[PATCH] [Sema] Don't permit dependent alignments on non-dependent typedef-names

[Richard Smith]

On Mon, Apr 6, 2015 at 5:39 PM, Aaron Ballman <aaron at aaronballman.com>
wrote:

> On Mon, Apr 6, 2015 at 8:36 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> > On Mon, Apr 6, 2015 at 4:35 PM, Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> On Mon, Apr 6, 2015 at 7:30 PM, Richard Smith <richard at metafoo.co.uk>
> >> wrote:
> >> > ================
> >> > Comment at: lib/Sema/SemaDeclAttr.cpp:2866
> >> > @@ -2865,1 +2865,3 @@
> >> >
> >> > +  if (E->isTypeDependent() || E->isValueDependent()) {
> >> > +    if (const auto *TND = dyn_cast<TypedefNameDecl>(D)) {
> >> > ----------------
> >> > The 'type dependent' check here is redundant.
> >> >
> >> > ================
> >> > Comment at: lib/Sema/SemaDeclAttr.cpp:2869
> >> > @@ +2868,3 @@
> >> > +      QualType T = TND->getUnderlyingType();
> >> > +      if (!T->isDependentType() &&
> !T->isInstantiationDependentType())
> >> > {
> >> > +        S.Diag(Attr.getLoc(),
> >> > diag::err_alignment_dependent_typedef_name)
> >> > ----------------
> >> > I don't think you should check instantiation-dependence here. I think
> >> > you should reject this, for instance:
> >> >
> >> >   template<int N> struct X {
> >> >     typedef __attribute__((aligned(N))) int X[sizeof(N)];
> >> >   };
> >> >
> >>
> >> Out of curiosity, why do you think that should be rejected?
> >
> >
> > For the same reason we're rejecting anything here: we'd otherwise create
> a
> > non-dependent type that has a dependent alignment.
>
> Huh, I guess I thought this type was dependent because of the
> sizeof(N) in the type.
>

For posterity, these are approximately-correct definitions:
 * value-dependent means "we don't know until instantiation whether this is
a constant expression or what it evaluates to if it is"
 * type-dependent means "we don't know until instantiation what the
(canonical) type of this expression is"
 * instantiation-dependent means "this refers to a template parameter in
any way (and thus instantiation might possibly fail)"

The relevant rule:

"A type is dependent if it is [...]  an array type constructed from any
dependent type or whose size is specified by a constant expression that is
value-dependent,"

sizeof(N) is instantiation-dependent but not value-dependent -- its value
is always sizeof(int) -- so the resulting type is an
instantiation-dependent type but not a dependent type.
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