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Date: Wed, 10 Jun 2009 15:28:00 -0000
From: Paolo Bonzini <paolo.bonzini@gmail.com>
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To: Ian Lance Taylor <iant@google.com>
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Subject: Re: skip_evaluation
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> In asking this, I'm particularly puzzled by code like this in
> build_base_path in cp/class.c:
> 
>   /* Don't bother with the calculations inside sizeof; they'll ICE if the
>      source type is incomplete and the pointer value doesn't matter.  */
>   if (skip_evaluation)
>     {
>       expr = build_nop (build_pointer_type (target_type), expr);
>       if (!want_pointer)
> 	expr = build_indirect_ref (EXPR_LOCATION (expr), expr, NULL);
>       return expr;
>     }
> 
> Presumably the early return is OK within a sizeof expression; it is OK
> within an expression like (0 ? x : y)?

 From reading the code, I'd say yes.  The bug that Jason fixed is 
related to stuff that cannot appear within a constant expression except 
within sizeof -- for example

    struct B {};
    struct D : public B {
        static const int i = sizeof((B*)(D*)0);
    };

    struct Z {};
    struct A : Z {};
    Z* implicitToZ (Z*);
    struct B : A {
        static const int i = sizeof(implicitToZ((B*)0));
    };

    struct B {};
    struct D;
    D* p;
    struct D: public B {
        static const int i = sizeof ((B*)p);
    };

(see PR27177).  All of these would still be forbidden within (0?x:y).

Paolo