Explain unresolvable references better

We run into problems when referring to a member of a self type of a class
that it not also a member of the class from outside via an asSeenFrom.

One example is in 11226.scala where we see:
```scala
trait ManagedActorClassification { this: ActorEventBus =>
  def unsubscribe(subscriber: Subscriber): Unit
}

class Unsubscriber(bus: ManagedActorClassification) {
  def test(a: ActorRef): Unit = bus.unsubscribe(a) // error
}
```
The problem is that `unsubscribe` refers to the type `Subscriber` which
is not resolvable as a member of `bus`. one idea could be to rule out
type signatures like `unsubscribe`, similar how we rule out public signatures
referring to private members. But this could rule out existing valid programs.
For instance, the `unsubscribe` signature is unproblematic if it gets only
called with prefixes that inherit `ActorEventBus`. You could say that the
problem was instead that the type of `bus` was not specific enough.

In the long term, maybe restructing the signature is the right move. But for
now, we just try to give better error messages in the case of existing failures.

Fixes #11226

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

@@ -57,17 +57,30 @@ end TypeError
 class MalformedType(pre: Type, denot: Denotation, absMembers: Set[Name])(using Context) extends TypeError:
   def toMessage(using Context) = em"malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}"
 
-class MissingType(pre: Type, name: Name)(using Context) extends TypeError:
-  private def otherReason(pre: Type)(using Context): String = pre match {
-    case pre: ThisType if pre.cls.givenSelfType.exists =>
-      i"\nor the self type of $pre might not contain all transitive dependencies"
-    case _ => ""
-  }
+class MissingType(val pre: Type, val name: Name)(using Context) extends TypeError:
+
+  def reason(using Context): String =
+    def missingClassFile =
+      "The classfile defining the type might be missing from the classpath"
+    val cls = pre.classSymbol
+    val givenSelf = cls match
+      case cls: ClassSymbol => cls.givenSelfType
+      case _ => NoType
+    pre match
+      case pre: ThisType if pre.cls.givenSelfType.exists =>
+        i"""$missingClassFile
+           |or the self type of $pre might not contain all transitive dependencies"""
+      case _ if givenSelf.exists && givenSelf.member(name).exists =>
+        i"""$name exists as a member of the self type $givenSelf of $cls
+           |but it cannot be referenced from a scope that does not extend that ${ctx.printer.kindString(cls)}"""
+      case _ =>
+        missingClassFile
+
 
   override def toMessage(using Context): Message =
     if ctx.debug then printStackTrace()
-    em"""cannot resolve reference to type $pre.$name
-        |the classfile defining the type might be missing from the classpath${otherReason(pre)}"""
+    em"""Cannot resolve reference to type $pre.$name.
+        |$reason."""
 end MissingType
 
 class RecursionOverflow(val op: String, details: => String, val previous: Throwable, val weight: Int)(using Context)

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -301,6 +301,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
     // these are usually easier to analyze. We exclude F-bounds since these would
     // lead to a recursive infinite expansion.
     object reported extends TypeMap, IdentityCaptRefMap:
+      var notes: String = ""
       def setVariance(v: Int) = variance = v
       val constraint = mapCtx.typerState.constraint
       var fbounded = false
@@ -318,6 +319,15 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
         case tp: LazyRef =>
           fbounded = true
           tp
+        case tp @ TypeRef(pre, _) =>
+          if pre != NoPrefix && !pre.member(tp.name).exists then
+            notes ++=
+              i"""
+                 |
+                 |Note that I could not resolve reference $tp.
+                 |${MissingType(pre, tp.name).reason}
+                 """
+          mapOver(tp)
         case _ =>
           mapOver(tp)
 
@@ -329,7 +339,7 @@ class TypeMismatch(val found: Type, expected: Type, val inTree: Option[untpd.Tre
       else (found1, expected1)
     val (foundStr, expectedStr) = Formatting.typeDiff(found2, expected2)
     i"""|Found:    $foundStr
-        |Required: $expectedStr"""
+        |Required: $expectedStr${reported.notes}"""
   end msg
 
   override def msgPostscript(using Context) =

tests/neg/i11226.check

@@ -0,0 +1,6 @@
+-- Error: tests/neg/i11226.scala:13:36 ---------------------------------------------------------------------------------
+13 |  def test(a: ActorRef): Unit = bus.unsubscribe(a) // error
+   |                                    ^
+   |               Cannot resolve reference to type (Unsubscriber.this.bus : ManagedActorClassification).Subscriber.
+   |               Subscriber exists as a member of the self type ActorEventBus of trait ManagedActorClassification
+   |               but it cannot be referenced from a scope that does not extend that trait.

tests/neg/i11226.scala

@@ -0,0 +1,14 @@
+trait ActorRef
+
+trait ActorEventBus {
+  type Subscriber = ActorRef
+}
+
+trait ManagedActorClassification { this: ActorEventBus =>
+  def unsubscribe(subscriber: Subscriber, from: Any): Unit
+  def unsubscribe(subscriber: Subscriber): Unit
+}
+
+class Unsubscriber(bus: ManagedActorClassification) {
+  def test(a: ActorRef): Unit = bus.unsubscribe(a) // error
+}

tests/neg/i11226a.check

@@ -0,0 +1,12 @@
+-- [E007] Type Mismatch Error: tests/neg/i11226a.scala:12:48 -----------------------------------------------------------
+12 |  def test(a: ActorRef): Unit = bus.unsubscribe(a) // error
+   |                                                ^
+   |                Found:    (a : ActorRef)
+   |                Required: Unsubscriber.this.bus.Subscriber
+   |
+   |                Note that I could not resolve reference Unsubscriber.this.bus.Subscriber.
+   |                Subscriber exists as a member of the self type ActorEventBus of trait ManagedActorClassification
+   |                but it cannot be referenced from a scope that does not extend that trait
+   |                                 
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i11226a.scala

@@ -0,0 +1,13 @@
+trait ActorRef
+
+trait ActorEventBus {
+  type Subscriber = ActorRef
+}
+
+trait ManagedActorClassification { this: ActorEventBus =>
+  def unsubscribe(subscriber: Subscriber): Unit
+}
+
+class Unsubscriber(bus: ManagedActorClassification) {
+  def test(a: ActorRef): Unit = bus.unsubscribe(a) // error
+}

tests/neg/i16407.check

@@ -1,12 +1,12 @@
 -- Error: tests/neg/i16407.scala:2:2 -----------------------------------------------------------------------------------
 2 |  f(g()) // error // error
   |  ^
-  |  cannot resolve reference to type (X.this : Y & X).A
-  |  the classfile defining the type might be missing from the classpath
-  |  or the self type of (X.this : Y & X) might not contain all transitive dependencies
+  |  Cannot resolve reference to type (X.this : Y & X).A.
+  |  The classfile defining the type might be missing from the classpath
+  |  or the self type of (X.this : Y & X) might not contain all transitive dependencies.
 -- Error: tests/neg/i16407.scala:2:4 -----------------------------------------------------------------------------------
 2 |  f(g()) // error // error
   |    ^
-  |    cannot resolve reference to type (X.this : Y & X).A
-  |    the classfile defining the type might be missing from the classpath
-  |    or the self type of (X.this : Y & X) might not contain all transitive dependencies
+  |    Cannot resolve reference to type (X.this : Y & X).A.
+  |    The classfile defining the type might be missing from the classpath
+  |    or the self type of (X.this : Y & X) might not contain all transitive dependencies.

tests/pos/i11226b.scala

@@ -0,0 +1,11 @@
+trait A {
+  class T()
+}
+trait B {
+  this: A =>
+  def f(a: Int = 0): Any
+}
+trait C extends B {
+  this: A =>
+  def f(t: T): Any
+}