ychen added inline comments.
================
Comment at: clang/lib/Sema/SemaTemplateDeduction.cpp:5455-5456
if (!ClangABICompat15) {
- // Consider this a fix for CWG1432. Similar to the fix for CWG1395.
auto *TST1 = T1->castAs<TemplateSpecializationType>();
auto *TST2 = T2->castAs<TemplateSpecializationType>();
+ const TemplateArgument &TA1 = TST1->template_arguments().back();
----------------
mizvekov wrote:
> ychen wrote:
> > ychen wrote:
> > > mizvekov wrote:
> > > > This is a bug, T1 and T2 are not in general canonical types for this
> > > > function, and this castAs can pick up an alias template, and these
> > > > should never participate in deduction.
> > > >
> > > > (Also, can you please add a test for that?)
> > > >
> > > > You should not need to keep sugar here, everything being tested below
> > > > is present on the canonical type, so this is simple to solve, as
> > > > suggested.
> > > I see. I'll add a test.
> > Looking more closely, `T1`/`T2` is mostly returned by
> > `ASTContext::getTemplateSpecializationType` and for injected template
> > specialization, always unqualified
> > https://github.com/llvm/llvm-project/blob/52dce8900c46d5842a021619537ede598983dfde/clang/include/clang/AST/Type.h#L5431-L5432
> > , it does not seem possible to have aliase template involved? I could've
> > missed something though.
> Yeah you are right, I missed that all the uses of this function, those types
> either come from an Injected TST, or a dependent TST created on the spot.
>
> But yeah, the change to a simple `cast` instead of `castAs` makes this more
> clear.
>
> Otherwise, if there could have been any sugar node on top of the TST, that
> sugar could have easily been an alias TST,
> and I think it's very unlikely one wouldn't care, as they are very different
> things.
>
> So I tend to view these as potential bugs, a simple `getAs` and `castAs` on
> TSTs, which does not check or keeps digging through alias templates is highly
> suspicious :)
>
> Thanks for taking a look at this though!
Thanks for the explanation. Yeah, `castAs` looks misleading indeed. I wasn't
aware of the subtle difference between `cast` and `castAs`. BTW, are you
comfortable accepting this patch?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D133683/new/
https://reviews.llvm.org/D133683
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