SI-7753 InstantiateDependentMap narrows type of unstable args #3518
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[Most of this comment and the initial fix were implemented by Jason Zaugg.
I just cleaned it up a bit.]
After a soundness fix in SI-3873, instantiation of dependent
method type results behaved differently depending on whether the argument
from which we were propagating information had a stable type
or not. This is particular to substitution into singleton types
over the parameter in question.
If the argument was stable, it was substituted into singleton
types, such as the one below in the prefix in `a.type#B`
(which is the longhand version of `a.B`)
scala> class A { type B >: Null <: AnyRef }
defined class A
scala> object AA extends A { type B = String }
defined object AA
scala> def foo(a: A): a.B = null
foo: (a: A)a.B
scala> foo(AA)
res0: AA.B = null
But what if it isn't stable?
scala> foo({def a = AA; a: A { type B <: String}})
res1: a.B = null
This commit changes that to:
scala> foo({def a = AA; a: A { type B <: String}})
res1: A{type B <: String}#B = null
|
LGTM (proxied from #3507) |
adriaanm
added a commit
that referenced
this pull request
Feb 13, 2014
SI-7753 InstantiateDependentMap narrows type of unstable args
|
Forgot to mention this also fixes SI-8223 |
|
An unforseen consequence of inferring the existential: milessabin/shapeless#79 |
This was referenced Apr 7, 2017
nicolasstucki
added a commit
to dotty-staging/scala
that referenced
this pull request
Nov 27, 2017
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Rebase of #3507, already LGTM'ed over there.
[Most of this comment and the initial fix were implemented by Jason Zaugg.
I just cleaned it up a bit.]
After a soundness fix in , instantiation of dependent
method type results behaved differently depending on whether the argument
from which we were propagating information had a stable type
or not. This is particular to substitution into singleton types
over the parameter in question.
If the argument was stable, it was substituted into singleton
types, such as the one below in the prefix in
a.type#B(which is the longhand version of
a.B)scala> class A { type B >: Null <: AnyRef }
defined class A
scala> object AA extends A { type B = String }
defined object AA
scala> def foo(a: A): a.B = null
foo: (a: A)a.B
scala> foo(AA)
res0: AA.B = null
But what if it isn't stable?
scala> foo({def a = AA; a: A { type B <: String}})
res1: a.B = null
This commit changes that to:
scala> foo({def a = AA; a: A { type B <: String}})
res1: A{type B <: String}#B = null