Fixes #8100 (#8101)

lib/pure/strformat.nim

@@ -258,7 +258,9 @@ template callFormat(res, arg) {.dirty.} =
     # workaround in order to circumvent 'strutils.format' which matches
     # too but doesn't adhere to our protocol.
     res.add arg
-  elif compiles(format(arg, res)):
+  elif compiles(format(arg, res)) and
+      # Check if format returns void
+      not (compiles do: discard format(arg, res)):
     format(arg, res)
   elif compiles(format(arg)):
     res.add format(arg)
@@ -684,6 +686,9 @@ when isMainModule:
   var nullTime: DateTime
   check &"{nullTime:yyyy-mm-dd}", "0000-00-00"
 
+  var tm = fromUnix(0)
+  discard &"{tm}"
+
   # Unicode string tests
   check &"""{"αβγ"}""", "αβγ"
   check &"""{"αβγ":>5}""", "  αβγ"