Backport "Inline transparent implicit parameters when typing Unapply trees" to LTS (#21048)

Backports #19646 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -5,6 +5,7 @@ package inlines
 import ast.*, core.*
 import Flags.*, Symbols.*, Types.*, Decorators.*, Constants.*, Contexts.*
 import StdNames.{tpnme, nme}
+import NameOps.*
 import typer.*
 import NameKinds.BodyRetainerName
 import SymDenotations.SymDenotation
@@ -54,6 +55,16 @@ object Inlines:
   def needsInlining(tree: Tree)(using Context): Boolean = tree match {
     case Block(_, expr) => needsInlining(expr)
     case _ =>
+      def isUnapplyExpressionWithDummy: Boolean =
+        // The first step of typing an `unapply` consists in typing the call
+        // with a dummy argument (see Applications.typedUnApply). We delay the
+        // inlining of this call.
+        def rec(tree: Tree): Boolean = tree match
+          case Apply(_, ProtoTypes.dummyTreeOfType(_) :: Nil) => true
+          case Apply(fn, _) => rec(fn)
+          case _ => false
+        tree.symbol.name.isUnapplyName && rec(tree)
+
       isInlineable(tree.symbol)
       && !tree.tpe.widenTermRefExpr.isInstanceOf[MethodOrPoly]
       && StagingLevel.level == 0
@@ -64,6 +75,7 @@ object Inlines:
       && !ctx.typer.hasInliningErrors
       && !ctx.base.stopInlining
       && !ctx.mode.is(Mode.NoInline)
+      && !isUnapplyExpressionWithDummy
   }
 
   private def needsTransparentInlining(tree: Tree)(using Context): Boolean =

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

@@ -1476,7 +1476,7 @@ trait Applications extends Compatibility {
 
         val dummyArg = dummyTreeOfType(ownType)
         val (newUnapplyFn, unapplyApp) =
-          val unapplyAppCall = withMode(Mode.NoInline):
+          val unapplyAppCall =
             typedExpr(untpd.TypedSplice(Apply(unapplyFn, dummyArg :: Nil)))
           inlinedUnapplyFnAndApp(dummyArg, unapplyAppCall)
 

tests/pos/i19623.scala

@@ -0,0 +1,40 @@
+import scala.compiletime.*
+import scala.language.dynamics
+
+abstract class % extends Selectable
+
+trait Select { type Out <: % }
+trait Selector extends Dynamic {
+  def selectDynamic[S <: Singleton & String](label: S): Any = ???
+
+  def unapply[R: RecordLike](record: R)(using
+      t: Select,
+      r: RecordLike[t.Out]
+  ): r.ElemTypes = ???
+}
+
+trait RecordLike[R] {
+  type ElemTypes <: Tuple
+}
+
+
+@main def Test = {
+  val r: %{ val name: String; } = ???
+
+  // originally derived in macro, use dummy instance instead
+  transparent inline given outputRecordLike[R <: %]: RecordLike[R] = null.asInstanceOf[
+    RecordLike[R] {
+      type ElemTypes = String *: EmptyTuple
+    }
+  ]
+
+  type FieldSelector = Select { type Out = % { val name: String } }
+  given fieldSelector: FieldSelector = ???
+  val selector: Selector = ???
+
+  val works = selector.unapply(r)
+  val works2 = selector.unapply(r)(using summon, fieldSelector, summon)
+  r match {
+    case selector(value) => value // compilation error
+  }
+}