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Simon Peyton Jones authored
This patch responds to Trac #15334 by making it an error to write an instance declaration for a tuple constraint like (Eq [a], Show [a]). I then discovered that instance validity checking was scattered betweeen TcInstDcls and TcValidity, so I took the time to bring it all together, into TcValidity.checkValidInstHead In doing so I discovered that there are lot of special cases. I have not changed them, but at least they are all laid out clearly now.
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