Pattern match in for comprehensions treats stable identifiers as variables

[scabug]

This seems related to #900, but a different issue.

I was trying to match on a case object in a for comprehension, but it kept matching all elements. I managed to reduce it to the following test case. It seems that even identifiers starting with a capital letter, or surrounded by back-ticks are treated as free variables. (There is no What identifier defined at all).

scala> case object Test
defined object Test

scala> for (Test <- List(1 ,2, 3)) println(Test)
1
2
3

scala> for (What <- List(1, 2, 3)) println(What)
1
2
3

scala> for (`What` <- List(1, 2, 3)) println(What)
1
2
3

I expected these to behave as in a regular pattern match. I get the same result in 2.11.5, 2.11.6, and 2.12.0-M1.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9324?orig=1
Reporter: Rumen Zarev (rumen.zarev)
Affected Versions: 2.11.5, 2.11.6, 2.12.0-M1
See #900

[scabug]

@retronym said:
The workaround is:

scala> for (t @ Test <- List(1 ,2, 3)) println(Test)
<console>:9: error: pattern type is incompatible with expected type;
 found   : Test.type
 required: Int
              for (t @ Test <- List(1 ,2, 3)) println(Test)
                       ^

[scabug]

@som-snytt said:
Is there a label for "has-stackoverflow-question"?

http://stackoverflow.com/q/33298595/1296806

[som-snytt]

Closing as duplicate of #900 under the rubric "aligns with valdef" as mentioned here.

In brief, "If p is some pattern other than a simple name or a name followed by a colon and a type," then you get pattern-matching behavior for either a val definition or a generator. The generator case falls under refutability, except that, narrowly construed, irrefutability requires a varid.

The alternatively syntaxes (workarounds) are similar in all cases, namely, to change the shape of the pattern.