cue/errors: don't equate error messages without line info

Previously, if two messages had no line and path information,
the two were equated and one was erased.

Not it falls back to the error message.

Change-Id: I9dff793777c2384939dac0873729dc77981d165a
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6482
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/testdata/script/eval_errs.txt

@@ -7,6 +7,9 @@ cmp stdout expect-stdout
 bar: empty disjunction: conflicting values int and "str" (mismatched types int and string):
     ./errs.cue:5:10
     ./errs.cue:6:16
+bar: empty disjunction: conflicting values string and 2 (mismatched types string and int):
+    ./errs.cue:5:21
+    ./errs.cue:6:26
 x.q: conflicting values "hello" and "goodbye":
     ./errs.cue:1:4
     ./errs.cue:2:4

cue/errors/errors.go

@@ -362,11 +362,7 @@ func (p *list) RemoveMultiples() {
 	var last Error
 	i := 0
 	for _, e := range *p {
-		pos := e.Position()
-		if last == nil ||
-			pos.Filename() != last.Position().Filename() ||
-			pos.Line() != last.Position().Line() ||
-			!equalPath(e.Path(), last.Path()) {
+		if last == nil || !approximateEqual(last, e) {
 			last = e
 			(*p)[i] = e
 			i++
@@ -375,6 +371,17 @@ func (p *list) RemoveMultiples() {
 	(*p) = (*p)[0:i]
 }
 
+func approximateEqual(a, b Error) bool {
+	aPos := a.Position()
+	bPos := b.Position()
+	if aPos == token.NoPos || bPos == token.NoPos {
+		return a.Error() == b.Error()
+	}
+	return aPos.Filename() == bPos.Filename() &&
+		aPos.Line() == bPos.Line() &&
+		equalPath(a.Path(), b.Path())
+}
+
 // An List implements the error interface.
 func (p list) Error() string {
 	format, args := p.Msg()

cuego/examples_test.go

@@ -42,7 +42,7 @@ func ExampleComplete_structTag() {
 	//Output:
 	// completed: cuego_test.Sum{A:1, B:5, C:6} (err: <nil>)
 	// completed: cuego_test.Sum{A:2, B:6, C:8} (err: <nil>)
-	// empty disjunction: conflicting values 5 and 2
+	// empty disjunction: conflicting values 5 and 2 (and 1 more errors)
 }
 
 func ExampleConstrain() {