[cfe-dev] Deducing a partial specialization fails

[Arthur O'Dwyer via cfe-dev]

On Tue, May 28, 2019 at 9:01 AM via cfe-dev <cfe-dev at lists.llvm.org> wrote:

> Please consider the example here:
>
> https://godbolt.org/z/4AwrXz
>
> Clang up to version 3.9.1 and all recent versions of other compilers
> accept the code. Clang version >=4.0 fails.
> If you move the definition of AccessorPointersT to Foo, it works again and
> the partial specialization is deduced correctly.
> Is this a bug or a pedantic interpretation of the c++ standard by clang?
>

Well, the compiler appears to be right that the partial specialization is
unreachable because the `IsConst` parameter can't be deduced.
Suppose you instantiated `Foo<A>`, where `A` was a class type defined as

struct A {};
template<> struct Bar<A> {
    template<bool> using AccessorPointersT = int;
};

Then `Foo<A>::AccessorPointersT<false>` and
`Foo<A>::AccessorPointersT<true>` denote the same type (namely, `int`). So
you can't use template "pattern matching" to differentiate them, because
they're identical.

(Would you expect the partial specialization to be chosen in this case? If
so, what would you expect it to deduce for the actual value of `IsConst`?)

If you move the definition of the AccessorPointers struct from Bar down
into Foo, then I believe the problem goes away, as you indicate. In that
case, `Foo<A>::AccessorPointersT<false>` and
`Foo<A>::AccessorPointersT<true>` definitely denote different types, and so
it's OK to use pattern matching on them.

–Arthur
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