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Fix #5521: prefix not checked in realizability check #5522

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Merged
merged 3 commits into from
Dec 2, 2018
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Fix #5521: prefix not checked in realizability check #5522

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a24218a
Fix #5521: prefix not checked in realizability check
abeln Nov 26, 2018
b66bcfe
Don't check prefix twice
Blaisorblade Nov 29, 2018
0eea5b3
Clean up with andAlso combinator
Blaisorblade Nov 29, 2018
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7 changes: 5 additions & 2 deletions compiler/src/dotty/tools/dotc/core/CheckRealizable.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -77,8 +77,11 @@ class CheckRealizable(implicit ctx: Context) {
else if (!isLateInitialized(sym)) realizability(tp.prefix)
else if (!sym.isEffectivelyFinal) new NotFinal(sym)
else realizability(tp.info).mapError(r => new ProblemInUnderlying(tp.info, r))
if (r == Realizable) sym.setFlag(Stable)
r
val r1 = if (r == Realizable) {
sym.setFlag(Stable)
realizability(tp.prefix)
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I looked at this time ago in #4036, I should look more closely, and I'm in a rush, but here are some initial comments.

I agree this is a bug and this change should give the correct behavior; but this change risks being pretty computationally expensive (possible fix below). This can duplicate recursive calls, and that risks causing exponential slowdowns. Triggering it might require a chain of final lazy vals or be impossible, but it's more robust to not duplicate calls.

If isLateInitialized(sym), this calls realizability(tp.prefix) again, and that can be too expensive. Caching only affects symbols, but we don't cache realizability on whole types; tp could not even be a TermRef and fall through boundsRealizability(tp).andAlso(memberRealizability(tp)).

I asked dottybot for a performance test. But maybe one could just reorganize the logic.
Looking at #4036 (which is where I "studied" this), I propose to add to your change the following:

-else if (!isLateInitialized(sym)) realizability(tp.prefix)
+else if (!isLateInitialized(sym)) Realizable

because in this branch we can certainly mark the symbol as stable (that PR also achieves this goal, though in a hackier way). If that works, this PR can go in with that fix.

Optional requests

Here's a couple of bonus requests one could fix here as well — if they are non-trivial, feel free to defer them. I should probably do these changes myself and PR them, I'm in a rush right now.

On performance: https://github.com/lampepfl/dotty/pull/4036/files#diff-0535d82f8f57a2204d6452bae575efc9 might be a useful test here (if it doesn't cause a stack overflow).

On error reporting, I think one could cherry-pick from #4036 the following:

-realizability(tp.prefix)
+realizability(tp.prefix).mapError(r => new ProblemInUnderlying(tp.prefix, r))

} else r
r1
}
case _: SingletonType | NoPrefix =>
Realizable
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12 changes: 12 additions & 0 deletions tests/neg/i5521.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
class Hello {
class Foo {
class Bar
final lazy val s: Bar = ???
}

lazy val foo: Foo = ???

val x: foo.s.type = ??? // error: `foo` must be final since it's a lazy val
val x2: foo.s.type = ??? // error: `foo` must be final since it's a lazy val
}