Add error message for dangling this in path selections

The following examples trigger the error message:

    val x: Foo.this = ???

    // Also triggers the error:
    import foo.this

    // Additionally, also slays the compiler
    type X = Foo.this.type

Author: b-studios

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -540,7 +540,7 @@ object Parsers {
       def handleThis(qual: Ident) = {
         in.nextToken()
         val t = atPos(start) { This(qual) }
-        if (!thisOK && in.token != DOT) syntaxError("`.' expected")
+        if (!thisOK && in.token != DOT) syntaxError(DangelingThisInPath(), start)
         dotSelectors(t, finish)
       }
       def handleSuper(qual: Ident) = {

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

@@ -901,4 +901,31 @@ object messages {
     val msg = hl"trying to define package with same name as `$existing`"
     val explanation = ""
   }
+
+  case class DangelingThisInPath()(implicit ctx: Context) extends Message(34) {
+    val kind = "Syntax"
+    val msg = hl"""Expected an additional member selection after the keyword ${"this"}"""
+
+    val importCode =
+      """import MyClass.this.member
+        |//                 ^^^^^^^
+      """
+
+    val typeCode =
+      """type T = MyClass.this.Member
+        |//                   ^^^^^^^
+      """
+
+    val explanation =
+      hl"""|Paths of imports and type selections must not end with the keyword ${"this"}.
+           |
+           |Maybe you forgot to select a member of ${"this"}?
+           |
+           |- Example for a valid import expression using a path
+           |${importCode}
+           |
+           |- Example for a valid type using a path
+           |${typeCode}
+           |"""
+  }
 }