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Avoid false warning when synthesising deferred givens #23087

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Avoid false warning when synthesising deferred givens #23087

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -577,7 +577,7 @@ class DependencyRecorder {
clazz
}

private var _responsibleForImports: Symbol = uninitialized
private[dotc] var _responsibleForImports: Symbol | Null = uninitialized
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If we're actually going to test whether it was initialized with an == null, then we should explicitly initialize it to null. uninitialized is supposed to be used only for things that will get assigned before every being read.

Suggested change
private[dotc] var _responsibleForImports: Symbol | Null = uninitialized
private[dotc] var _responsibleForImports: Symbol | Null = null


/** Top level import dependencies are registered as coming from a first top level
* class/trait/object declared in the compilation unit. If none exists, issue a warning and return NoSymbol.
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16 changes: 16 additions & 0 deletions compiler/src/dotty/tools/dotc/typer/Typer.scala
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Expand Up @@ -3158,10 +3158,26 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
val usingParamAccessors = cls.paramAccessors.filter(_.is(Given))
val paramScope = newScopeWith(usingParamAccessors*)
val searchCtx = ctx.outer.fresh.setScope(paramScope)

// Before losing the reference to ctx.owner
// when calling implicitArgTree with searchCtx,
// let's store ctx.owner as the fallback "responsibleForImports"
// in DependencyRecorder. That way, if we end up recording any dependencies
// we use ctx.owner as the "fromClass" rather than emitting a warning
// (because ctx.compilationUnit.tpdTree is still EmptyTree during typer).
// For example, to record mirror dependencies, see i23049.
val depRecorder = ctx.compilationUnit.depRecorder
val responsibleForImports = depRecorder._responsibleForImports
if responsibleForImports == null then
depRecorder._responsibleForImports = ctx.owner

val rhs = implicitArgTree(target, cdef.span,
where = i"inferring the implementation of the deferred ${dcl.showLocated}"
)(using searchCtx)

if responsibleForImports == null then
depRecorder._responsibleForImports = null

val impl = dcl.copy(cls,
flags = dcl.flags &~ (HasDefault | Deferred) | Final | Override,
info = target,
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12 changes: 12 additions & 0 deletions tests/warn/i23049.scala
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@@ -0,0 +1,12 @@
trait TC[X]
object TC {
given [X: scala.deriving.Mirror.ProductOf]: TC[X] = ???
}

trait Base[T] {
given TC[T] = scala.compiletime.deferred
}

case class P(x: Int)

object A extends Base[P]
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