Explain unresolvable references better

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 called on a receiver whose type does not extend $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) =

compiler/test/dotc/neg-best-effort-pickling.blacklist

@@ -14,6 +14,7 @@ i17121.scala
 illegal-match-types.scala
 i13780-1.scala
 i20317a.scala
+i11226.scala
 
 # semantic db generation fails in the first compilation
 i1642.scala

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 called on a receiver whose type does not extend trait ManagedActorClassification.

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 called on a receiver whose type does not extend trait ManagedActorClassification
+   |                                
+   |
+   | 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
+}