PartialTypeSignatures mishandles type variables that begin with an underscore
The following GHCi exchange was reported here (you need not follow the link to understand this ticket, though).
Prelude Language.Haskell.TH> runQ [d|foo :: forall _a . _a -> _a;foo x = x|] [SigD foo_6 (ForallT [PlainTV _a_5]  (AppT (AppT ArrowT (WildCardT (Just _a_5))) (WildCardT (Just _a_5)))),FunD foo_6 [Clause [VarP x_7] (NormalB (VarE x_7)) ]]
The problem is that the quoted expression uses no wildcards, yet the TH output does. That quoted expression just uses a type variable that begins with an underscore. Such an expression was allowed in Haskell long before wildcards.