syncs: fix AtomicValue.CompareAndSwap (#16137)

Fix CompareAndSwap in the edge-case where
the underlying sync.AtomicValue is uninitialized
(i.e., Store was never called) and
the oldV is the zero value,
then perform CompareAndSwap with any(nil).

Also, document that T must be comparable.
This is a pre-existing restriction.

Fixes #16135

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/syncs_test.go

@@ -64,6 +64,23 @@ func TestAtomicValue(t *testing.T) {
 			t.Fatalf("LoadOk = (%v, %v), want (nil, true)", got, gotOk)
 		}
 	}
+
+	{
+		c1, c2, c3 := make(chan struct{}), make(chan struct{}), make(chan struct{})
+		var v AtomicValue[chan struct{}]
+		if v.CompareAndSwap(c1, c2) != false {
+			t.Fatalf("CompareAndSwap = true, want false")
+		}
+		if v.CompareAndSwap(nil, c1) != true {
+			t.Fatalf("CompareAndSwap = false, want true")
+		}
+		if v.CompareAndSwap(c2, c3) != false {
+			t.Fatalf("CompareAndSwap = true, want false")
+		}
+		if v.CompareAndSwap(c1, c2) != true {
+			t.Fatalf("CompareAndSwap = false, want true")
+		}
+	}
 }
 
 func TestMutexValue(t *testing.T) {