Template Haskell drops unary tuples

Summary

Template Haskell drops unary tuples

Steps to reproduce

foo = $( return (TupE [VarE 'undefined]) )

bar = foo `seq` "hi"

and evaluate bar. You will observe undefined is evaluated leading to an exception.

Expected behavior

I expect Template Haskell to throw an exception, saying that unary tuples do not exist.

Environment

• GHC version used: 8.6

Commentary

Convert.hs drops unary tuple constructors. Of course, we don't have unary boxed tuples, but if foo is accepted above, I would surely expect bar to be "hi". Instead, we should just reject foo outright.