Avoid spurious error spans

Parser.syntaxError contains a tweak that underlines the whole span
of the current token, if it has a name. Tho make this work reliably
we need to reset the name to null when reading a new token, since not every
token sets a name.

Author: odersky

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

@@ -407,6 +407,7 @@ object Scanners {
      */
     protected final def fetchToken(): Unit = {
       offset = charOffset - 1
+      name = null
       (ch: @switch) match {
         case ' ' | '\t' | CR | LF | FF =>
           nextChar()

tests/neg/trailingCommas.scala

@@ -18,7 +18,7 @@ trait SimpleExpr { (23, "bar", ) } // error
 trait TypeArgs { def f: ValidGeneric[Int, String, ] } // error
 
 trait TypeParamClause { type C[A, B, ] } // error
-trait FunTypeParamClause { def f[A, B, ] } // error
+trait FunTypeParamClause { def f[A, B, ] } // error // error
 
 trait SimpleType { def f: (Int, String, ) } // error
 trait FunctionArgTypes { def f: (Int, String, ) => Boolean } // error