iterable[T] (#17196)

* fix failing test toSeq in manual which now works
* changelog
* reject proc fn(a: iterable)
* add iterable to spec
* remove MCS/UFCS limitation that now works

tests/types/titerable.nim

@@ -0,0 +1,143 @@
+discard """
+  targets: "c js"
+"""
+
+from stdtest/testutils import accept, reject, whenVMorJs
+
+# toSeq-like templates
+
+template toSeq2(a: iterable): auto =
+  var ret: seq[typeof(a)]
+  for ai in a: ret.add ai
+  ret
+
+template toSeq3(a: iterable[string]): auto =
+  var ret: seq[typeof(a)]
+  for ai in a: ret.add ai
+  ret
+
+template toSeq4[T](a: iterable[T]): auto =
+  var ret: seq[typeof(a)]
+  for ai in a: ret.add ai
+  ret
+
+template toSeq5[T: SomeInteger](a: iterable[T]): auto =
+  var ret: seq[typeof(a)]
+  for ai in a: ret.add ai
+  ret
+
+template toSeq6(a: iterable[int | float]): auto =
+  var ret: seq[typeof(a)]
+  for ai in a: ret.add ai
+  ret
+
+template toSeq7(a: iterable[seq]): auto =
+  var ret: seq[typeof(a)]
+  for ai in a: ret.add ai
+  ret
+
+template fn7b(a: untyped) = discard
+template fn7c(a: typed) = discard
+template fn7d(a: auto) = discard
+template fn7e[T](a: T) = discard
+
+template fn8a(a: iterable) = discard
+template fn8b[T](a: iterable[T]) = discard
+template fn8c(a: iterable[int]) = discard
+
+template bad1 =
+  template fn4(a: int, b: iterable[float, int]) =
+    discard
+
+template bad2 =
+  proc fn4(a: iterable) = discard
+
+template bad3 =
+  proc fn4(a: iterable[int]) = discard
+
+template good4 =
+  template fn1(a: iterable) = discard
+  template fn2(a: iterable[int]) = discard
+
+# iterators
+iterator iota(n: int): auto =
+  for i in 0..<n: yield i
+
+iterator repeat[T](a: T, n: int): T =
+  for i in 0..<n:
+    yield a
+
+iterator myiter(n: int): auto =
+  for i in 0..<n: yield $(i*2)
+
+when not defined(js):
+  iterator iotaClosure(n: int): auto {.closure.} =
+    for i in 0..<n: yield i
+
+template main() =
+  let expected1 = @[0, 1, 2]
+  let expected2 = @["0", "2"]
+
+  doAssert toSeq2(myiter(2)) == expected2
+  doAssert toSeq2(iota(3)) == expected1
+  doAssert toSeq2(1.1.repeat(2)) == @[1.1, 1.1]
+
+  whenVMorJs: discard
+  do:
+    doAssert toSeq2(iotaClosure(3)) == expected1
+
+  when true:
+    # MCS/UFCS
+    doAssert iota(3).toSeq2() == expected1
+
+  doAssert toSeq3(myiter(2)) == expected2
+  accept toSeq3(myiter(2))
+  reject toSeq3(iota(3))
+
+  doAssert toSeq4(iota(3)) == expected1
+  doAssert toSeq4(myiter(2)) == expected2
+  
+  doAssert toSeq4(0..2) == expected1
+  doAssert toSeq4(items(0..2)) == expected1
+  doAssert toSeq4(items(@[0,1,2])) == expected1
+  reject toSeq4(@[0,1,2])
+  reject toSeq4(13)
+
+  block:
+    accept fn8a(iota(3))
+    accept fn7b(iota(3))
+    reject fn7c(iota(3))
+    reject fn7d(iota(3))
+    reject fn7e(iota(3))
+
+  block:
+    fn8a(iota(3))
+    reject fn8a(123)
+    reject fn8c(123)
+    reject fn8c(123.3)
+    accept fn8c(items(@[1,2]))
+
+  block:
+    # shows that iterable is more restrictive than untyped
+    reject fn8a(nonexistant)
+    accept fn7b(nonexistant)
+    reject fn7c(nonexistant)
+    reject fn7d(nonexistant)
+    reject fn7e(nonexistant)
+
+  doAssert toSeq5(iota(3)) == expected1
+  reject toSeq5(myiter(2))
+
+  doAssert toSeq6(iota(3)) == expected1
+  reject toSeq6(myiter(2))
+
+  doAssert toSeq2("abc".repeat(3)) == @["abc", "abc", "abc"]
+  doAssert toSeq2(@['x', 'y'].repeat(2)) == @[@['x', 'y'], @['x', 'y']]
+
+  reject bad1
+  reject bad2
+  reject bad3
+  accept good4
+
+static: main()
+main()