time: be consistent about representation of UTC location in Time struct

In the zero Time, the (not user visible) nil *Location indicates UTC.
In the result of t.UTC() and other ways to create times in specific
zones, UTC is indicated by a non-nil *Location, specifically &utcLoc.
This creates a representation ambiguity exposed by comparison with ==
or reflect.DeepEqual or the like.

Change time.Time representation to use only nil, never &utcLoc,
to represent UTC. This eliminates the ambiguity.

Fixes #15716.

Change-Id: I7dcc2c20ce6b073e1daae323d3e49d17d1d52802
Reviewed-on: https://go-review.googlesource.com/31144
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/time/format.go b/src/time/format.go
index ea95f0be44..0ad3cf64e4 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -1021,12 +1021,12 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 		// If that zone was in effect at the given time, use it.
 		name, offset, _, _, _ := local.lookup(t.sec + internalToUnix)
 		if offset == zoneOffset && (zoneName == "" || name == zoneName) {
-			t.loc = local
+			t.setLoc(local)
 			return t, nil
 		}
 
 		// Otherwise create fake zone to record offset.
-		t.loc = FixedZone(zoneName, zoneOffset)
+		t.setLoc(FixedZone(zoneName, zoneOffset))
 		return t, nil
 	}
 
@@ -1037,7 +1037,7 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 		offset, _, ok := local.lookupName(zoneName, t.sec+internalToUnix)
 		if ok {
 			t.sec -= int64(offset)
-			t.loc = local
+			t.setLoc(local)
 			return t, nil
 		}
 
@@ -1046,7 +1046,7 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 			offset, _ = atoi(zoneName[3:]) // Guaranteed OK by parseGMT.
 			offset *= 3600
 		}
-		t.loc = FixedZone(zoneName, offset)
+		t.setLoc(FixedZone(zoneName, offset))
 		return t, nil
 	}