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digitalmars.D.bugs - [Issue 1818] New: storage class in type tuples is discarded for function and delegate declarations

reply d-bugmail puremagic.com writes:

           Summary: storage class in type tuples is discarded for function
                    and delegate declarations
           Product: D
           Version: 2.010
          Platform: PC
        OS/Version: All
            Status: NEW
          Keywords: wrong-code, spec
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: kamm-removethis incasoftware.de

Type tuples do contain storage class information when obtained via is(func
argstuple == function) or std.traits.ParameterTypeTuple. However, this
information is discarded if the tuple is used for a function declaration even
though the ParameterTypeTuple retains the storage class sometimes.

void foo(ref int i) { ++i; }
void bar(ParameterTypeTuple!(foo) f) { foo(f); }

void main() {
  int i = 0;
  writefln(i); // prints 1

// but incorrectly:

pragma(msg, ParameterTypeTuple!(foo).stringof); // prints "(ref int)"
pragma(msg, ParameterTypeTuple!(bar).stringof); // prints "(ref int)"

I think the frontend code responsible for this misbehaviour is func.c around
line 760. There the storage class of the tuple is propagated to each of the
tuple members, replacing existing storage classes.

Fixing this bug will allow compile-time reflection and manipulation of
parameter storage classes. It would still be impossible to create type tuples
containing non-default storage classes directly, but using code similar to this
will allow indirect manipulation via argument tuples:

template appendArg(X, T...) {
    alias ParameterTypeTuple!(void delegate(T, X)) appendArg;

template appendRefArg(X, T...) {
    alias ParameterTypeTuple!(void delegate(T, ref X)) appendRefArg;

alias appendRefArg!(int) refint;
alias appendArg!(real, refint) refint_real;
alias appendRefArg!(long, refint_real) refint_real_outlong;

pragma(msg, refint.stringof ~ " " ~ refint_real.stringof ~ " " ~
// current output: (ref int) (int, real) (int, real, ref long)
// desired output: (ref int) (ref int, real) (ref int, real, ref long)

Consequently this would also allow the implementation of the identity function
template that Andrei recommended as a testcase a while back:

ReturnType!(func) ident(alias func)(ParameterTypeTuple!(func) args) { return
func(args); }

Currently this fails for functions with arguments using non-default storage

Finally, this change might break some code that expects storage classes to be
lost. It also interacts with bugs 1411 and 1424.

Feb 05 2008
next sibling parent d-bugmail puremagic.com writes:

------- Comment #1 from kamm-removethis incasoftware.de  2008-02-07 13:10
I have commented out the mentioned code in func.c and tested this with gdc.
While it works in the sense that

void foo(ref int i) { ++i; }
void bar(ParameterTypeTuple!(foo) f) { foo(f); }

now yields a bar taking a ref int, there are other issues. Most notably

template printT(T...)
  pragma(msg, T.stringof);
  alias T printT;
alias printT!(ParameterTypeTuple!(foo)) printed;

prints a mere (int). This seems to happen because tuples are split into types
when instantiating a template, losing their extra information. (template.c
TemplateInstance::semanticTiargs I think).

Looking at the whole issue again, it may be more sensible to remove storage
class information from type tuples altogether. After all, they are supposed to
hold types and not argument-specifications. As storage class manipulation is
essential in some instances, a new kind of tuple could be introduced:

Feb 07 2008
prev sibling parent d-bugmail puremagic.com writes:

yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
                 CC|                            |dbiehl gmx.net

--- Comment #2 from yebblies <yebblies gmail.com> 2011-06-10 10:47:20 PDT ---
*** Issue 1424 has been marked as a duplicate of this issue. ***

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Jun 10 2011