patch: clarify missing template arguments when parsing base specifier

[Chandler Carruth]

On Mon, Aug 26, 2013 at 11:40 PM, Nick Lewycky <nlewycky at google.com> wrote:

> On 26 August 2013 23:22, Chandler Carruth <chandlerc at google.com> wrote:
>
>>
>> On Mon, Aug 26, 2013 at 11:09 PM, Nick Lewycky <nlewycky at google.com>wrote:
>>
>>> On 26 August 2013 22:53, Chandler Carruth <chandlerc at google.com> wrote:
>>>
>>>> I think this case has more problems than just verbosity...
>>>>
>>>> On Mon, Aug 26, 2013 at 10:38 PM, Nick Lewycky <nlewycky at google.com>wrote:
>>>>
>>>>> a.cc:1:56: error: no template named 'Foox'; did you mean 'Foo'?
>>>>>  template <typename T> class Foo {}; class Bar : public Foox {};
>>>>> [point at 'Foox' suggest 'Foo']
>>>>>
>>>>
>>>> Why does it assume Foox is a template?
>>>>
>>>
>>> It's already proven that it's not not-a-template.
>>>
>>>  a.cc:1:29: note: 'Foo' declared here
>>>>> template <typename T> class Foo {}; class Bar : public Foox {};
>>>>> [point at 'Foo']
>>>>> a.cc:1:56: error: expected template argument list after template-id
>>>>> template <typename T> class Foo {}; class Bar : public Foox {};
>>>>> [point at 'Foox']
>>>>>
>>>>
>>>> And given that we then hit this error, why do we even consider the Foo
>>>> typo correction? Do we prefer that over a "Fooxie" class due to shorter
>>>> edit distance? That doesn't seem right. I would intuitively expect the lack
>>>> of "<..." to be a stronger signal than any edit distance, and thus
>>>> disqualify template-ids from the typo correction candidate set.
>>>>
>>>
>>> No. We only go down this patch after we've done a lookup and typo
>>> correction on non-templates, and found nothing.
>>>
>>
>> I'm suggesting that a missing header or exceeding the maximum edit
>> distance threshold seems just as plausible as using a template without
>> template arguments. I'm not claiming that I have some strong reason to
>> believe one interpretation or the other to be more likely, only that it
>> doesn't seem clear-cut in either direction to me.
>>
>
> ... if it can't find a template, then it doesn't mention templates:
>
> $ echo 'class Bar : public Foo {};' | llvm/Debug+Asserts/bin/clang -x c++ -
> <stdin>:1:20: error: expected class name
> class Bar : public Foo {};
>                    ^
> 1 error generated.
>
> $ echo 'template<typename T> class Fooa; class Foob {}; class Bar : public
> Foo {};' | llvm/Debug+Asserts/bin/clang -x c++ -
> <stdin>:1:68: error: unknown class name 'Foo'; did you mean 'Foob'?
> template<typename T> class Fooa; class Foob {}; class Bar : public Foo {};
>                                                                    ^~~
>                                                                    Foob
> <stdin>:1:40: note: 'Foob' declared here
> template<typename T> class Fooa; class Foob {}; class Bar : public Foo {};
>                                        ^
> 1 error generated.
>
> Today, clang emits the exact same diagnostic "expected class name" even
> when you do have Foo declared as a template. That's the only thing I'm
> trying to fix, but it has this weird side-effect in that even asking Sema
> isTemplateName() causes does typo-correction, and when that typo-correction
> succeeds it issues this "no template" diagnostic.
>

I really do understand that, and I don't think your patch is wrong. I just
think the problem in typo correction it exposes is severe enough to warrant
fixing.

Specifically, I don't think we should be typo-correcting to a template-id
when we have syntactic indicators that a template-id isn't valid. For
example, given "foo->baz", we should only correct to a method name "bar" if
we can also correct to "bar()" because it has a zero-argument overload
(modulo weird cases where the expression "foo->bar" is valid). There are
likely other places (maybe even with methods!) where we get this wrong, but
I think all of them are pretty dubious. Here, I think the lack of "<" is a
*really* strong signal as there is just no way to use a template-id in that
context. If all template parameters had default arguments, perhaps 'Foo<>'
would be a good correction, but that's a more clever step.
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