conversions to unsigned numbers are not checked anymore; implements /… (#12688) [backport]

* conversions to unsigned numbers are not checked anymore; implements / fixes https://github.com/nim-lang/RFCs/issues/175

* change the spec yet again to be less consistent but to make more sense; updated the changelog

1 files changed, 14 insertions, 0 deletions

diff --git a/doc/manual.rst b/doc/manual.rst
index c869b391f..82487a385 100644
--- a/doc/manual.rst
+++ b/doc/manual.rst
@@ -3188,6 +3188,7 @@ has lots of advantages:
 
 Type conversions
 ----------------
+
 Syntactically a `type conversion` is like a procedure call, but a
 type name replaces the procedure name. A type conversion is always
 safe in the sense that a failure to convert a type to another
@@ -3207,6 +3208,19 @@ A type conversion can also be used to disambiguate overloaded routines:
   let procVar = (proc(x: string))(p)
   procVar("a")
 
+Since operations on unsigned numbers wrap around and are unchecked so are
+type conversion to unsigned integers and between unsigned integers. The
+rationale for this is mostly better interoperability with the C Programming
+language when algorithms are ported from C to Nim.
+
+Exception: Values that are converted to an unsigned type at compile time
+are checked so that code like ``byte(-1)`` does not compile.
+
+**Note**: Historically the operations
+were unchecked and the conversions were sometimes checked but starting with
+the revision 1.0.4 of this document and the language implementation the
+conversions too are now *always unchecked*.
+
 
 Type casts
 ----------