[cfe-dev] [ycm-dev] libclang: diagnostics not reported for template instantiations

[Val Markovic via cfe-dev]

Yeah, that's almost certainly it then. The thing is, that flag gives
something like a 10x perf boost when compiling large TUs. I think we almost
certainly want to have it on by default, though we *may* want to expose a
user-level setting to turn it off. That's a big "may" since we've been
using the flag for something like a year if not more and nobody has
complained about this until now.

On Sun, Nov 1, 2015 at 9:22 AM, Ben Jackson <puremourning at gmail.com> wrote:

> Val,
>
> I think you might be right. I will try it out to confirm, but the docs
> look to point to exactly that:
>
> *CXTranslationUnit_Incomplete*
>
> Used to indicate that the translation unit is incomplete.
>
> When a translation unit is considered "incomplete", semantic analysis that
> is typically performed at the end of the translation unit will be
> suppressed. For example, this suppresses the completion of tentative
> declarations in C and of instantiation of implicitly-instantiation function
> templates in C++. This option is typically used when parsing a header
> with the intent of producing a precompiled header.
>
> Thanks,
> Ben
>
>
> On 1 Nov 2015, at 17:12, Val Markovic <val at markovic.io> wrote:
>
> Could this be related to the Incomplete flag we started passing to
> libclang a while ago?
>
> On Sun, Nov 1, 2015 at 9:11 AM, Manuel Klimek <klimek at google.com> wrote:
>
>> +Benjamin
>>
>> On Tue, Oct 27, 2015 at 5:15 PM Val Markovic <val at markovic.io> wrote:
>>
>>> +klimek
>>>
>>> On Mon, Oct 26, 2015 at 3:09 PM, Ben Jackson <puremourning at gmail.com>
>>> wrote:
>>>
>>>> Hi there,
>>>>
>>>> An issue was raised against ycmd that we were not reporting errors as a
>>>> result of failed template instantiations. This is because, seemingly,
>>>> libclang does not report diagnostics for template instantiations where
>>>> instantiation fails (in the sense that the generated code is not valid).
>>>>
>>>> A fairly canonical test case is as follows:
>>>>
>>>>     template<typename T>
>>>>     void get_size(const T& t)
>>>>     {
>>>>         return t.size();
>>>>     }
>>>>
>>>>     struct Test
>>>>     {
>>>>         void not_size() const;
>>>>     };
>>>>
>>>>     int run_test()
>>>>     {
>>>>         Test t;
>>>>
>>>>         get_size(t);
>>>>
>>>>         return "not_an_int";
>>>>     }
>>>>
>>>> Expected: a diagnostic reported at the line ‘get_size(t)’ as struct
>>>> Test does not have a size()const method. However, no diagnostic is reported.
>>>>
>>>> The ‘return “not_an_int”’ is just to prove that diagnostics reporting
>>>> is working in general in the test case.
>>>>
>>>> When running the above with ‘c-index-test -index-file’, grepping for
>>>> ‘[diagnostic]’, the result is :
>>>>
>>>>     $ ./bin/c-index-test -index-file
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp  |
>>>> grep '\[diagnostic\]'
>>>>
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp:18:12:
>>>> error: cannot initialize return object of type 'int' with an lvalue of type
>>>> 'const char [11]'
>>>> [diagnostic]:
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp:18:12:
>>>> error: cannot initialize return object of type ‘int' with an lvalue of type
>>>> 'const char [11]'
>>>>
>>>> However, when running clang directly, we get the diagnostic for
>>>> get_size(t):
>>>>
>>>>      clang++
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp
>>>>
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp:18:12:
>>>> error: cannot initialize return object of type 'int' with an lvalue of type
>>>> 'const char [11]'
>>>>         return "not_an_int";
>>>>            ^~~~~~~~~~~~
>>>>
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp:4:14:
>>>> error: no member named 'size' in 'Test'
>>>>         return t.size();
>>>>            ~ ^
>>>>
>>>> ../llvm/tools/clang/test/Index/diagnostics-template-instantiation.cpp:16:5:
>>>> note: in instantiation of function template specialization 'get_size<Test>'
>>>> requested here
>>>>         get_size(t);
>>>>     ^
>>>>     2 errors generated.
>>>>
>>>> Is this a bug or simply a limitation of the libclang interface? Or is
>>>> there something required to get these diagnostics to report. Apologies, I
>>>> must confess I have not looked into this in detail within clang.
>>>>
>>>
>> I'd expect there to be a way to drive libclang to get those diagnostics.
>> On the other hand, it would probably mean a significant slow-down, because
>> template instantiation can be a significant amount of the parse time.
>>
>>
>>>
>>>> Many thanks and kind regards,
>>>> Ben
>>>>
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>>>
>>>
>
>
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