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Make toplevel opaque types transparent to the whole file #21530

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Make toplevel opaque types transparent to the whole file #21530

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1 change: 1 addition & 0 deletions compiler/src/dotty/tools/dotc/core/TypeComparer.scala
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Expand Up @@ -1620,6 +1620,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling

def findEnclosingThis(moduleClass: Symbol, from: Symbol): Type =
if ((from.owner eq moduleClass) && from.isPackageObject && from.is(Opaque)) from.thisType
else if (from eq moduleClass.owner) && moduleClass.isTopLevelDefinitionsObject && moduleClass.is(Opaque) then moduleClass.thisType
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Isn't that going to create a C$.this reference outside of C$? If yes, I don't think that's supposed to be possible/valid.

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The reference wasn't outside when the user wrote it. The compiler moved it so it's nested in a package object, but to the user the call site isn't from "outside".

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@sjrd sjrd Sep 2, 2024 •
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I understand, but still ... from an elaborated, TASTy-level point of view, this is invalid. A C.this reference has no meaning outside of C. This is worse than being "not accessible" because of protected or even "not visible" because of private; here it does not even exist.

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I'd argue that C.this, where C is a singleton module is just an alternative to C.type. Which, btw, is what widenInferred does as a final cleanup.

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I'd argue that C.this, where C is a singleton module is just an alternative to C.type.

It represents the same set of values at run-time, sure, but it's not valid everywhere C.type is valid.

Which, btw, is what widenInferred does as a final cleanup.

AFAICT, widentInferred turns pre.C$ into pre.C; not C$.this into pre.C nor the converse.

else if (from.is(Package)) tp
else if ((from eq moduleClass) && from.is(Opaque)) from.thisType
else if (from eq NoSymbol) tp
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7 changes: 7 additions & 0 deletions tests/pos/i15050.ex1.scala
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@@ -0,0 +1,7 @@
import OpaqueBug.*
def g(n: Counter): Counter = n
object OpaqueBug:
opaque type Counter = Int
val initial: Counter = 42
def f(n: Int): Int = g(n) + initial
@main def run = println(f(21))
6 changes: 6 additions & 0 deletions tests/pos/i15050.ex2.scala
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@@ -0,0 +1,6 @@
opaque type Counter = Int
def g(n: Counter): Counter = n
object OpaqueBug:
val initial: Counter = 42 // was error: Found: (42 : Int)
def f(n: Int): Int = g(n) + initial // was error: Found: (n : Int) Required: Counter
@main def run = println(f(21))
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