[cfe-dev] Bug in template parsing?

[John McCall]

On Mar 8, 2013, at 9:08 AM, Jordan Rose <jordan_rose at apple.com> wrote:
> On Mar 8, 2013, at 3:41 , Nicola Gigante <nicola.gigante at gmail.com> wrote:
>> Hello.
>> 
>> I'm not sure why this piece of code doesn't compile, but I suspect a bug.
>> This happens with the clang trunk (r176686) (C++11 mode)
>> 
>> The compilation succeeds if value is made const static OR if I put parenthesis around test<1, N>::value.
>> 
>> template<int I, int N>
>> struct test
>> {
>>    bool value = test<1, N>::value;
>> };
>> 
>> template<int N>
>> struct test<1, N>
>> {
>>    static const bool value = true;
>> };
>> 
>> $ ./llvm/build/bin/clang++ -std=c++11 -fsyntax-only test.cpp
>> test.cpp:4:26: error: declaration of 'N' shadows template parameter
>>    bool value = test<1, N>::value;
>>                         ^
>> test.cpp:1:21: note: template parameter is declared here
>> template<int I, int N>
>>                    ^
>> test.cpp:4:27: error: expected ';' at end of declaration list
>>    bool value = test<1, N>::value;
>>                          ^
>>                          ;
>> test.cpp:4:24: error: expected '>'
>>    bool value = test<1, N>::value;
>>                       ^
>> 3 errors generated.
>> 
>> Am I missing something (or is it already known)?
> 
> Clang is parsing this as:
> 
> bool value = (test < 1), N > ::value;
> 
> The first error is because it thinks you're declaring "bool N", the second because "> ::value" isn't a valid initializer for "bool N", and the third because "test < 1" isn't a complete template reference.
> 
> I can't be sure whether this is correct, and clearly we could have better recovery, but it explains what's going on here. I think it's due to the fact that we don't know if "test" is a template until after it's been parsed—it could be an integer that we're trying to compare against 1.

Ugh.  I *think* this just a bug and doesn't rise to the level of a standards defect.  I did a cursory check and couldn't find an existing defect.

The lookup rules for non-static data member initializers are equivalent to the rules for being in a member function, i.e. lookup is delayed until the class is complete.  However, collecting tokens and delaying their parsing is an unambiguous process for member functions, because a member function body can be assumed to have balanced delimiters.  That is at least not obviously true for non-static data member initializers, because < and > may or may not be delimiters depending on whether what came before them was a template-id, and we can't decide whether something was a template-id without parsing.

So what's apparently currently happening is that we're just collecting tokens until we get to a semicolon or comma (but skipping over balanced parens, brackets, and braces) and then treating that all as a delayed initializer.

Now, I don't know that there's a formal ambiguity here, because a template-argument has to be a constant-expression, and a constant-expression is just a semantically-constrained conditional-expression, which does not include the assignment operator.  So, for example, this:
  bool a = test<1, b=2>::value;
cannot be parsed as this:
  bool a = test<1, (b=2)>::value;
because that's not a grammatically valid template argument.  (It is with parens, but then it's almost certainly a *semantically* invalid template argument, unless somebody's got a constexpr assignment operator...)

So I think we might be able to tweak our current rule so that we only cut off after a comma if what follows looks like a declarator followed by '='.

John.
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