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Harden erasure of TermRefs #15658

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Jul 14, 2022
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Harden erasure of TermRefs #15658

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Harden erasure of TermRefs
odersky Jul 12, 2022
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odersky Jul 13, 2022
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19 changes: 17 additions & 2 deletions compiler/src/dotty/tools/dotc/core/TypeErasure.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -602,7 +602,9 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
else if (tp.isRepeatedParam) apply(tp.translateFromRepeated(toArray = sourceLanguage.isJava))
else if (semiEraseVCs && isDerivedValueClass(tycon.classSymbol)) eraseDerivedValueClass(tp)
else apply(tp.translucentSuperType)
case _: TermRef | _: ThisType =>
case tp: TermRef =>
this(underlyingOfTermRef(tp))
case _: ThisType =>
this(tp.widen)
case SuperType(thistpe, supertpe) =>
SuperType(this(thistpe), this(supertpe))
Expand Down Expand Up @@ -687,6 +689,19 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
tp
}

/** Widen term ref, skipping any `()` parameter of an eventual getter. Used to erase a TermRef.
* Since getters are introduced after erasure, one would think that erasing a TermRef
* could just use `widen`. However, it's possible that the TermRef got read from a class
* file after Getters (i.e. in the backend). In that case, the reference will not get
* an earlier denotation even when time travelling forward to erasure. Hence, we
* need to take the extra precaution of going from nullary method types to their resuls.
* A test case where this is needed is pos/i15649.scala, which fails non-deterministically
* if `underlyingOfTermRef` is replaced by `widen`.
*/
private def underlyingOfTermRef(tp: TermRef)(using Context) = tp.widen match
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This probably deserves a comment referencing the issue. Otherwise someone will come here later, figure that this is useless, and remove it.

case tpw @ MethodType(Nil) if tp.symbol.isGetter => tpw.resultType
case tpw => tpw

private def eraseArray(tp: Type)(using Context) = {
val defn.ArrayOf(elemtp) = tp: @unchecked
if (isGenericArrayElement(elemtp, isScala2 = sourceLanguage.isScala2)) defn.ObjectType
Expand Down Expand Up @@ -832,7 +847,7 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
case JavaArrayType(elem) =>
sigName(elem) ++ "[]"
case tp: TermRef =>
sigName(tp.widen)
sigName(underlyingOfTermRef(tp))
case ExprType(rt) =>
sigName(defn.FunctionOf(Nil, rt))
case tp: TypeVar =>
Expand Down
11 changes: 11 additions & 0 deletions tests/pos/i15649.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
trait ConfigSourceModule:
object ConfigSource:
class R

object M extends ConfigSourceModule

object Foo:
implicit class FromConfigSource(c: M.ConfigSource.type)

object FooBar: // problem disappears if we rename as `Bar`
def foo: M.ConfigSource.R = new M.ConfigSource.R // problem disappears if we use `???` as rhs