deprecate multi-parameter infix operations

[adriaanm]

use a tuple instead

this enables a better story for auto-tupling (scala/scala3#4311)

deprecation path (for a given -Xsource level):

• 2.14:
  • deprecation warning: application involving a multi-param infix operation
• 3.0:
  • behavior change: application involving a multi-param infix operation is only allowed if the method takes a single argument, args are tupled
  • warning: definition of a symbolic method with multiple params ("cannot be used infix")

[adriaanm]

@smarter suggests we start warning on definition in 2.13, this way library authors can either mark symbolic multi-param methods as @deprecated in their library (and thus not get the deprecation warning that scalac emits)

[adriaanm]

note that the 2.14 warning shouldn't fire on method that take a single argument list, since those will still be ok in 3.0

[adriaanm]

the main goal being that the following applications will remain accepted during the whole upgrade (for time-varying reasons)

(x, y) :: xs
a == (b ,c)

the following will warn / error over time:

x += (1, 2, 3)
x add (a, b, c)

(because they take multiple arguments)

[SethTisue]

new proposal at scala/scala3#4311 (comment)

[SethTisue]

note that new proposal includes a 2.13 change (warning on definition)

[SethTisue]

@adriaanm where do we stand on this for RC1?

[adriaanm]

This will need to wait until later in 2.13.x

[eed3si9n]

Since we are shipping a bunch of collection changes in 2.13, and multi-parameter +s are already on deprecation track, I sent a PR to replace them with custom compilation error telling ppl to use ++ instead - scala/scala#7790