Repro case for redundant constraint bug.

Makefile

all:
	ghc --version
	ghc -Wredundant-constraints -Werror -O0 Main.hs
	ghc -Wredundant-constraints -Werror -O2 Main.hs

output

make
ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.10.2
ghc -Wredundant-constraints -Werror -O0 Main.hs
Loaded package environment from /home/nicuveo/.ghc/x86_64-linux-8.10.2/environments/default
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
ghc -Wredundant-constraints -Werror -O2 Main.hs
Loaded package environment from /home/nicuveo/.ghc/x86_64-linux-8.10.2/environments/default
[1 of 1] Compiling Main             ( Main.hs, Main.o ) [flags changed]

Main.hs:12:1: error: [-Wredundant-constraints, -Werror=redundant-constraints]
    • Redundant constraint: Show Int
    • In the type signature for:
           foo :: Show Int => Int -> IO ()
   |
12 | foo :: Show Int => Int -> IO ()
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
make: *** [Makefile:4: all] Error 1

RedundantConstraintBug.hs

{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies        #-}

module Main where

bar :: forall a. Show a => a -> IO ()
bar = print

foo :: Show Int => Int -> IO ()
foo = bar

main :: IO ()
main = foo 42

Can repro on the 9.1.20201218 ghc I'm using too

I added a comment to https://gitlab.haskell.org/ghc/ghc/-/issues/16932.