Ambiguous parsing of `x!==nothing`

[StevenWhitaker]

The following demonstrates a parser ambiguity that I think should throw a ParseError similar to how 1.+1 throws an error:

julia> x! = y -> y[1] = 1
#1 (generic function with 1 method)

julia> x! == nothing
false

julia> x!==nothing
ERROR: UndefVarError: `x` not defined
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

julia> versioninfo()
Julia Version 1.10.4
Commit 48d4fd48430 (2024-06-04 10:41 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 32 × 13th Gen Intel(R) Core(TM) i9-13900HX
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, goldmont)
Threads: 32 default, 0 interactive, 16 GC (on 32 virtual cores)
Environment:
  JULIA_NUM_THREADS = 32
  JULIA_PKG_USE_CLI_GIT = true

In particular, does x!==nothing mean x! == nothing or x !== nothing? The parser silently assumes the latter. I personally always include whitespace, thus circumventing the issue, but I ran across a line of code in NonlinearSolve.jl that does not have whitespace, and so I was surprised the syntax was allowed!

[nsajko]

xref https://github.com/JuliaLang/julia/issues/46411

[c42f]

Yes, the tokenizer assumes != should always be parsed as a token if it's present, taking precedence over a ! on the end of an identifier.

I'm not sure x!=1 should be an error (and it would be breaking to make it one - probably relatively high on the breaking things scale if I had to guess).

I personally agree it's not great style, but I'm not even sure this should be a warning - I'm somewhat inclined to say this belongs in a linter (and code formatting tools) rather than in the parser itself.