Error tree on outdent is zero extent

Recovery on syntax error may consume remaining input,
with an OUTDENT at EOF, so that the parent tree does
not actually extend to EOF.

Do not report spurious `def <error>` as ambiguous.

Author: som-snytt

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

@@ -409,7 +409,9 @@ object Parsers {
       false
     }
 
-    def errorTermTree(start: Offset): Tree = atSpan(Span(start, in.offset)) { unimplementedExpr }
+    def errorTermTree(start: Offset): Tree =
+      val end = if in.token == OUTDENT then start else in.offset
+      atSpan(Span(start, end)) { unimplementedExpr }
 
     private var inFunReturnType = false
     private def fromWithinReturnType[T](body: => T): T = {

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

@@ -307,7 +307,7 @@ object Scanners {
         println(s"\nSTART SKIP AT ${sourcePos().line + 1}, $this in $currentRegion")
       var noProgress = 0
         // Defensive measure to ensure we always get out of the following while loop
-        // even if source file is weirly formatted (i.e. we never reach EOF)
+        // even if source file is weirdly formatted (i.e. we never reach EOF)
       var prevOffset = offset
       while !atStop && noProgress < 3 do
         nextToken()

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -1339,7 +1339,7 @@ trait Checking {
     typr.println(i"check no double declarations $cls")
 
     def checkDecl(decl: Symbol): Unit = {
-      for (other <- seen(decl.name) if !decl.isAbsent() && !other.isAbsent()) {
+      for other <- seen(decl.name) if decl.name != nme.ERROR && !decl.isAbsent() && !other.isAbsent() do
         typr.println(i"conflict? $decl $other")
         def javaFieldMethodPair =
           decl.is(JavaDefined) && other.is(JavaDefined) &&
@@ -1356,7 +1356,6 @@ trait Checking {
         if decl.hasDefaultParams && other.hasDefaultParams then
           report.error(em"two or more overloaded variants of $decl have default arguments", decl.srcPos)
           decl.resetFlag(HasDefaultParams)
-      }
       if (!excludeFromDoubleDeclCheck(decl))
         seen(decl.name) = decl :: seen(decl.name)
     }

tests/neg/i23729.scala

@@ -0,0 +1,19 @@
+
+trait Collection[Self, Element]:
+  type Index
+  extension (self: Self)
+    def start: Index
+
+sealed trait Tree[+T]
+object Tree:
+  case object Empty extends Tree[Nothing]
+  case class Node[+T](value: T, lhs: Tree[T], rhs: Tree[T]) extends Tree[T]
+
+enum Direction:
+  case Left, Right, Here
+given [T]: Collection[Tree[T], T] with
+  type Index = List[Direction]
+  extension (self: Tree[T])
+    def start: List[Direction] = match self // error syntax
+      case Empty => Nil // error poor recovery
+      case Node(_, l, _) => l.start :+ Left // error poor recovery