encoding/json: simplify dominantField

Fixes #18037

Change-Id: I20e27bcc013b00b726eb348daf5ca86b138ddcc2
Reviewed-on: https://go-review.googlesource.com/107598
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/json/encode.go

@@ -1237,32 +1237,10 @@ func typeFields(t reflect.Type) []field {
 // will be false: This condition is an error in Go and we skip all
 // the fields.
 func dominantField(fields []field) (field, bool) {
-	// The fields are sorted in increasing index-length order. The winner
-	// must therefore be one with the shortest index length. Drop all
-	// longer entries, which is easy: just truncate the slice.
-	length := len(fields[0].index)
-	tagged := -1 // Index of first tagged field.
-	for i, f := range fields {
-		if len(f.index) > length {
-			fields = fields[:i]
-			break
-		}
-		if f.tag {
-			if tagged >= 0 {
-				// Multiple tagged fields at the same level: conflict.
-				// Return no field.
-				return field{}, false
-			}
-			tagged = i
-		}
-	}
-	if tagged >= 0 {
-		return fields[tagged], true
-	}
-	// All remaining fields have the same length. If there's more than one,
-	// we have a conflict (two fields named "X" at the same level) and we
-	// return no field.
-	if len(fields) > 1 {
+	// The fields are sorted in increasing index-length order, then by presence of tag.
+	// That means that the first field is the dominant one. We need only check
+	// for error cases: two fields at top level, either both tagged or neither tagged.
+	if len(fields) > 1 && len(fields[0].index) == len(fields[1].index) && fields[0].tag == fields[1].tag {
 		return field{}, false
 	}
 	return fields[0], true