Disallow exports of extension methods in extension methods

Revert "Add note on export of extensions"

This reverts commit 77447c8.

Author: odersky

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

@@ -1114,6 +1114,8 @@ class Namer { typer: Typer =>
           Skip
         else if pathMethod.exists && mbr.isType then
           No("is a type, so it cannot be exported as extension method")
+        else if pathMethod.exists && sym.is(ExtensionMethod) then
+          No("is already an extension method, cannot be exported into another one")
         else
           Yes
       }

docs/_docs/reference/other-new-features/export.md

@@ -162,11 +162,9 @@ extension (x: String)
   private def moreOps = new StringOps(x)
   export moreOps.*
 ```
-In this case the qualifier expression must be an identifier that refers to a unique parameterless extension method in the same extension clause.
-
-An export will then create extension methods for all accessible term members,
-matching the selectors, in the result of the qualifier path.
-For instance, the extension above would be expanded to
+In this case the qualifier expression must be an identifier that refers to a unique parameterless extension method in the same extension clause. The export will create
+extension methods for all accessible term members
+in the result of the qualifier path. For instance, the extension above would be expanded to
 ```scala
 extension (x: String)
   def take(n: Int): String = x.substring(0, n)
@@ -176,38 +174,6 @@ extension (x: String)
   def capitalize: String = moreOps.capitalize
 ```
 
-### A Note on Exporting Extension Methods
-**Note:** extension methods can have surprising results if exported from within an extension (i.e. they
-are exported as-if they were an ordinary method). Observe the following example:
-```scala
-class StringOps:
-  extension (x: String) def capitalize: String = ...
-  def zero: String = ""
-
-extension (s: String)
-  private def moreOps = new StringOps()
-  export moreOps.*
-```
-this would be expanded to
-```scala
-extension (s: String)
-  private def moreOps = new StringOps()
-  def capitalize(x: String): String = moreOps.capitalize(x)
-  ...
-```
-observe the extra String parameter on `capitalize`.
-It is instead recommended to export extension methods from
-outside of an extension, and to use a renaming selector to avoid them, e.g.:
-```scala
-private val myStringOps = new StringOps()
-
-extension (s: String)
-  private def moreOps = myStringOps
-  export moreOps.{capitalize as _, *}
-
-export myStringOps.capitalize
-```
-
 ## Syntax changes:
 
 ```

tests/neg/export-extension-in-extension.scala

@@ -0,0 +1,16 @@
+class StringOps:
+  extension (x: String)
+    def capitalize: String = ???
+    def foo: String = ???
+
+  def foo: String = ???
+
+
+extension (s: String)
+  private def moreOps = new StringOps()
+  export moreOps.capitalize // error: no eligible member capitalize at moreOps
+
+extension (s: String)
+  private def moreOps2= new StringOps()
+  export moreOps2.foo // OK
+