Fix #8181: Fix isExprIntro predicate to exclude extension on

Author: odersky

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

@@ -213,7 +213,9 @@ object Parsers {
     } && !in.isSoftModifierInModifierPosition
 
     def isExprIntro: Boolean =
-      in.canStartExprTokens.contains(in.token) && !in.isSoftModifierInModifierPosition
+      in.canStartExprTokens.contains(in.token)
+      && !in.isSoftModifierInModifierPosition
+      && !(isIdent(nme.extension) && followingIsExtension())
 
     def isDefIntro(allowedMods: BitSet, excludedSoftModifiers: Set[TermName] = Set.empty): Boolean =
       in.token == AT

tests/pos/i8181.scala

@@ -0,0 +1,6 @@
+object Main:
+  def main(args: Array[String]): Unit =
+    extension on (a: AnyRef):
+      def putout(): Unit = println(a)
+
+    "blub".putout()