Merge pull request #12294 from dotty-staging/fix-12247

Refrain from instantiating type variables to undetermined types

Author: smarter

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -433,6 +433,10 @@ object Types {
     /** Is this a higher-kinded type lambda with given parameter variances? */
     def isDeclaredVarianceLambda: Boolean = false
 
+    /** Does this type contain wildcard types? */
+    final def containsWildcardTypes(using Context) =
+      existsPart(_.isInstanceOf[WildcardType], stopAtStatic = true)
+
 // ----- Higher-order combinators -----------------------------------
 
     /** Returns true if there is a part of this type that satisfies predicate `p`.
@@ -4465,13 +4469,30 @@ object Types {
     def instantiate(fromBelow: Boolean)(using Context): Type =
       instantiateWith(avoidCaptures(TypeComparer.instanceType(origin, fromBelow)))
 
+    /** For uninstantiated type variables: the entry in the constraint (either bounds or
+     *  provisional instance value)
+     */
+    private def currentEntry(using Context): Type = ctx.typerState.constraint.entry(origin)
+
     /** For uninstantiated type variables: Is the lower bound different from Nothing? */
-    def hasLowerBound(using Context): Boolean =
-      !ctx.typerState.constraint.entry(origin).loBound.isExactlyNothing
+    def hasLowerBound(using Context): Boolean = !currentEntry.loBound.isExactlyNothing
 
     /** For uninstantiated type variables: Is the upper bound different from Any? */
-    def hasUpperBound(using Context): Boolean =
-      !ctx.typerState.constraint.entry(origin).hiBound.isRef(defn.AnyClass)
+    def hasUpperBound(using Context): Boolean = !currentEntry.hiBound.isRef(defn.AnyClass)
+
+    /** For uninstantiated type variables: Is the lower bound different from Nothing and
+     *  does it not contain wildcard types?
+     */
+    def hasNonWildcardLowerBound(using Context): Boolean =
+      val lo = currentEntry.loBound
+      !lo.isExactlyNothing && !lo.containsWildcardTypes
+
+    /** For uninstantiated type variables: Is the upper bound different from Any and
+     *  does it not contain wildcard types?
+     */
+    def hasNonWildcardUpperBound(using Context): Boolean =
+      val hi = currentEntry.hiBound
+      !hi.isRef(defn.AnyClass) && !hi.containsWildcardTypes
 
     /** Unwrap to instance (if instantiated) or origin (if not), until result
      *  is no longer a TypeVar

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

@@ -173,12 +173,14 @@ object Inferencing {
         && ctx.typerState.constraint.contains(tvar)
         && {
           val direction = instDirection(tvar.origin)
-          if direction != 0 then
+          if minimizeSelected then
+            if direction <= 0 && tvar.hasNonWildcardLowerBound then
+              instantiate(tvar, fromBelow = true)
+            else if direction >= 0 && tvar.hasNonWildcardUpperBound then
+              instantiate(tvar, fromBelow = false)
+            // else hold off instantiating unbounded unconstrained variable
+          else if direction != 0 then
             instantiate(tvar, fromBelow = direction < 0)
-          else if minimizeSelected then
-            if tvar.hasLowerBound then instantiate(tvar, fromBelow = true)
-            else if tvar.hasUpperBound then instantiate(tvar, fromBelow = false)
-            else () // hold off instantiating unbounded unconstrained variables
           else if variance >= 0 && (force.ifBottom == IfBottom.ok || tvar.hasLowerBound) then
             instantiate(tvar, fromBelow = true)
           else if variance >= 0 && force.ifBottom == IfBottom.fail then

tests/pos/i12247.scala

@@ -0,0 +1,27 @@
+sealed abstract class CtorType
+object CtorType {
+  final class Props extends CtorType
+  sealed trait Summoner { type CT <: CtorType }
+  implicit def summonP: Summoner {type CT = Props} = ???
+}
+
+final case class Builder() {
+  def build(using ctorType: CtorType.Summoner): Component[ctorType.CT] = ???
+}
+
+final class Component[CT <: CtorType]
+
+object Test {
+
+  def assertTypeOf[A](a: => A) = new TestDsl[A]
+  class TestDsl[A] {
+    def is[B](implicit ev: A =:= B): Unit = ()
+  }
+
+  type Expect = Component[CtorType.Props]
+
+  assertTypeOf( Builder().build ).is[Expect] // error
+
+  val x = Builder().build
+  assertTypeOf(x).is[Expect] // ok
+}