Emit tab warning promptly

p = let  x = 4
[tab]y = 5
    in 12

produces error: parse error on input ‘y’ and nothing else. The problem is caused by a tab appearing to the user to line up x with y but actually placing y in the wrong column. The user typically will scratch their head and then post a question on StackOverflow. There they'll be told to use -fwarn-tabs, but ''that option won't help'' (and it's probably already on) because GHC will exit with a parse error before it gets around to giving the warning. I suspect the solution is probably to emit the tab warning in the lexer, but one of the parsing gurus will probably know better.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Parser)
Test case
Differential revisions
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Architecture