NondecreasingIndentation is problematic when the indentation can't be decreased

Consider the following program, which was deliberately mis-indented (someone playing with an exercise with deliberate errors and being confused by the result):

module Print3Broken where

printSecond :: IO ()
printSecond = do
putStrLn greeting -- Yarrrrr out of scope

main :: IO ()
main = do
  putStrLn greeting
  printSecond
  where greeting = "Yarrrrr"

With -XNondecreasingIndentation on by default, this will parse as long as the definition of main (or any other definition) is not present after the do block. But there is no legal way to have a definition after the do because it is not possible to unindent from column 1 --- and the resulting parse error is a bit confusing and gives no hint that NondecreasingIndentation applied to the do is the cause.

I suspect NondecreasingIndentation should not be applied when it is impossible to decrease the indentation.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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