Make type parameters after extension method def illegal

We'd have to at least check that there's no use before definition.
Forbid for now, and come back once we support multiple type parameter
clauses.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -910,9 +910,8 @@ object desugar {
   /** Transform extension construct to list of extension methods */
   def extMethods(ext: ExtMethods)(using Context): Tree = flatTree {
     for mdef <- ext.methods yield
-      if ext.tparams.nonEmpty && mdef.tparams.nonEmpty then
-        ctx.error(em"extension method cannot have type parameters since some were already given in extension clause",
-          mdef.tparams.head.sourcePos)
+      if mdef.tparams.nonEmpty then
+        ctx.error("no type parameters allowed here", mdef.tparams.head.sourcePos)
       defDef(
         cpy.DefDef(mdef)(
           name = mdef.name.toExtensionName,

docs/docs/reference/contextual/extension-methods.md

@@ -85,7 +85,8 @@ Extensions can also take using clauses. For instance, the `+` extension above co
     def - (y: T): T = n.minus(x, y)
 ```
 
-**Note**: If an extension defines type parameters in its prefix, the extension method itself is not allowed to have additional type parameters. This restriction might be lifted in the future once we support multiple type parameter clauses in a method.
+**Note**: Type parameters have to be given after the `extension` keyword;
+they cannot be given after the `def`. This restriction might be lifted in the future once we support multiple type parameter clauses in a method.
 
 ### Collective Extensions
 

tests/neg-custom-args/extmethods-tparams.scala

@@ -1,2 +1,2 @@
-def (self: T).foo[T] = ??? // error
-def [T1](self: T1) bar[T2] = ??? // error // error
+extension (self: T) def foo[T] = ??? // error
+extension [T1](self: T1) def bar[T2] = ??? // error