Special error message for extension (...) with

Author: odersky

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

@@ -3619,8 +3619,11 @@ object Parsers {
         isUsingClause(extParams)
       do ()
       leadParamss ++= paramClauses(givenOnly = true, numLeadParams = nparams)
-      if in.token == COLON then
-        syntaxError("no `:` expected here")
+      if in.token == WITH then
+        syntaxError(
+          i"""No `with` expected here.
+             |
+             |An extension clause is simply followed by one or more method definitions.""")
         in.nextToken()
       val methods =
         if isDefIntro(modifierTokens) then

tests/neg/extension-colon.check

@@ -0,0 +1,6 @@
+-- Error: tests/neg/extension-with.scala:1:19 --------------------------------------------------------------------------
+1 |extension (x: Int) with // error
+  |                   ^^^^
+  |                   No `with` expected here.
+  |
+  |                   An extension clause is simply followed by one or more method definitions.

tests/neg/extension-colon.scala

@@ -0,0 +1,2 @@
+extension (x: Int) with // error
+  def foo = x