From d47605d29d47d1394378a2986e03a1c5ec4f813f Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Mon, 11 Dec 2023 18:29:01 +0000
Subject: [PATCH] Fix type inferencing (constraining) regressions

---
 .../dotc/core/PatternTypeConstrainer.scala    | 39 ++++++++-----------
 .../dotty/tools/dotc/core/TypeComparer.scala  |  9 ++++-
 .../src/dotty/tools/dotc/typer/Typer.scala    |  6 ++-
 .../dotc/semanticdb/SemanticdbTests.scala     |  2 +-
 tests/neg/i18453.min.scala                    | 11 ++++++
 tests/{pos => neg}/i18453.scala               |  6 ++-
 tests/neg/i5976.scala                         |  2 +-
 tests/pos/i18453.zio.scala                    | 33 ++++++++++++++++
 tests/pos/i19001.case1.scala                  | 19 +++++++++
 tests/pos/i19001.case2.scala                  | 16 ++++++++
 tests/pos/i19001.case3.scala                  | 12 ++++++
 tests/pos/i19009.case1.scala                  | 18 +++++++++
 tests/pos/i19009.case2.scala                  | 10 +++++
 tests/pos/i19009.case3.scala                  | 31 +++++++++++++++
 tests/pos/i19009.min3.scala                   |  9 +++++
 tests/semanticdb/metac.expect                 |  5 ++-
 16 files changed, 198 insertions(+), 30 deletions(-)
 create mode 100644 tests/neg/i18453.min.scala
 rename tests/{pos => neg}/i18453.scala (53%)
 create mode 100644 tests/pos/i18453.zio.scala
 create mode 100644 tests/pos/i19001.case1.scala
 create mode 100644 tests/pos/i19001.case2.scala
 create mode 100644 tests/pos/i19001.case3.scala
 create mode 100644 tests/pos/i19009.case1.scala
 create mode 100644 tests/pos/i19009.case2.scala
 create mode 100644 tests/pos/i19009.case3.scala
 create mode 100644 tests/pos/i19009.min3.scala

diff --git a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
index 4e3596ea8814..38f8e19e2737 100644
--- a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
+++ b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
@@ -263,29 +263,22 @@ trait PatternTypeConstrainer { self: TypeComparer =>
 
     trace(i"constraining simple pattern type $tp >:< $pt", gadts, (res: Boolean) => i"$res gadt = ${ctx.gadt}") {
       (tp, pt) match {
-        case (AppliedType(tyconS, argsS), AppliedType(tyconP, argsP)) =>
-          val saved = state.nn.constraint
-          val result =
-            ctx.gadtState.rollbackGadtUnless {
-              tyconS.typeParams.lazyZip(argsS).lazyZip(argsP).forall { (param, argS, argP) =>
-                val variance = param.paramVarianceSign
-                if variance == 0 || assumeInvariantRefinement ||
-                  // As a special case, when pattern and scrutinee types have the same type constructor,
-                  // we infer better bounds for pattern-bound abstract types.
-                  argP.typeSymbol.isPatternBound && patternTp.classSymbol == scrutineeTp.classSymbol
-                then
-                  val TypeBounds(loS, hiS) = argS.bounds
-                  val TypeBounds(loP, hiP) = argP.bounds
-                  var res = true
-                  if variance <  1 then res &&= isSubType(loS, hiP)
-                  if variance > -1 then res &&= isSubType(loP, hiS)
-                  res
-                else true
-              }
-            }
-          if !result then
-            constraint = saved
-          result
+        case (AppliedType(tyconS, argsS), AppliedType(tyconP, argsP)) => rollbackConstraintsUnless:
+          tyconS.typeParams.lazyZip(argsS).lazyZip(argsP).forall { (param, argS, argP) =>
+            val variance = param.paramVarianceSign
+            if variance == 0 || assumeInvariantRefinement ||
+              // As a special case, when pattern and scrutinee types have the same type constructor,
+              // we infer better bounds for pattern-bound abstract types.
+              argP.typeSymbol.isPatternBound && patternTp.classSymbol == scrutineeTp.classSymbol
+            then
+              val TypeBounds(loS, hiS) = argS.bounds
+              val TypeBounds(loP, hiP) = argP.bounds
+              var res = true
+              if variance <  1 then res &&= isSubType(loS, hiP)
+              if variance > -1 then res &&= isSubType(loP, hiS)
+              res
+            else true
+          }
         case _ =>
           // Give up if we don't get AppliedType, e.g. if we upcasted to Any.
           // Note that this doesn't mean that patternTp, scrutineeTp cannot possibly
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 7c1b6570b0e8..931196c0065a 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1010,7 +1010,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         if (tp2a ne tp2) // Follow the alias; this might avoid truncating the search space in the either below
           return recur(tp1, tp2a)
 
-        // Rewrite (T111 | T112) & T12 <: T2 to (T111 & T12) <: T2 and (T112 | T12) <: T2
+        // Rewrite (T111 | T112) & T12 <: T2 to (T111 & T12) <: T2 and (T112 & T12) <: T2
         // and analogously for T11 & (T121 | T122) & T12 <: T2
         // `&' types to the left of <: are problematic, because
         // we have to choose one constraint set or another, which might cut off
@@ -1982,6 +1982,13 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     else op2
   end necessaryEither
 
+  inline def rollbackConstraintsUnless(inline op: Boolean): Boolean =
+    val saved = constraint
+    var result = false
+    try result = ctx.gadtState.rollbackGadtUnless(op)
+    finally if !result then constraint = saved
+    result
+
   /** Decompose into conjunction of types each of which has only a single refinement */
   def decomposeRefinements(tp: Type, refines: List[(Name, Type)]): Type = tp match
     case RefinedType(parent, rname, rinfo) =>
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index a1ef6c0b2f25..2f03c79754e8 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -4194,7 +4194,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           val funExpected = functionExpected
           val arity =
             if funExpected then
-              defn.functionArity(ptNorm)
+              if !isFullyDefined(pt, ForceDegree.none) && isFullyDefined(wtp, ForceDegree.none) then
+                // if method type is fully defined, but expected type is not,
+                // prioritize method parameter types as parameter types of the eta-expanded closure
+                0
+              else defn.functionArity(ptNorm)
             else
               val nparams = wtp.paramInfos.length
               if nparams > 1
diff --git a/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala b/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala
index 8839a6cd03b1..4db047d0951e 100644
--- a/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala
+++ b/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala
@@ -102,7 +102,7 @@ class SemanticdbTests:
       |inspect with:
       |  diff $expect ${expect.resolveSibling("" + expect.getFileName + ".out")}
       |Or else update all expect files with
-      |  sbt 'scala3-compiler-bootstrapped/test:runMain dotty.tools.dotc.semanticdb.updateExpect'""".stripMargin)
+      |  sbt 'scala3-compiler-bootstrapped/Test/runMain dotty.tools.dotc.semanticdb.updateExpect'""".stripMargin)
     Files.walk(target).sorted(Comparator.reverseOrder).forEach(Files.delete)
     if errors.nonEmpty then
       fail(s"${errors.size} errors in expect test.")
diff --git a/tests/neg/i18453.min.scala b/tests/neg/i18453.min.scala
new file mode 100644
index 000000000000..e63a818e8f71
--- /dev/null
+++ b/tests/neg/i18453.min.scala
@@ -0,0 +1,11 @@
+// Slightly nicer version of i18453
+// which uses a non-abstract type Foo instead
+trait Box[T]
+
+trait Foo
+
+class Test:
+  def meth[A](func: A => A & Foo)(using boxA: Box[A]): Unit = ???
+  def test[B]                    (using boxB: Box[B]): Unit =
+    def nest(p: B): B & Foo = ???
+    meth(nest) // error
diff --git a/tests/pos/i18453.scala b/tests/neg/i18453.scala
similarity index 53%
rename from tests/pos/i18453.scala
rename to tests/neg/i18453.scala
index 40dd14935a10..9a865b420f65 100644
--- a/tests/pos/i18453.scala
+++ b/tests/neg/i18453.scala
@@ -1,3 +1,7 @@
+// Would be nice if this compiled
+// but it doesn't
+// because of how we constrain `A`
+// and then try to "minimise" its instantiation
 trait Box[T]
 
 class Test:
@@ -5,4 +9,4 @@ class Test:
 
   def g[X, Y](using bx: Box[X]): Unit =
     def d(t: X): X & Y = t.asInstanceOf[X & Y]
-    f(d)
+    f(d) // error
diff --git a/tests/neg/i5976.scala b/tests/neg/i5976.scala
index ef2e743e39fe..0b037f50a4ea 100644
--- a/tests/neg/i5976.scala
+++ b/tests/neg/i5976.scala
@@ -1,6 +1,6 @@
 object Test {
   def f(i: => Int) = i + i
-  val res = List(42).map(f)
+  val res = List(42).map(f) // error
 
   val g: (=> Int) => Int = f
   val h: Int => Int = g // error
diff --git a/tests/pos/i18453.zio.scala b/tests/pos/i18453.zio.scala
new file mode 100644
index 000000000000..32a9ebd0321c
--- /dev/null
+++ b/tests/pos/i18453.zio.scala
@@ -0,0 +1,33 @@
+// Minimised from zio's ZLayer ++
+
+// In an attempt to fix i18453
+// this would break zio's ZLayer
+// in the "would-error" cases
+class Cov[+W]:
+  def add[X >: W, Y](y: Cov[Y]): Cov[X & Y] = ???
+  def pre[Y >: W, X](x: Cov[X]): Cov[X & Y] = ???
+
+class Test:
+  def a1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & B & C]       = a.add(b).add(c)
+  def a2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A with B with C] = a.add(b).add(c) // would-error
+
+  def b1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.add(b).add(c) // would-error (a2)
+  def b2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.add(b).add(c)
+  def b3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.add(b.add(c))
+  def b4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.add(b.add(c))
+
+
+  def c3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & B & C]       = a.pre(b).pre(c)
+  def c4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A with B with C] = a.pre(b).pre(c) // would-error
+
+  def d1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.pre(b).pre(c) // would-error (c4)
+  def d2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.pre(b).pre(c)
+  def d3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.pre(b.pre(c))
+  def d4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.pre(b.pre(c))
+
+
+  def add[X, Y](x: Cov[X], y: Cov[Y]): Cov[X & Y] = ???
+  def e1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = add(add(a, b), c) // alt assoc: ok!
+  def e2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = add(add(a, b), c) // reg assoc: ok
+  def e3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = add(a, add(b, c)) // reg assoc: ok
+  def e4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = add(a, add(b, c)) // alt assoc: ok!
diff --git a/tests/pos/i19001.case1.scala b/tests/pos/i19001.case1.scala
new file mode 100644
index 000000000000..3e1a67caf308
--- /dev/null
+++ b/tests/pos/i19001.case1.scala
@@ -0,0 +1,19 @@
+import java.util.concurrent.CompletionStage
+import scala.concurrent.Future
+
+trait ActorRef[-T]:
+   def ask[Res](replyTo: ActorRef[Res] => T): Future[Res] = ???
+
+implicit final class FutureOps[T](private val f: Future[T]) extends AnyVal:
+  def asJava: CompletionStage[T] = ???
+
+class AskPattern[Req, Res]:
+  val actor: ActorRef[Req] = ???
+  val messageFactory: ActorRef[Res] => Req = ???
+
+  def failing(): CompletionStage[Res] = actor.ask(messageFactory.apply).asJava
+  def workaround1(): CompletionStage[Res] = actor.ask[Res](messageFactory.apply).asJava
+  def workaround2(): CompletionStage[Res] = actor.ask(messageFactory).asJava
+
+  val jMessageFactory: java.util.function.Function[ActorRef[Res], Req] = ???
+  def originalFailingCase(): CompletionStage[Res] = actor.ask(jMessageFactory.apply).asJava
diff --git a/tests/pos/i19001.case2.scala b/tests/pos/i19001.case2.scala
new file mode 100644
index 000000000000..547441c58ff2
--- /dev/null
+++ b/tests/pos/i19001.case2.scala
@@ -0,0 +1,16 @@
+import scala.util.{Try, Success, Failure}
+
+trait ActorRef[-T]
+trait ActorContext[T]:
+  def ask[Req, Res](target: ActorRef[Req], createRequest: ActorRef[Res] => Req)(mapResponse: Try[Res] => T): Unit
+
+@main def Test =
+  val context: ActorContext[Int] = ???
+  val askMeRef: ActorRef[Request] = ???
+
+  case class Request(replyTo: ActorRef[Int])
+
+  context.ask(askMeRef, Request.apply) {
+    case Success(res) => res // error: expected Int, got Any
+    case Failure(ex)  => throw ex
+  }
diff --git a/tests/pos/i19001.case3.scala b/tests/pos/i19001.case3.scala
new file mode 100644
index 000000000000..cc3f8e558fc8
--- /dev/null
+++ b/tests/pos/i19001.case3.scala
@@ -0,0 +1,12 @@
+trait IO[A]:
+  def map[B](f: A => B): IO[B] = ???
+
+trait RenderResult[T]:
+  def value: T
+
+def IOasync[T](f: (Either[Throwable, T] => Unit) => Unit): IO[T] = ???
+
+def render[T]: IO[T] = {
+  def register(cb: Either[Throwable, RenderResult[T]] => Unit): Unit = ???
+  IOasync(register).map(_.value) // map should take RenderResult[T], but uses Any
+}
diff --git a/tests/pos/i19009.case1.scala b/tests/pos/i19009.case1.scala
new file mode 100644
index 000000000000..84738dcf384a
--- /dev/null
+++ b/tests/pos/i19009.case1.scala
@@ -0,0 +1,18 @@
+trait Player[+P]
+trait RatingPeriod[P]:
+  def games: Map[P, Vector[ScoreVsPlayer[P]]]
+
+trait ScoreVsPlayer[+P]
+
+def updated[P](playerID: P, matchResults: IndexedSeq[ScoreVsPlayer[P]], lookup: P => Option[Player[P]]): Player[P] = ???
+
+trait Leaderboard[P]:
+  def  playersByIdInNoParticularOrder: Map[P, Player[P]]
+
+  def after[P2 >: P](ratingPeriod: RatingPeriod[? <: P]): Leaderboard[P2] =
+    val competingPlayers = ratingPeriod.games.iterator.map { (id, matchResults) =>
+      updated(id, matchResults, playersByIdInNoParticularOrder.get) // error
+      // workaround:
+      updated[P](id, matchResults, playersByIdInNoParticularOrder.get)
+    }
+    ???
diff --git a/tests/pos/i19009.case2.scala b/tests/pos/i19009.case2.scala
new file mode 100644
index 000000000000..8c395aa48e46
--- /dev/null
+++ b/tests/pos/i19009.case2.scala
@@ -0,0 +1,10 @@
+object NodeOrdering:
+  def postOrderNumbering[NodeType](cfgEntry: NodeType, expand: NodeType => Iterator[NodeType]): Map[NodeType, Int]  = ???
+
+trait CfgNode
+trait Method extends CfgNode
+
+def postOrder =
+  def method: Method = ???
+  def expand(x: CfgNode): Iterator[CfgNode] = ???
+  NodeOrdering.postOrderNumbering(method, expand)
diff --git a/tests/pos/i19009.case3.scala b/tests/pos/i19009.case3.scala
new file mode 100644
index 000000000000..b2b17b312af0
--- /dev/null
+++ b/tests/pos/i19009.case3.scala
@@ -0,0 +1,31 @@
+trait Bound[+E]
+
+trait SegmentT[E, +S]
+object SegmentT:
+  trait WithPrev[E, +S] extends SegmentT[E, S]
+
+trait SegmentSeqT[E, +S]:
+  def getSegmentForBound(bound: Bound[E]): SegmentT[E, S] with S
+
+abstract class AbstractSegmentSeq[E, +S] extends SegmentSeqT[E, S]
+
+trait MappedSegmentBase[E, S]
+
+type MappedSegment[E, S] = AbstractMappedSegmentSeq.MappedSegment[E, S]
+
+object AbstractMappedSegmentSeq:
+  type MappedSegment[E, S] = SegmentT[E, MappedSegmentBase[E, S]] with MappedSegmentBase[E, S]
+
+abstract class AbstractMappedSegmentSeq[E, S]
+    extends AbstractSegmentSeq[E, MappedSegmentBase[E, S]]:
+  def originalSeq: SegmentSeqT[E, S]
+
+  final override def getSegmentForBound(bound: Bound[E]): MappedSegment[E, S] =
+    searchFrontMapper(frontMapperGeneral, originalSeq.getSegmentForBound(bound))
+
+  protected final def frontMapperGeneral(original: SegmentT[E, S]): MappedSegment[E, S] = ???
+
+  protected def searchFrontMapper[Seg >: SegmentT.WithPrev[E, S] <: SegmentT[E, S], R](
+      mapper: Seg => R,
+      original: Seg
+  ): R = ???
diff --git a/tests/pos/i19009.min3.scala b/tests/pos/i19009.min3.scala
new file mode 100644
index 000000000000..f59a4485b219
--- /dev/null
+++ b/tests/pos/i19009.min3.scala
@@ -0,0 +1,9 @@
+trait Foo[A]
+trait Bar[B] extends Foo[B]
+
+class Test[C]:
+  def put[X >: Bar[C]](fn: X => Unit, x1: X): Unit = ()
+  def id(foo: Foo[C]): Foo[C] = foo
+
+  def t1(foo2: Foo[C]): Unit =
+    put(id, foo2) // was: error: exp: Bar[C], got (foo2 : Foo[C])
diff --git a/tests/semanticdb/metac.expect b/tests/semanticdb/metac.expect
index 2fd8eca47a7b..c8bb3d3e4ec4 100644
--- a/tests/semanticdb/metac.expect
+++ b/tests/semanticdb/metac.expect
@@ -1094,7 +1094,7 @@ Language => Scala
 Symbols => 181 entries
 Occurrences => 159 entries
 Diagnostics => 1 entries
-Synthetics => 5 entries
+Synthetics => 6 entries
 
 Symbols:
 _empty_/Enums. => final object Enums extends Object { self: Enums.type => +30 decls }
@@ -1277,7 +1277,7 @@ _empty_/Enums.unwrap().(ev) => implicit given param ev: <:<[A, Option[B]]
 _empty_/Enums.unwrap().(opt) => param opt: Option[A]
 _empty_/Enums.unwrap().[A] => typeparam A 
 _empty_/Enums.unwrap().[B] => typeparam B 
-local0 => param x: A
+local0 => param x: Option[B]
 
 Occurrences:
 [0:7..0:12): Enums <- _empty_/Enums.
@@ -1445,6 +1445,7 @@ Diagnostics:
 
 Synthetics:
 [52:9..52:13):Refl => *.unapply[Option[B]]
+[52:31..52:50):identity[Option[B]] => *[Function1[A, Option[B]]]
 [54:14..54:18):Some => *.apply[Some[Int]]
 [54:14..54:34):Some(Some(1)).unwrap => *(given_<:<_T_T[Option[Int]])
 [54:19..54:23):Some => *.apply[Int]