Refrain from instantiating type variables to undetermined types

When instantiating type variables before implicit search, refrain instantiating
them with bounds that are either Nothing or Any or that contain wildcard types.

Fixes #12247

Author: odersky

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

@@ -429,6 +429,10 @@ 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`.
@@ -4461,13 +4465,30 @@ 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
+}