DAA and 'out' parameters (#20506)

* DAA and 'out' parameters

* progress

* documented strictDefs and out parameters

* docs, tests and a bugfix

* fixes silly regression

tests/init/tinitchecks_v2.nim

@@ -0,0 +1,59 @@
+discard """
+cmd: "nim check $file"
+action: "compile"
+"""
+
+{.experimental: "strictDefs".}
+
+proc myopen(f: out File; s: string): bool =
+  f = default(File)
+  result = false
+
+proc main =
+  var f: File
+  if myopen(f, "aarg"):
+    f.close
+
+proc invalid =
+  var s: seq[string]
+  s.add "abc" #[tt.Warning
+  ^ use explicit initialization of 's' for clarity [Uninit] ]#
+
+proc valid =
+  var s: seq[string] = @[]
+  s.add "abc" # valid!
+
+main()
+invalid()
+valid()
+
+proc branchy(cond: bool) =
+  var s: seq[string]
+  if cond:
+    s = @["y"]
+  else:
+    s = @[]
+  s.add "abc" # valid!
+
+branchy true
+
+proc p(x: out int; y: out string; cond: bool) = #[tt.Warning
+                   ^ Cannot prove that 'y' is initialized. This will become a compile time error in the future. [ProveInit] ]#
+  x = 4
+  if cond:
+    y = "abc"
+  # error: not every path initializes 'y'
+
+var gl: int
+var gs: string
+p gl, gs, false
+
+proc canRaise(x: int): int =
+  result = x
+  raise newException(ValueError, "wrong")
+
+proc currentlyValid(x: out int; y: out string; cond: bool) =
+  x = canRaise(45)
+  y = "abc" # <-- error: not every path initializes 'y'
+
+currentlyValid gl, gs, false

tests/init/toutparam_subtype.nim

@@ -0,0 +1,24 @@
+discard """
+cmd: "nim check $file"
+action: "compile"
+errormsg: "type mismatch: got <Subclass[system.int]>"
+line: 21
+"""
+
+{.experimental: "strictDefs".}
+
+type
+  Superclass[T] = object of RootObj
+    a: T
+  Subclass[T] = object of Superclass[T]
+    s: string
+
+proc init[T](x: out Superclass[T]) =
+  x = Superclass(a: 8)
+
+proc subtypeCheck =
+  var v: Subclass[int]
+  init(v)
+  echo v.s # the 's' field was never initialized!
+
+subtypeCheck()

tests/init/tuninit1.nim

@@ -4,7 +4,7 @@ discard """
 """
 
 import strutils
-
+{.experimental: "strictDefs".}
 {.warning[Uninit]:on.}
 
 proc p =