Avoid false warning when synthesising deferred givens (#23087)

Fixes #23049

Author: sjrd

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -577,7 +577,7 @@ class DependencyRecorder {
     clazz
   }
 
-  private var _responsibleForImports: Symbol = 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.

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3205,10 +3205,26 @@ 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,

tests/warn/i23049.scala

@@ -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]