https://issues.dlang.org/show_bug.cgi?id=24176

          Issue ID: 24176
           Summary: Parameters of opApply delegate don't have to be `ref`
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dlang.org
          Assignee: nobody@puremagic.com
          Reporter: nick@geany.org

The spec should be clearer on that.
Also a `ref` parameter of the delegate can accept non-ref foreach variables.

PR incoming.

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