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digitalmars.D.bugs - [Issue 16672] New: Deprecate "block only" delegate syntax


          Issue ID: 16672
           Summary: Deprecate "block only" delegate syntax
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: schveiguy yahoo.com

Currently, this code:

{int x = 5; return x;}

Can either be a block (if written by itself), or a delegate, if used in an
expression. e.g.:

auto dg = {int x = 5; return x;}

This particular ambiguity becomes horrible if combined with the single
expression lambda syntax:

auto x = (a => {return a;})(1);

What this looks like is an immediately called lambda that returns its parameter
(a). However, what it *REALLY* returns is a delegate that will return 1 when

This confusion leads to many problems, as people never want this result.

The solution is simple -- deprecate the shortened brace syntax, and require ()
before the braces. e.g.:

auto dg = () {int x = 5; return x; }

Without the parens, the above should be an error. The drawback is obviously
that one needs to insert parentheses wherever a no-param delegate needs
declaring. The cost of doing this, IMO, is low compared to the regular reports
we get of the x => {...} syntax "not working".

Nov 08 2016