Allow erased as extension parameter

Author: odersky

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

@@ -2961,7 +2961,7 @@ object Parsers {
         if in.token == RPAREN && !prefix && !impliedMods.is(Given) then Nil
         else
           val clause =
-            if prefix && !isIdent(nme.using) then param() :: Nil
+            if prefix && !isIdent(nme.using) && !isIdent(nme.erased) then param() :: Nil
             else
               paramMods()
               if givenOnly && !impliedMods.is(Given) then

tests/pos/i11743.scala

@@ -0,0 +1,8 @@
+import language.experimental.erasedTerms
+import scala.compiletime.erasedValue
+type UnivEq[A]
+object UnivEq:
+  erased def force[A]: UnivEq[A] = erasedValue
+  extension [A](erased proof: UnivEq[A])
+    inline def univEq(a: A, b: A): Boolean =
+      a == b