Fix #1665: Check that != has an operand on the left.

[nicolasstucki]

No description provided.

[DarkDimius]

why? this is supported by scalac.

[nicolasstucki]

In scalac there is a warning and then it is converted to true

<console>:12: warning: comparing values of types type and Int using `!=' will always yield true
       !=(1)
         ^
res0: Boolean = true

I could:

• Make it a warning and collapse it to true if in Scala 2 mode.
• Or make it a warning and collapse it to true in all cases.

[DarkDimius]

this should also handle idents. Here's an example illustrating how to do it: https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/backend/jvm/DottyBackendInterface.scala#L428-L438

[nicolasstucki]

Ok, I understand the point. The I misinterpreted the scalac error message where it says types type.

[nicolasstucki]

Here if I don't add rhs != defn.NullType && lhs != defn.NullType I start getting failures when comparing java.lang.Object some other types including types in scala.collection.
@DarkDimius I wonder if this is a bug or am I testing it the wrong way?

[DarkDimius]

I'm not sure I understood what failures did you get. Could you please elaborate?

[nicolasstucki]

For example if I run partestwithout the rhs != defn.NullType && lhs != defn.NullType I get the following two warnings (with partest-only-no-bootsrtap I only get the second one):

[info] Test test.TestBCode.nullChecks started
-- Warning: 
class Foo {
  def foo(x: AnyRef): Int = {
    val bool = x == null
    if (x != null) 1
    else 0
  }
}

5 |    if (x != null) 1
  |       ^^^^^^^^^^^
  |       comparing values of types Object and Null using `!=' will always yield true

[info] Test dotc.tests.repl_all started
input differs from transcript:
scala> val Const,x = 0
Const: Int = 0
x: Int = 0
scala> val (Const, List(`x`, _, a), b) = (0, List(0, 1337, 1), 2)
-- Warning: <console> --------------------------------------------------------------------------------------------------
6 |val (Const, List(`x`, _, a), b) = (0, List(0, 1337, 1), 2)
  |                 ^
  |         comparing values of types scala.collection.immutable.List[Int] and Null using `!=' will always yield true
a: Int = 1
b: Int = 2
scala> val a@b = 0
a: Int @unchecked = 0
b: Int @unchecked = 0
scala> :quit

Are those non nullable AnyRefs?

[DarkDimius]

I think this is a bug in IsInstanceOfEvaluator.

[odersky]

We should have a discussion whether we need IsInstanceOfEvaluator or whether this can be rolled into multiversal equality checks.

[nicolasstucki]

/rebuild

[DarkDimius]

Otherwise LGTM

[DarkDimius]

name isn't used in either of two cases.

[nicolasstucki]

Removed

[DarkDimius]

why does this test need scala2 reflection?

[nicolasstucki]

Just because it is the code that was in issue #1665.

[felixmulder]

@nicolasstucki - could you update this test to use Java reflection instead? Cheers from Dima

[nicolasstucki]

I removed the test as the other test file contains all the tests needed.

[DarkDimius]

LGTM