Lack of support for QuantifiedConstraints is breaking tool support (such as stylish-haskell) for GHC 8.6. Is there any way I could help get this merged?

I didn't see this PR. Looks reasonable to me but CI is failing.

Looks like some warnings on older GHCs, shouldn't be too hard.

Before I fix it, is it intended that haskell-src-exts support GHC versions as far back as 7.6? Note that 7.10 (which this PR works with) was released almost four years ago.

I suppose 7.6 is quite old now but seeing as it wasn't much effort to keep supporting I didn't see any reason to remove it. It's probably fine to remove releases before 7.10.

OK, so that fixed GHC 7.8. On GHC 7.6, two tests cause stack overflow.

Any update on this?

Associating this with related issues #433 and #419.

This would be a very useful feature for me. Is there update on whether it can be merged?

It can! It's been on my backlog for a while. I had a mergeable version of this (see #441) about a month ago. I went ahead and merged & released as haskell-src-exts-1.22.0.

I made a slight mistake with the release in that the full changelog updates for this release were not properly published. See the in-repo changelog for more complete details.