Handle Scalac variance unsoundness with regards to constructors.

[odersky]

See SI-9549 for a summary. Review by @retronym

[DarkDimius]

println left in code.

[retronym]

LGTM.

I checked out this branch to test that:

class C[+A] { class Inner { def this(a: A) = this() } } // only allowed under the migration

I do see an seemingly spurious warning about procedure syntax, is that a known issue?

⚡ ./bin/dotc -migration -language:Scala2 sandbox/test.scala
sandbox/test.scala:1: warning: Procedure syntax no longer supported; `=' should be inserted here
class C[+A] { class Inner { def this(a: A) = this() } }
                                           ^
sandbox/test.scala:1: warning: According to new variance rules, this is no longer accepted; need to annotate with @uncheckedVariance:
covariant type A occurs in contravariant position in type A of value a
class C[+A] { class Inner { def this(a: A) = this() } }
                                       ^
two warnings found

This version is correctly allowed:

class C[+A] { def this(a: A) = this() }

During my testing, I also noticed another difference wrt scalac:

class C[A] { class A } // sandbox/test.scala:1: error: A is already defined as type A

[odersky]

The procedure syntax warning seems to be a bug. I'll follow up on this.

On Fri, Nov 6, 2015 at 10:07 PM, Jason Zaugg [email protected]
wrote:

LGTM.

I checked out this branch to test that:

class C[+A] { class Inner { def this(a: A) = this() } } // only allowed under the migration

I do see an seemingly spurious warning about procedure syntax, is that a
known issue?

⚡ ./bin/dotc -migration -language:Scala2 sandbox/test.scala
sandbox/test.scala:1: warning: Procedure syntax no longer supported; `=' should be inserted here
class C[+A] { class Inner { def this(a: A) = this() } }
^
sandbox/test.scala:1: warning: According to new variance rules, this is no longer accepted; need to annotate with @uncheckedVariance:
covariant type A occurs in contravariant position in type A of value a
class C[+A] { class Inner { def this(a: A) = this() } }
^
two warnings found

This version is allowed:

class C[+A] { def this(a: A) = this() }

During my testing, I also noticed another difference wrt scalac:

class C[A] { class A } // sandbox/test.scala:1: error: A is already defined as type A

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#926 (comment).

Martin Odersky
EPFL