REPL: Import shadowing issue

[allanrenucci]

scala> object A { def f = 1 }                                                                                                                                                                                   
// defined object A

scala> import A._; def f = 2
def f: Int

scala> f                                                                                                                                                                                                        
val res0: Int = 1 // should be 2

[allanrenucci]

This has to do with the way we handle user defined imports and wrap expression in an object:

When we evaluate an expression, we need to imports previously evaluated expressions:
https://github.com/lampepfl/dotty/blob/86c4e72e3a87aae12c57e2c887231b77717438a3/compiler/src/dotty/tools/repl/ReplCompiler.scala#L50-L66

We first import the wrapper object and then the user defined imports in it. So a user defined import might shadow a definition even if the import precedes the definition

[SethTisue]

this is a regression/change since Scala 2; all Scala 2 versions I tested print 2

(you never know, but sometimes @som-snytt gets interested in these sort of problems...)

[SethTisue]

there is some discussion and analysis on #14951. I wrote:

Dale suggested maybe just forbidding mixing import with other things in a single REPL entry.

It also occurred to both of us that another solution path could be to notice when import is present and automatically split the entry up into multiple entries.

[som-snytt]

Today, it would be more natural to say that import makes symbols available in the current line, and export makes them available to subsequent lines.