Wrong type inferenced for locally defined function

Consider the following example.

data A = A
    deriving Show
data B = B
    -- deriving Show is missing here.

main :: IO ()
main = do
    let myPrint = putStrLn . show
    myPrint A
    myPrint B

The Show instance for B is missing and the most general type for myPrint is Show a => a -> IO ().

Ghc, however, reports the following error:

file.hs:10:13: error:
    • Couldn't match expected type ‘A’ with actual type ‘B’
    • In the first argument of ‘myPrint’, namely ‘B’
      In a stmt of a 'do' block: myPrint B
      In the expression:
        do { let myPrint = putStrLn . show;
             myPrint A;
             myPrint B }

This error won't help the programmer find the missing deriving Show declaration. Instead, the error should read something like No instance for (Show B) arising from a use of ‘myPrint’.

Weirdly, when I type this main function into ghci, the error is 'correct'.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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