Re: [patch] for PR 18040

[Zack Weinberg <zack at codesourcery dot com>]

kenner@vlsi1.ultra.nyu.edu (Richard Kenner) writes:

>     If you provide an example which produces a type conversion in the
>     middle of a chain of dereference expressions, I will endeavor to
>     clarify what I mean.
>
> I'll try, but, as I said, I don't write Ada very well ...
>
> 	with unchecked_conversion;
> 	function foo return integer is
> 	    type arr is array (1 .. 100) of integer;
> 	    type r is record
> 		f1: integer;
> 		f2: array (1..99) of integer;
> 	    end record;
> 	    function uc is new unchecked_conversion (arr, r);
> 	    type q is record f1: arr; end record;
>
> 	    var: q;
> 	begin
> 	    return (uc (q.f1).f1;
> 	end foo;

This isn't a great example because everything in here is addressable,
so one could equally well solve the problem by decomposing the chain
of references.

Florian's example is better in this regard.  I showed elsewhere how to
solve it by promoting the transformation to an addressable level.
With the suggestion that COMPONENT_REFS would implicitly convert,
you'd have something which I can no longer write in any source
language, so here it is in pseudo-dump_tree() notation (just the body
of the function):

  <modify_expr type <void>
     <var_decl type <U> var2>
     <component_ref type <U> 
        <field_decl type <U> B>
        <component_ref type <Baz> 
           <field_decl type <T> B>
           <var_decl type <Foo> Var1>>>>

The crucial bit being that the inner COMPONENT_REF has a different
type than its FIELD_DECL, indicating an implicit conversion to that
type.

zw