Fix #10123: Try to fully define qualifier type

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

@@ -81,6 +81,28 @@ object Inferencing {
     if (depVars.nonEmpty) instantiateSelected(tp, depVars.toList)
   }
 
+  /** If `tp` is top-level type variable with a lower bound in the current constraint,
+   *  instantiate it from below. We also look for TypeVars whereever their instantiation
+   *  could uncover new type members.
+   */
+  def couldInstantiateTypeVar(tp: Type)(using Context): Boolean = tp.dealias match
+    case tvar: TypeVar
+    if !tvar.isInstantiated
+       && ctx.typerState.constraint.contains(tvar)
+       && tvar.hasLowerBound =>
+      tvar.instantiate(fromBelow = true)
+      true
+    case AppliedType(tycon, _) =>
+      couldInstantiateTypeVar(tycon)
+    case RefinedType(parent, _, _) =>
+      couldInstantiateTypeVar(parent)
+    case tp: AndOrType =>
+      couldInstantiateTypeVar(tp.tp1) || couldInstantiateTypeVar(tp.tp2)
+    case AnnotatedType(tp, _) =>
+      couldInstantiateTypeVar(tp)
+    case _ =>
+      false
+
   /** The accumulator which forces type variables using the policy encoded in `force`
    *  and returns whether the type is fully defined. The direction in which
    *  a type variable is instantiated is determined as follows:

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

@@ -12,6 +12,7 @@ import config.Printers.typr
 import ast.Trees._
 import NameOps._
 import ProtoTypes._
+import Inferencing.couldInstantiateTypeVar
 import collection.mutable
 import reporting._
 import Checking.{checkNoPrivateLeaks, checkNoWildcard}
@@ -159,6 +160,9 @@ trait TypeAssigner {
       TryDynamicCallType
     else if (qualType.isErroneous || name.toTermName == nme.ERROR)
       UnspecifiedErrorType
+    else if couldInstantiateTypeVar(qualType) then
+      // try again with more defined qualifier type
+      selectionType(tree, qual1)
     else if (name == nme.CONSTRUCTOR)
       errorType(ex"$qualType does not have a constructor", tree.srcPos)
     else {

tests/pos/i10123.scala

@@ -0,0 +1,19 @@
+class D
+class C[+T](x: T)
+
+class Foo() {
+  val status: Int = 0
+}
+
+object Main {
+  implicit class RichC[T](c: C[T]) {
+    def await(implicit d: D = ???): T = ???
+  }
+
+  def test1: Int = {
+    val foo = new C(new Foo()).await
+    foo.status
+  }
+
+  val test2 = new C(new Foo()).await.status
+}

tests/pos/i9567.scala

@@ -0,0 +1,22 @@
+// object Test {
+//   val x: Int => Int = identity
+// }
+
+trait Foo[F[_]] {
+  def foo[G[x] >: F[x]]: G[Unit]
+}
+
+trait M[A] {
+  def bla: Int = 1
+  def baz(f: Int => Int): Int = f(1)
+}
+
+object Test {
+  def bar(x: Foo[M]): Unit = {
+    // error: value bla is not a member of G[Unit], where:    G is a type variable with constraint >: M and <: [x] =>> Any
+    x.foo.bla
+
+    // error: value bla is not a member of G[Unit], where:    G is a type variable with constraint >: M and <: [x] =>> Any
+    x.foo.baz(x => x)
+  }
+}