SI-6728 Fixes crash in parser on incomplete for expression

The parser was trying to take the position of an empty expression
which would crash. Son on the empty expression case in TreeBuilder#
makeWhile it tries to do that and, if that failed, gets a position
that wraps both the condition and the body.

I also made a slight improvement to the UnsupportedOperationEx
messages in Position.

4 files changed, 14 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala b/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala
index f94055f666..ac8ab493e0 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala
@@ -258,7 +258,7 @@ abstract class TreeBuilder {
 
   /** Create tree representing a while loop */
   def makeWhile(lname: TermName, cond: Tree, body: Tree): Tree = {
-    val continu = atPos(o2p(body.pos.endOrPoint)) { Apply(Ident(lname), Nil) }
+    val continu = atPos(o2p(body.pos pointOrElse wrappingPos(List(cond, body)).pos.endOrPoint)) { Apply(Ident(lname), Nil) }
     val rhs = If(cond, Block(List(body), continu), Literal(Constant()))
     LabelDef(lname, Nil, rhs)
   }
diff --git a/src/reflect/scala/reflect/internal/util/Position.scala b/src/reflect/scala/reflect/internal/util/Position.scala
index 3d10d4c9ce..8f287a1640 100644
--- a/src/reflect/scala/reflect/internal/util/Position.scala
+++ b/src/reflect/scala/reflect/internal/util/Position.scala
@@ -91,7 +91,7 @@ abstract class Position extends scala.reflect.api.Position { self =>
   /** An optional value containing the source file referred to by this position, or
    *  None if not defined.
    */
-  def source: SourceFile = throw new UnsupportedOperationException("Position.source")
+  def source: SourceFile = throw new UnsupportedOperationException(s"Position.source on ${this.getClass}")
 
   /** Is this position neither a NoPosition nor a FakePosition?
    *  If isDefined is true, offset and source are both defined.
@@ -111,19 +111,19 @@ abstract class Position extends scala.reflect.api.Position { self =>
   def makeTransparent: Position = this
 
   /** The start of the position's range, error if not a range position */
-  def start: Int = throw new UnsupportedOperationException("Position.start")
+  def start: Int = throw new UnsupportedOperationException(s"Position.start on ${this.getClass}")
 
   /** The start of the position's range, or point if not a range position */
   def startOrPoint: Int = point
 
   /**  The point (where the ^ is) of the position */
-  def point: Int = throw new UnsupportedOperationException("Position.point")
+  def point: Int = throw new UnsupportedOperationException(s"Position.point on ${this.getClass}")
 
   /**  The point (where the ^ is) of the position, or else `default` if undefined */
   def pointOrElse(default: Int): Int = default
 
   /** The end of the position's range, error if not a range position */
-  def end: Int = throw new UnsupportedOperationException("Position.end")
+  def end: Int = throw new UnsupportedOperationException(s"Position.end on ${this.getClass}")
 
   /** The end of the position's range, or point if not a range position */
   def endOrPoint: Int = point
diff --git a/test/files/neg/t6728.check b/test/files/neg/t6728.check
new file mode 100644
index 0000000000..d853d6f724
--- /dev/null
+++ b/test/files/neg/t6728.check
@@ -0,0 +1,4 @@
+t6728.scala:4: error: '(' expected but '}' found.
+  }
+  ^
+one error found
diff --git a/test/files/neg/t6728.scala b/test/files/neg/t6728.scala
new file mode 100644
index 0000000000..ba0b1a0fdf
--- /dev/null
+++ b/test/files/neg/t6728.scala
@@ -0,0 +1,5 @@
+object X {
+  while(true) {
+    for
+  }
+}