Turning on -fglasgow-exts makes f x = id$x break. I propose having a flag to warn about things like this, enabled by -Wall. To be precise, "stolen syntax" is syntax that means something valid in (usually) Haskell98 and something different with some extension enabled. If there are syntax-stealing extensions implemented by other non-GHC compilers, we may want to warn about those too. (This includes all keywords, possibly other words like "forall", "exists", "family", unicode symbols for -> and others...)

If anyone actually agrees on or implements a (optional) change to unary-minus, this would subsume the warning aspects of #1318 (closed).