Refine parameter adaptation logic for arrays (#23591)

Fixes #23179

Since JDK 9, argument checks for LambdaMetaFactory have become stricter.
Which is to say that its JDK 8 version was too lax.

We apply adaptation now in case the samType is an array, but the
implType is not.

Author: bracevac

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -483,7 +483,9 @@ object Erasure {
 
       val paramAdaptationNeeded =
         implParamTypes.lazyZip(samParamTypes).exists((implType, samType) =>
-          !sameClass(implType, samType) && !autoAdaptedParam(implType))
+          !sameClass(implType, samType) && (!autoAdaptedParam(implType)
+            // LambdaMetaFactory cannot auto-adapt between Object and Array types
+            || samType.isInstanceOf[JavaArrayType]))
       val resultAdaptationNeeded =
         !sameClass(implResultType, samResultType) && !autoAdaptedResult
 

compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala

@@ -3,6 +3,7 @@ package dotc
 package transform
 
 import core.*
+import core.TypeErasure.erasure
 import Scopes.newScope
 import Contexts.*, Symbols.*, Types.*, Flags.*, Decorators.*, StdNames.*, Constants.*
 import MegaPhase.*
@@ -69,14 +70,50 @@ class ExpandSAMs extends MiniPhase:
             case _ => tp
           val tpe1 = collectAndStripRefinements(tpe)
           val Seq(samDenot) = tpe1.possibleSamMethods
-          cpy.Block(tree)(stats,
-            transformFollowingDeep:
-              AnonClass(List(tpe1),
-                List(samDenot.symbol.asTerm.name -> fn.symbol.asTerm),
-                refinements.toList,
-                adaptVarargs = true
-              )
-          )
+          val implSym = fn.symbol.asTerm
+
+          // Check if SAM param erases to array but impl param doesn't (see #23179).
+          // E.g. SAM expects Array[AnyRef] (erases to Array[Object]) but impl has
+          // Array[? >: AnyRef] (erases to Object). In this case we need to create
+          // a wrapper with the SAM signature that casts arguments before forwarding.
+          val samInfo = samDenot.info
+          val implInfo = implSym.info
+          val needsArrayAdaptation = (samInfo, implInfo) match
+            case (samMt: MethodType, implMt: MethodType) =>
+              samMt.paramInfos.lazyZip(implMt.paramInfos).exists { (samPt, implPt) =>
+                erasure(samPt).isInstanceOf[JavaArrayType] && !erasure(implPt).isInstanceOf[JavaArrayType]
+              }
+            case _ => false
+
+          if needsArrayAdaptation then
+            // Create a wrapper method with SAM's signature that casts args to impl's types
+            val wrapperSym = newSymbol(
+              implSym.owner, implSym.name.asTermName, Synthetic | Method,
+              samInfo, coord = implSym.span).entered.asTerm
+            val wrapperDef = DefDef(wrapperSym, paramss => {
+              val implMt = implInfo.stripPoly.asInstanceOf[MethodType]
+              val adaptedArgs = paramss.head.lazyZip(implMt.paramInfos).map { (arg, implPt) =>
+                if arg.tpe =:= implPt then arg else arg.cast(implPt)
+              }
+              ref(implSym).appliedToTermArgs(adaptedArgs)
+            })
+            cpy.Block(tree)(fn :: wrapperDef :: Nil,
+              transformFollowingDeep:
+                AnonClass(List(tpe1),
+                  List(samDenot.symbol.asTerm.name -> wrapperSym),
+                  refinements.toList,
+                  adaptVarargs = true
+                )
+            )
+          else
+            cpy.Block(tree)(stats,
+              transformFollowingDeep:
+                AnonClass(List(tpe1),
+                  List(samDenot.symbol.asTerm.name -> implSym),
+                  refinements.toList,
+                  adaptVarargs = true
+                )
+            )
       }
     case _ =>
       tree

tests/neg/i23179-reverse.scala

@@ -0,0 +1,22 @@
+// Test that reverse cases of #23179 are properly rejected by the type checker.
+// The original issue was about SAM expecting Array[T] but impl having Array[? >: T].
+// Due to array invariance, the reverse cases cannot occur.
+
+object TestReverse {
+  trait A { def f(a: Array[? <: AnyRef]): Any }
+  def g(a: A) = a.f(Array.empty[AnyRef])
+
+  def test(): Unit = {
+    g((x: Array[AnyRef]) => x.headOption) // error: Array[AnyRef] is not a subtype of Array[? <: AnyRef]
+  }
+}
+
+object TestResult {
+  trait A { def f(a: Int): Array[AnyRef] }
+  def g(a: A) = a.f(1)
+
+  def test(): Unit = {
+    val arr: Array[? >: AnyRef] = Array("hello")
+    g((x: Int) => arr) // error: Array[? >: AnyRef] is not a subtype of Array[AnyRef]
+  }
+}

tests/run/i23179.scala

@@ -0,0 +1,31 @@
+object Test {
+  // Basic case: SAM expects Array[AnyRef], impl has Array[? >: AnyRef]
+  trait A { def f(a: Array[AnyRef]): Any }
+  def g(a: A) = a.f(Array.empty[AnyRef])
+
+  // Multiple array parameters
+  trait MultiParam { def f(a: Array[AnyRef], b: Array[AnyRef]): Any }
+  def gMulti(a: MultiParam) = a.f(Array.empty[AnyRef], Array.empty[AnyRef])
+
+  // Mixed parameters: only some need adaptation
+  trait MixedParam { def f(a: Array[AnyRef], b: Int): Any }
+  def gMixed(a: MixedParam) = a.f(Array.empty[AnyRef], 42)
+
+  // Generic return type
+  trait GenericReturn[T] { def f(a: Array[AnyRef]): T }
+  def gGeneric[T](a: GenericReturn[T]): T = a.f(Array.empty[AnyRef])
+
+  def main(args: Array[String]): Unit = {
+    // Basic case
+    g((x: Array[? >: AnyRef]) => x.headOption)
+
+    // Multiple array parameters - both need adaptation
+    gMulti((x: Array[? >: AnyRef], y: Array[? >: AnyRef]) => (x.headOption, y.headOption))
+
+    // Mixed - only first param needs adaptation
+    gMixed((x: Array[? >: AnyRef], n: Int) => (x.headOption, n))
+
+    // Generic return type
+    val result: Option[?] = gGeneric((x: Array[? >: AnyRef]) => x.headOption)
+  }
+}