Backport "Update phrasing for NotClassType explain error message" to …

…LTS (#20923)

Backports #19635 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

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

@@ -2749,7 +2749,11 @@ extends SyntaxMsg(TargetNameOnTopLevelClassID):
 class NotClassType(tp: Type)(using Context)
 extends TypeMsg(NotClassTypeID), ShowMatchTrace(tp):
   def msg(using Context) = i"$tp is not a class type"
-  def explain(using Context) = ""
+  def explain(using Context) =
+    i"""A class type includes classes and traits in a specific order. Defining a class, even an anonymous class,
+        |requires specifying a linearization order for the traits it extends. For example, `A & B` is not a class type
+        |because it doesn't specify which trait takes precedence, A or B. For more information about class types, please see the Scala Language Specification.
+        |Class types also can't have refinements."""
 
 class NotConstant(suffix: String, tp: Type)(using Context)
 extends TypeMsg(NotConstantID), ShowMatchTrace(tp):

docs/_docs/contributing/testing.md

@@ -141,6 +141,11 @@ checkfiles with the test outputs.
 $ sbt
 > testCompilation --update-checkfiles
 ```
+Or for ScalaJS
+```bash
+$ sbt
+> sjsCompilerTests/testOnly -- -Ddotty.tests.updateCheckfiles=TRUE
+```
 
 Use `--help` to see all the options
 ```bash

tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.check

@@ -6,14 +6,20 @@
 14 |  val b = js.constructorOf[NativeJSObject.type] // error
    |                           ^^^^^^^^^^^^^^^^^^^
    |                           NativeJSObject.type is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:16:27 -----------------------------
 16 |  val c = js.constructorOf[NativeJSClass with NativeJSTrait] // error
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                           NativeJSClass & NativeJSTrait is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:17:27 -----------------------------
 17 |  val d = js.constructorOf[NativeJSClass { def bar: Int }] // error
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                           NativeJSClass{def bar: Int} is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:19:27 -----------------------------------------
 19 |  val e = js.constructorOf[JSTrait] // error
    |                           ^^^^^^^
@@ -22,19 +28,29 @@
 20 |  val f = js.constructorOf[JSObject.type] // error
    |                           ^^^^^^^^^^^^^
    |                           JSObject.type is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:22:27 -----------------------------
 22 |  val g = js.constructorOf[JSClass with JSTrait] // error
    |                           ^^^^^^^^^^^^^^^^^^^^
    |                           JSClass & JSTrait is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:23:27 -----------------------------
 23 |  val h = js.constructorOf[JSClass { def bar: Int }] // error
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^
    |                           JSClass{def bar: Int} is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:25:42 -----------------------------
 25 |  def foo[A <: js.Any] = js.constructorOf[A] // error
    |                                          ^
    |                                          A is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructorof-error-in-prepjsinterop.scala:26:66 -----------------------------
 26 |  def bar[A <: js.Any: scala.reflect.ClassTag] = js.constructorOf[A] // error
    |                                                                  ^
    |                                                                  A is not a class type
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.check

@@ -6,14 +6,20 @@
 14 |  val b = js.constructorTag[NativeJSObject.type] // error
    |                                                ^
    |                                                NativeJSObject.type is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:16:61 ----------------------------
 16 |  val c = js.constructorTag[NativeJSClass with NativeJSTrait] // error
    |                                                             ^
    |                                                             NativeJSClass & NativeJSTrait is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:17:59 ----------------------------
 17 |  val d = js.constructorTag[NativeJSClass { def bar: Int }] // error
    |                                                           ^
    |                                                           NativeJSClass{def bar: Int} is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:19:36 ----------------------------------------
 19 |  val e = js.constructorTag[JSTrait] // error
    |                                    ^
@@ -22,19 +28,29 @@
 20 |  val f = js.constructorTag[JSObject.type] // error
    |                                          ^
    |                                          JSObject.type is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:22:49 ----------------------------
 22 |  val g = js.constructorTag[JSClass with JSTrait] // error
    |                                                 ^
    |                                                 JSClass & JSTrait is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:23:53 ----------------------------
 23 |  val h = js.constructorTag[JSClass { def bar: Int }] // error
    |                                                     ^
    |                                                     JSClass{def bar: Int} is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:25:45 ----------------------------
 25 |  def foo[A <: js.Any] = js.constructorTag[A] // error
    |                                             ^
    |                                             A is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg-scalajs/jsconstructortag-error-in-prepjsinterop.scala:26:69 ----------------------------
 26 |  def bar[A <: js.Any: scala.reflect.ClassTag] = js.constructorTag[A] // error
    |                                                                     ^
    |                                                                     A is not a class type
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/classOf.check

@@ -2,13 +2,19 @@
 6 |  def f1[T] = classOf[T] // error
   |                      ^
   |                      T is not a class type
+  |
+  | longer explanation available when compiling with `-explain`
 -- [E170] 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
+  |
+  | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg/classOf.scala:9:18 ---------------------------------------------------------------------
 9 |  val y = classOf[C { type I = String }] // error
   |                  ^^^^^^^^^^^^^^^^^^^^^
   |                  Test.C{type I = String} is not a class type
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i13808.check

@@ -2,7 +2,11 @@
 13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
    |                                     ^^^^^^^^^^
    |                                     OpaqueTypes.OpaqueType is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] 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
+   |
+   | longer explanation available when compiling with `-explain`