fix #8485 ; make default public instead of redefined everywhere; us…

…e better implementation

lib/core/seqs.nim

@@ -88,10 +88,6 @@ proc grow*[T](x: var seq[T]; newLen: int; value: T) =
     x.data[i] = value
   x.len = newLen
 
-template default[T](t: typedesc[T]): T =
-  var v: T
-  v
-
 proc setLen*[T](x: var seq[T]; newLen: int) {.deprecated.} =
   if newlen < x.len: shrink(x, newLen)
   else: grow(x, newLen, default(T))

lib/pure/collections/queues.nim

@@ -154,10 +154,6 @@ proc add*[T](q: var Queue[T], item: T) =
   q.data[q.wr] = item
   q.wr = (q.wr + 1) and q.mask
 
-template default[T](t: typedesc[T]): T =
-  var v: T
-  v
-
 proc pop*[T](q: var Queue[T]): T {.inline, discardable.} =
   ## Remove and returns the first (oldest) element of the queue `q`.
   emptyCheck(q)

lib/pure/collections/sets.nim

@@ -41,11 +41,6 @@ type
 
 {.deprecated: [TSet: HashSet].}
 
-template default[T](t: typedesc[T]): T =
-  ## Used by clear methods to get a default value.
-  var v: T
-  v
-
 proc clear*[A](s: var HashSet[A]) =
   ## Clears the HashSet back to an empty state, without shrinking
   ## any of the existing storage. O(n) where n is the size of the hash bucket.
@@ -281,10 +276,6 @@ template doWhile(a, b) =
     b
     if not a: break
 
-template default[T](t: typedesc[T]): T =
-  var v: T
-  v
-
 proc exclImpl[A](s: var HashSet[A], key: A) : bool {. inline .} =
   assert s.isValid, "The set needs to be initialized."
   var hc: Hash

lib/pure/collections/tableimpl.nim

@@ -116,10 +116,6 @@ template hasKeyOrPutImpl(enlarge) {.dirty.} =
     maybeRehashPutImpl(enlarge)
   else: result = true
 
-template default[T](t: typedesc[T]): T =
-  var v: T
-  v
-
 template delImplIdx(t, i) =
   let msk = maxHash(t)
   if i >= 0:

lib/system.nim

@@ -4193,3 +4193,19 @@ when defined(genode):
       componentConstructHook(env)
         # Perform application initialization
         # and return to thread entrypoint.
+
+# needed pending https://github.com/nim-lang/Nim/issues/8486
+proc defaultImpl[T]():T=
+  var a: T
+  result = a
+
+template default*[T](t: typedesc[T]): T =
+  ## returns default value for type ``T``, and can work at compile time; V2
+  runnableExamples:
+    const a = (1,2).type.default
+    doAssert a == (0, 0)
+  defaultImpl[T]()
+
+when isMainModule:
+  # needed pending https://github.com/nim-lang/Nim/issues/7280
+  discard int.default

tests/destructor/tcustomseqs.nim

@@ -98,10 +98,6 @@ proc grow*[T](x: var myseq[T]; newLen: int; value: T) =
     x.data[i] = value
   x.len = newLen
 
-template default[T](t: typedesc[T]): T =
-  var v: T
-  v
-
 proc setLen*[T](x: var myseq[T]; newLen: int) {.deprecated.} =
   if newlen < x.len: shrink(x, newLen)
   else: grow(x, newLen, default(T))

tests/system/tsystem_misc.nim

@@ -102,3 +102,21 @@ proc foo(a: openArray[byte]) =
 
 let str = "0123456789"
 foo(toOpenArrayByte(str, 0, str.high))
+
+
+# BUG:`block default2:` causes compile time error:
+# attempting to call undeclared routine: 'default'
+block default2:
+  doAssert int.default == 0
+
+  # test case from https://github.com/nim-lang/Nim/issues/8485
+  proc len(T: typedesc[tuple]):auto=
+    const t=default(T)
+    var n=0
+    for _ in t.fields:
+      inc n
+    return n
+
+  var a=(1,2,3)
+  const length = a.type.len
+  doAssert length == 3

tests/vm/tvmmisc.nim

@@ -13,10 +13,10 @@ block:
 
 # #4412
 block:
-  proc default[T](t: typedesc[T]): T {.inline.} = discard
+  proc default2[T](t: typedesc[T]): T {.inline.} = discard
 
   static:
-    var x = default(type(0))
+    var x = default2(type(0))
 
 # #6379
 import algorithm