Inline transparent implicit parameters when typing Unapply trees

We needed to delay the inlining of the transparent inline when typing
the unapply function application. We used the NoInline mode, but this
also stopped the inlining of the arguments of the unapply. To fix this
we target more precisely the inlining of the unapply method and not the
implicit arguments. To do this we detect the dummy argument that is used
type the unapply as an application, before it is transformed into a pattern.

Fixes #19623

Author: nicolasstucki

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

@@ -1492,7 +1492,7 @@ 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
+  }
+}