Show a match type trace for "not a class type" errors

Closes #15155

Author: odersky

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

@@ -183,7 +183,8 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case LossyWideningConstantConversionID // errorNumber: 167
   case ImplicitSearchTooLargeID // errorNumber: 168
   case TargetNameOnTopLevelClassID // errorNumber: 169
-
+  case NotClassTypeID // errorNumber 170
+  
   def errorNumber = ordinal - 1
 
 object ErrorMessageID:

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

@@ -2521,3 +2521,8 @@ import transform.SymUtils._
           |  $annot $symbol { ... }
           |
           |${hl("export")} Wrapper.${symbol.name}  ${hl("// optional")}"""
+
+  class NotClassType(tp: Type)(using Context)
+  extends TypeMsg(NotClassTypeID), ShowMatchTrace(tp):
+    def msg = ex"$tp is not a class type"
+    def explain = ""

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

@@ -937,7 +937,7 @@ trait Checking {
         if (stablePrefixReq && ctx.phase <= refchecksPhase) checkStable(tref.prefix, pos, "class prefix")
         tp
       case _ =>
-        report.error(ex"$tp is not a class type", pos)
+        report.error(NotClassType(tp), pos)
         defn.ObjectType
     }
 

tests/neg/classOf.check

@@ -1,14 +1,14 @@
--- Error: tests/neg/classOf.scala:6:22 ---------------------------------------------------------------------------------
+-- [E079] Type Error: tests/neg/classOf.scala:6:22 ---------------------------------------------------------------------
 6 |  def f1[T] = classOf[T] // error
   |                      ^
   |                      T is not a class type
--- Error: tests/neg/classOf.scala:7:32 ---------------------------------------------------------------------------------
+-- [E079] Type Error: tests/neg/classOf.scala:7:32 ---------------------------------------------------------------------
 7 |  def f2[T <: String] = classOf[T] // error
   |                                ^
   |                                T is not a class type
   |
   |                                where:    T is a type in method f2 with bounds <: String
--- Error: tests/neg/classOf.scala:9:18 ---------------------------------------------------------------------------------
+-- [E079] Type Error: tests/neg/classOf.scala:9:18 ---------------------------------------------------------------------
 9 |  val y = classOf[C { type I = String }] // error
   |                  ^^^^^^^^^^^^^^^^^^^^^
   |                  Test.C{I = String} is not a class type

tests/neg/i13808.check

@@ -1,8 +1,8 @@
--- Error: tests/neg/i13808.scala:13:37 ---------------------------------------------------------------------------------
+-- [E079] Type Error: tests/neg/i13808.scala:13:37 ---------------------------------------------------------------------
 13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
    |                                     ^^^^^^^^^^
    |                                     OpaqueTypes.OpaqueType is not a class type
--- Error: tests/neg/i13808.scala:13:49 ---------------------------------------------------------------------------------
+-- [E079] Type Error: tests/neg/i13808.scala:13:49 ---------------------------------------------------------------------
 13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
    |                                                 ^^^
    |                                                 FooModule.Foo is not a class type

tests/neg/i15155.check

@@ -0,0 +1,11 @@
+-- [E079] Type Error: tests/neg/i15155.scala:10:33 ---------------------------------------------------------------------
+10 |  val EnumerationClass = classOf[EnumValue[E]] // error
+   |                                 ^^^^^^^^^^^^
+   |                                 EnumValue[E] is not a class type
+   |
+   |                                 Note: a match type could not be fully reduced:
+   |
+   |                                   trying to reduce  EnumValue[E]
+   |                                   failed since selector  E
+   |                                   does not match  case Aux[a] => a
+   |                                   and cannot be shown to be disjoint from it either.

tests/neg/i15155.scala

@@ -0,0 +1,11 @@
+import scala.reflect.ClassTag
+// https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala-3/org/json4s/ext/package.scala#L4-L8
+type Aux[A] = { type Value = A }
+type EnumValue[A <: Enumeration] = A match {
+  case Aux[a] => a
+}
+
+// https://github.com/json4s/json4s/blob/355d8751391773e0d79d04402a4f9fb7bfc684ec/ext/src/main/scala/org/json4s/ext/EnumSerializer.scala#L25-L26
+class EnumSerializer[E <: Enumeration: ClassTag](enumeration: E) {
+  val EnumerationClass = classOf[EnumValue[E]] // error
+}