[cfe-dev] Endless recursion due to return type deduction of mutually-recursive templates

[Yaron Keren]

Hi,

I am not sure if this should go the llvm-cfe or here as this is a LLVM file
but clang problem.

Anyway here is a patch file for HandleLLVMOptions.cmake to increase the
stack size with Visual C++ to 2MB, which is more than enough to handle the
default 256 template instantiations limit.

Yaron



2013/9/20 Yaron Keren <yaron.keren at gmail.com>

> Compiling under Visual C++ 2012 with the default stack = 1MB, the stack
> overflows at depth of 216 instantiations, well before the 256 limit. Either
> the maximum depth needs to be lower or the Visual C++ stack needs to be
> larger using the /STACK:number_of_bytes linker parameter.
>
> In any case, the "recursive template instantiation exceeded maximum depth"
> error is followed by 256 notes where the instantiations happens...
>
> Yaron
>
>
>
> 2013/9/20 Richard Smith <richard at metafoo.co.uk>
>
>> This is not endless recursion, just a stack overflow. I get:
>>
>> <stdin>:2:6: fatal error: recursive template instantiation exceeded
>> maximum depth of 256
>> auto g(T x);
>>      ^
>> <stdin>:15:14: note: in instantiation of function template specialization
>> 'f<double>' requested here
>>     return x*f(x-1);
>>              ^
>> <stdin>:7:14: note: in instantiation of function template specialization
>> 'g<double>' requested here
>>     return x*g(x-1);
>>              ^
>> [...]
>>
>> This is not ideal, though. (There are a couple of other contexts where
>> this kind of thing can happen, with constexpr and with decltype.) We should
>> detect when we're recursively instantiating a function template
>> specialization from within itself and bail out.
>>
>
>
> On Thu, Sep 19, 2013 at 1:34 AM, Yaron Keren <yaron.keren at gmail.com>wrote:
>
>> Hello,
>>
>> The following code causes endless recursion leading to seg fault with the
>> latest SVN.
>>
>> clang tries to deduce g return type, which requires f return type, which
>> requires g return type, repeats endlessly.
>>
>> If the templates are modified to regular functions taking double, clang
>> correctly errors upon parse:
>> error: function 'g' with deduced return type cannot be used before it is
>> defined
>>     return x*g(x-1);
>>
>> but with templates there is no error parsing, just seg fault running.
>>
>> Yaron
>>
>>
>> // Endless recursion with revision 190992.
>>
>> template <class T>
>> auto g(T x);
>>
>> template <class T>
>> auto f(T x) {
>>   if (x>0)
>>     return x*g(x-1);
>>   else
>>     return 1.0;
>> }
>>
>> template <class T>
>> auto g(T x) {
>>   if (x>0)
>>     return x*f(x-1);
>>   else
>>     return 1.0;
>> }
>>
>> int main() {
>>   f(9.0);
>> }
>>
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
>>
>
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