Issue with multiple mid-stream variables in for comprehensions

[ftucky]

Compiler version

3.2.0

Minimized code

  for k -> v <- Map(0->"A")
  dummy = 0
  yield k->v
// Result is a Map[Int,String] (expected)


  for 
    k -> v <- Map(0->"A")
    dummy  = 0
    dummy2 = 2
  yield k->v
// Result is a List[(Int,String)] (*UNEXPECTED*)

  for 
    k -> v <- Map(0->"A")
    dummy  = 0
    u <- 1::Nil 
    dummy2 = 2
  yield k->v
// Result is a Map[Int,String]  (expected)

Output

Map(0 -> A)
List((0,A))
Map(0 -> A)

Expectation

Map(0->A)
Map(0->A)
Map(0->A)

The presence of two successive midstream variable bindings seems to unsettle the compiler.

The type of the for expression is expected to be the one of the first (outer) container, not a List as in the second example.

Note that when the two bindings are separated by a Generator (as in the 3rd example) the expected behavior is restored.

Same observation in scala2 (2.11.2) :

for (u <- Map(0->"A");v=1) yield u
// scala.collection.immutable.Map[Int,String] = Map(0 -> A)   // (expected)

for (u <- Map(0->"A");v=1;w=2) yield u
// scala.collection.immutable.Iterable[(Int, String)] = List((0,A)) // (*UNEXPECTED*)

[som-snytt]

The classic Scala 2 ticket is scala/bug#7515 which will turn 10 years old soon.

The workaround is to tuple midstream.

[ftucky]

My bad, I only searched dotty issues: I'll be slapped on the fingers.

Please close as duplicate of scala/bug#7515,
or keep open in dotty backlog as a gentle reminder,
at your convenience.