Move reader comparability test to var

sdk/metric/manual_reader.go

@@ -39,8 +39,8 @@ type manualReader struct {
 	aggregationSelector AggregationSelector
 }
 
-// Compile time check the manualReader implements Reader.
-var _ Reader = &manualReader{}
+// Compile time check the manualReader implements Reader and is comparable.
+var _ = map[Reader]struct{}{&manualReader{}: {}}
 
 // NewManualReader returns a Reader which is directly called to collect metrics.
 func NewManualReader(opts ...ManualReaderOption) Reader {

sdk/metric/periodic_reader.go

@@ -149,6 +149,9 @@ type periodicReader struct {
 	shutdownOnce sync.Once
 }
 
+// Compile time check the periodicReader implements Reader and is comparable.
+var _ = map[Reader]struct{}{&periodicReader{}: {}}
+
 // newTicker allows testing override.
 var newTicker = time.NewTicker
 

sdk/metric/reader_test.go

@@ -216,10 +216,3 @@ func TestDefaultTemporalitySelector(t *testing.T) {
 		assert.Equal(t, metricdata.CumulativeTemporality, DefaultTemporalitySelector(ik))
 	}
 }
-
-func TestReadersAreComparable(t *testing.T) {
-	var _ map[Reader]struct{} = map[Reader]struct{}{
-		NewManualReader():                  {},
-		NewPeriodicReader(new(fnExporter)): {},
-	}
-}