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

2 files changed, 28 insertions, 1 deletions

diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 18f0b42f5..5604cb9e4 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/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(DanglingThisInPath(), start)
         dotSelectors(t, finish)
       }
       def handleSuper(qual: Ident) = {
diff --git a/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala b/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
index 25db28e4a..9ba771be9 100644
--- a/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala
@@ -966,4 +966,31 @@ object messages {
            |    ${"val foo: Int = 3"}
            |"""
   }
+
+  case class DanglingThisInPath()(implicit ctx: Context) extends Message(36) {
+    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}
+           |"""
+  }
 }