Add migration helpers for named tuples

[odersky]

Compiler version

3.6

Minimized example

From the SIP 58 proposal, which was recently accepted:

There are some source incompatibilities involving named tuples of length one.
First, what was previously classified as an assignment could now be interpreted as a named tuple. Example:

var age: Int
(age = 1)

This was an assignment in parentheses before, and is a named tuple of arity one now. It is however not idiomatic Scala code, since assignments are not usually enclosed in parentheses.

Second, what was a named argument to an infix operator can now be interpreted as a named tuple.

class C:
  infix def f(age: Int)
val c: C

then

c f (age = 1)

will now construct a tuple as second operand instead of passing a named parameter.

Expectation

These problems can be detected and diagnosed fairly straightforwardly: When faced with a unary named tuple, try to interpret it as an assignment, and if that succeeds, issue a migration error and suggest a workaround of these kinds:

  {age = 1}     // ok
  c.f(age = 1)  // ok

[odersky]

Maybe suitable for the next Spree?

[som-snytt]

Discussion of infix named args #19072

and #21565 was a quickie deprecation from last month.