compiler due to a clash with another type variable, issue a warning that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall.

Alternative suggestion: if an error is displayed, where the error contains a renamed type variable, issue a hint that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall.