SI-9847 Nuance pure expr statement warning

[som-snytt]

Clarify the current warning, which means that an expression
split over multiple lines may not be parsed as naively expected.

When typing a block, attempt minor nuance. For instance, a
single expression is not in need of parens. Try to avoid
duplicate warnings for expressions that were adapted away
from result position.

[som-snytt]

Passing a flag to suppress the check for blocks is suboptimal.

An alternative would be just to shorten the original warning and not be fancy; and avoid lint territory.

[som-snytt]

This failing checkfile change for run/t7747-repl.scala happened on another PR, too. It passes on my machine:

-res3: List[Product with Serializable] = List(BippyBups(), PuppyPups(), Bingo())
+res3: List[Serializable with Product] = List(BippyBups(), PuppyPups(), Bingo())

[lrytz]

Isn't there some logic in place to only report a single warning per position?

I wonder why we get both warnings here. With M5 I don't get the discarded non-Unit value one:

Welcome to Scala 2.12.0-M5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> def f(): Unit = 42
<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
       def f(): Unit = 42
                       ^
f: ()Unit

[som-snytt]

AbstractReporter suppresses multiple ERROR or lower-priority messages (warnings in presence of errors).

The value discard is a flag:

$ scala -Ywarn-value-discard
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> def f: Unit = 42
<console>:11: warning: discarded non-Unit value
       def f: Unit = 42
                     ^
<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
       def f: Unit = 42
                     ^
f: Unit

scala> :quit
$ scalam -Ywarn-value-discard
Welcome to Scala 2.12.0-M5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> def f: Unit = 42
<console>:11: warning: discarded non-Unit value
       def f: Unit = 42
                     ^
<console>:11: warning: a pure expression does nothing in statement position; you may be omitting necessary parentheses
       def f: Unit = 42
                     ^
f: Unit

scala> :quit
$ skala -Ywarn-value-discard
Welcome to Scala 2.12.0-20160708-085210-f805cf5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> def f: Unit = 42
<console>:11: warning: discarded non-Unit value
       def f: Unit = 42
                     ^
<console>:11: warning: a pure expression does nothing in statement position
       def f: Unit = 42
                     ^
f: Unit

scala> :quit

[adriaanm]

(Adding WIP label to indicate this is awaiting an A for the reviewer's Q)

[som-snytt]

I tried to add the WTF label, but I guess I don't have the authority.

[adriaanm]

And that's why.

[adriaanm]

LGTM