Backport "fix(#18265): crash on extension method without type nor RHS" to LTS

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

@@ -2607,6 +2607,8 @@ object Parsers {
  parents match {
  case parent :: Nil if !in.isNestedStart =>
  reposition(if (parent.isType) ensureApplied(wrapNew(parent)) else parent)
+ case tkn if in.token == INDENT =>
+ New(templateBodyOpt(emptyConstructor, parents, Nil))
  case _ =>
  New(reposition(templateBodyOpt(emptyConstructor, parents, Nil)))
  }

tests/neg/i18265.check

@@ -0,0 +1,6 @@
+-- [E019] Syntax Error: tests/neg/i18265.scala:5:13 --------------------------------------------------------------------
+5 | def twice // error
+ | ^
+ | Missing return type
+ |
+ | longer explanation available when compiling with `-explain`

tests/neg/i18265.scala

@@ -0,0 +1,5 @@
+trait Foo
+
+val foo = new Foo:
+ extension (s: String)
+ def twice // error

tests/semanticdb/expect/StructuralTypes.expect.scala

@@ -16,7 +16,7 @@ object StructuralTypes/*<-example::StructuralTypes.*/:
 
  val V/*<-example::StructuralTypes.V.*/: Object/*->java::lang::Object#*/ {
  def scalameta/*<-local4*/: String/*->scala::Predef.String#*/
- } = /*<-local6*/new:
- def scalameta/*<-local5*/ = "4.0"
+ } = new:
+ /*<-local6*/def scalameta/*<-local5*/ = "4.0"
  V/*->example::StructuralTypes.V.*/.scalameta/*->scala::reflect::Selectable#selectDynamic().*/
 end StructuralTypes/*->example::StructuralTypes.*/

tests/semanticdb/metac.expect

@@ -3472,7 +3472,7 @@ Occurrences:
 [16:9..16:15): Object -> java/lang/Object#
 [17:8..17:17): scalameta <- local4
 [17:19..17:25): String -> scala/Predef.String#
-[18:6..18:6): <- local6
+[19:4..19:4): <- local6
 [19:8..19:17): scalameta <- local5
 [20:2..20:3): V -> example/StructuralTypes.V.
 [20:4..20:13): scalameta -> scala/reflect/Selectable#selectDynamic().