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a template-argument cannot be a type that contains 'auto' in C++20

May 2, 2022 at 4:31pm
JUANDENT (411)
In C++20 we are supposed to be able to use concepts like this:

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//type requirement
template<typename T>
concept containerTypes = requires (T t)
{
	typename T::value_type;
	typename T::size_type;
	typename T::allocator_type;
	typename T::iterator;
	typename T::const_iterator;
};

auto max_value(containerTypes auto& cont)	-> typename std::remove_cvref_t<decltype(cont)>::value_type              // this is not OK
{
        typename std::remove_cvref_t<decltype(cont)>::value_type* pp;  // this is OK
	auto x = std::max_element(cont.begin(), cont.end());
	return *x;
}
void useAdd()
{
	std::vector v{ 10,20,30,50 };
	auto m = max_value(v);
}


I get the error in the return type:


a template-argument cannot be a type that contains 'auto'


This error goes away when I remove the trailing return type and leave the compiler to return auto!!

What's happening?

Last edited on May 2, 2022 at 8:48pm
May 3, 2022 at 9:47am
TheIdeasMan (6829)
I wonder if it is this:

https://stackoverflow.com/questions/60358154/auto-as-a-template-argument-placeholder-for-a-function-parameter

Also, when one has problems with complex templates, sometimes the answer is to make more use of auto, so if you have removed the trailing return type and just use auto, and it works, why not go with that?
May 3, 2022 at 10:56am
JUANDENT (411)
Because I want to know what is wrong!!
May 3, 2022 at 11:00am
seeplus (6613)
Which compiler/os are you using?

PS I get the same error with VS2022 using preview/latest.
Last edited on May 3, 2022 at 4:05pm
May 3, 2022 at 3:52pm
Peter87 (11243)
I don't know if it should compile or not but at least it compiles with GCC and Clang. https://godbolt.org/z/zjjad86cY
Last edited on May 3, 2022 at 4:00pm
May 3, 2022 at 4:29pm
seeplus (6613)
godbolt using VS doesn't compile. So possibly a bug with VS2022. I've had issues with VS concepts previously - where godbolt compiles but VS doesn't. Twice MS agreed it was a problem and fixed it and once stated they adhered to the standard and gcc had an 'extension'.

@JUANDENT I suggest you raise this as an issue with MS (VS/help/SendFeedback/reportaproblem ) and include the godbolt link.
May 3, 2022 at 5:43pm
JLBorges (13770)
C++20:
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template < std::ranges::forward_range RANGE >
std::ranges::range_reference_t<RANGE> max_value( RANGE& range )
{
    [[maybe_unused]] std::ranges::range_value_t<RANGE>* ptr = nullptr ; // this is OK
    if( std::ranges::empty(range) ) throw std::invalid_argument("empty");
    return *std::ranges::max_element(range) ;
}

http://coliru.stacked-crooked.com/a/aa052ddfa9f5332f
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