Friendlier error message when a : is given after extension clause

Author: odersky

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

@@ -3565,6 +3565,9 @@ object Parsers {
       val tparams = typeParamClauseOpt(ParamOwner.Def)
       val extParams = paramClause(0, prefix = true)
       val givenParamss = paramClauses(givenOnly = true)
+      if in.token == COLON then
+        syntaxError("no `:` expected here")
+        in.nextToken()
       val methods =
         if isDefIntro(modifierTokens) then
           extMethod() :: Nil

tests/neg/extension-colon.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/extension-colon.scala:1:18 -------------------------------------------------------------------------
+1 |extension (x: Int): // error
+  |                  ^
+  |                  no `:` expected here

tests/neg/extension-colon.scala

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