document 'var T' and 'typedesc' restriction in generics; closes #1156

doc/manual.rst

@@ -4387,6 +4387,34 @@ be inferred to have the equivalent of the `any` type class and thus they will
 match anything without discrimination.
 
 
+Generic inference restrictions
+------------------------------
+
+The types ``var T`` and ``typedesc[T]`` cannot be inferred in a generic
+instantiation. The following is not allowed:
+
+.. code-block:: nim
+    :test: "nim c $1"
+    :status: 1
+
+  proc g[T](f: proc(x: T); x: T) =
+    f(x)
+
+  proc c(y: int) = echo y
+  proc v(y: var int) =
+    y += 100
+  var i: int
+
+  # allowed: infers 'T' to be of type 'int'
+  g(c, 42)
+
+  # not valid: 'T' is not inferred to be of type 'var int'
+  g(v, i)
+
+  # also not allowed: explict instantiation via 'var int'
+  g[var int](v, i)
+
+
 Concepts
 --------