haddock parsing fails with valid input

Reported here: https://github.com/haskell/haddock/issues/798

This is really a GHC parsing issue, even though it only shows up in the haddock "route."

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import parser labels

changed milestone to %8.8.1

These won't be addressed by GHC 8.6.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

This bug makes using doctest painful as it gives parse errors for programs that ghc accepts. See also https://github.com/haskell/haddock/issues/798

added Pnormal label

removed milestone

I keep bumping into this when having -haddock enabled, and it's extremely frustrating that a valid comment can break compilation. Eg, commenting out the $ (+ 1) line in the following:

blah 
 = id
 $ (+ 1)
 $ 6

results in

blah 
 = id
 -- $ (+ 1)
 $ 6

which then causes a parse error. This behavior breaks a good amount of editor tooling, such as "comment out line."

Emitting a warning instead of a parse error would be an acceptable workaround.

Allegedly this is fixed by !2377 (closed)