Better positions for infix term operations.

Preserving the position of infix operators is useful for IDEs'
type-at-point. We also preserve the position of the untyped lhs of
right-associative operators, this is useful both for IDEs and for error
messages, before:
4 |val x: List[Int] = "foo" :: List(1)
  |                                   ^
  |                                   found:    String($1$)
  |                                   required: Int
  |

After:
scala> val x: List[Int] = "foo" :: List(1)
-- [E007] Type Mismatch Error: <console> ---------------------------------------
4 |val x: List[Int] = "foo" :: List(1)
  |                   ^^^^^
  |                   found:    String($1$)
  |                   required: Int
  |

Note: It would be even nicer if we displayed "String" instead of
"String($1$)" since $1$ is synthetic, this commit does not address this.

Author: smarter

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -750,12 +750,14 @@ object desugar {
           case Tuple(args) => args mapConserve assignToNamedArg
           case _ => right :: Nil
         }
-        Apply(Select(left, op.name), args)
+        val selectPos = Position(left.pos.start, op.pos.end, op.pos.start)
+        Apply(Select(left, op.name).withPos(selectPos), args)
       } else {
         val x = ctx.freshName().toTermName
+        val selectPos = Position(op.pos.start, right.pos.end, op.pos.start)
         new InfixOpBlock(
           ValDef(x, TypeTree(), left).withMods(synthetic),
-          Apply(Select(right, op.name), Ident(x)))
+          Apply(Select(right, op.name).withPos(selectPos), Ident(x).withPos(left.pos)))
       }
     }
 

tests/repl/errmsgs.check

@@ -78,4 +78,11 @@ scala> class Foo() { def bar: Int = 1 }; val foo = new Foo(); foo.barr
 4 |class Foo() { def bar: Int = 1 }; val foo = new Foo(); foo.barr
   |                                                       ^^^^^^^^
   |        value `barr` is not a member of Foo(foo) - did you mean `foo.bar`?
+scala> val x: List[Int] = "foo" :: List(1)
+-- [E007] Type Mismatch Error: <console> ---------------------------------------
+4 |val x: List[Int] = "foo" :: List(1)
+  |                   ^^^^^
+  |                   found:    String($1$)
+  |                   required: Int
+  |                   
 scala> :quit