More careful ClassTag instantiation (#23659)

We now use a blend of the new scheme and a backwards compatible special
case if type variables as ClassTag arguments are constrained by further
type variables.

Fixes #23611

Author: odersky

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

@@ -18,6 +18,7 @@ import annotation.{tailrec, constructorOnly}
 import ast.tpd
 import ast.tpd.*
 import Synthesizer.*
+import TypeComparer.{fullLowerBound, fullUpperBound}
 
 /** Synthesize terms for special classes */
 class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
@@ -36,10 +37,32 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         // bounds are usually widened during instantiation.
         instArg(tp.tp1)
       case tvar: TypeVar if ctx.typerState.constraint.contains(tvar) =>
+      	// If tvar has a lower or upper bound:
+      	//   1. If the bound is not another type variable, use this as approximation.
+      	//   2. Otherwise, if the type can be forced to be fully defined, use that type
+      	//      as approximation.
+      	//   3. Otherwise leave argument uninstantiated.
+      	// The reason for (2) is that we observed complicated constraints in i23611.scala
+      	// that get better types if a fully defined type is computed than if several type
+      	// variables are approximated incrementally. This is a minimization of some ZIO code.
+      	// So in order to keep backwards compatibility (where before we _only_ did 2) we
+      	// add that special case.
+        def isGroundConstr(tp: Type): Boolean = tp.dealias match
+          case tvar: TypeVar if ctx.typerState.constraint.contains(tvar) => false
+          case pref: TypeParamRef if ctx.typerState.constraint.contains(pref) => false
+          case tp: AndOrType => isGroundConstr(tp.tp1) && isGroundConstr(tp.tp2)
+          case _ => true
         instArg(
-            if tvar.hasLowerBound then tvar.instantiate(fromBelow = true)
-            else if tvar.hasUpperBound then tvar.instantiate(fromBelow = false)
-            else NoType)
+            if tvar.hasLowerBound then
+              if isGroundConstr(fullLowerBound(tvar.origin)) then tvar.instantiate(fromBelow = true)
+              else if isFullyDefined(tp, ForceDegree.all) then tp
+              else NoType
+            else if tvar.hasUpperBound then
+              if isGroundConstr(fullUpperBound(tvar.origin)) then tvar.instantiate(fromBelow = false)
+              else if isFullyDefined(tp, ForceDegree.all) then tp
+              else NoType
+            else
+              NoType)
       case _ =>
         tp
 

tests/pos/i23611.scala

@@ -0,0 +1,26 @@
+import java.io.{File, IOException}
+import java.net.URI
+import java.nio.file.{Path, Paths}
+import scala.reflect.ClassTag
+
+trait FileConnectors {
+  def listPath(path: => Path): ZStream[Any, IOException, Path]
+
+  final def listFile(file: => File): ZStream[Any, IOException, File] =
+    for {
+      path <- null.asInstanceOf[ZStream[Any, IOException, Path]]
+      r    <- listPath(path).mapZIO(a => ZIO.attempt(a.toFile).refineToOrDie)
+    } yield r
+}
+
+sealed trait ZIO[-R, +E, +A]
+extension [R, E <: Throwable, A](self: ZIO[R, E, A])
+  def refineToOrDie[E1 <: E: ClassTag]: ZIO[R, E1, A] = ???
+
+object ZIO:
+  def attempt[A](code: => A): ZIO[Any, Throwable, A] = ???
+
+sealed trait ZStream[-R, +E, +A]:
+  def map[B](f: A => B): ZStream[R, E, B] = ???
+  def flatMap[R1 <: R, E1 >: E, B](f: A => ZStream[R1, E1, B]): ZStream[R1, E1, B]
+  def mapZIO[R1 <: R, E1 >: E, A1](f: A => ZIO[R1, E1, A1]): ZStream[R1, E1, A1]

tests/pos/i23611a.scala

@@ -0,0 +1,30 @@
+import java.io.{File, IOException}
+import java.net.URI
+import java.nio.file.{Path, Paths}
+import scala.reflect.ClassTag
+
+trait FileConnectors {
+  def listPath(path: => Path): ZStream[Any, IOException, Path]
+
+  final def listFile(file: => File): ZStream[Any, IOException, File] =
+    for {
+      path <- null.asInstanceOf[ZStream[Any, IOException, Path]]
+      r    <- listPath(path).mapZIO(a => ZIO.attempt(a.toFile).refineToOrDie)
+    } yield r
+}
+
+sealed abstract class CanFail[-E]
+object CanFail:
+  given [E]: CanFail[E] = ???
+
+sealed trait ZIO[-R, +E, +A]
+extension [R, E <: Throwable, A](self: ZIO[R, E, A])
+  def refineToOrDie[E1 <: E: ClassTag](using CanFail[E]): ZIO[R, E1, A] = ???
+
+object ZIO:
+  def attempt[A](code: => A): ZIO[Any, Throwable, A] = ???
+
+sealed trait ZStream[-R, +E, +A]:
+  def map[B](f: A => B): ZStream[R, E, B] = ???
+  def flatMap[R1 <: R, E1 >: E, B](f: A => ZStream[R1, E1, B]): ZStream[R1, E1, B]
+  def mapZIO[R1 <: R, E1 >: E, A1](f: A => ZIO[R1, E1, A1]): ZStream[R1, E1, A1]