document default (#16942)

lib/system.nim

@@ -954,8 +954,23 @@ when defined(nimHasDefault):
     ##   echo @a # => @[1, 3, 5]
     ##   echo @b # => @['f', 'o', 'o']
 
-  proc default*(T: typedesc): T {.magic: "Default", noSideEffect.}
-    ## returns the default value of the type ``T``.
+  proc default*(T: typedesc): T {.magic: "Default", noSideEffect.} =
+    ## returns the default value of the type `T`.
+    runnableExamples:
+      assert (int, float).default == (0, 0.0)
+      # note: `var a = default(T)` is usually the same as `var a: T` and (currently) generates
+      # a value whose binary representation is all 0, regardless of whether this
+      # would violate type constraints such as `range`, `not nil`, etc. This
+      # property is required to implement certain algorithms efficiently which
+      # may require intermediate invalid states.
+      type Foo = object
+        a: range[2..6]
+      var a1: range[2..6] # currently, this compiles
+      # var a2: Foo # currently, this errors: Error: The Foo type doesn't have a default value.
+      # var a3 = Foo() # ditto
+      var a3 = Foo.default # this works, but generates a `UnsafeDefault` warning.
+    # note: the doc comment also explains why `default` can't be implemented
+    # via: `template default*[T](t: typedesc[T]): T = (var v: T; v)`
 
   proc reset*[T](obj: var T) {.noSideEffect.} =
     ## Resets an object `obj` to its default value.