Type deduction for non-viable function templates

Question

In his answer to this question and the comment section, Johannes Schaub says there's a "match error" when trying to do template type deduction for a function template that requires more arguments than have been passed:

template<class T>
void foo(T, int);

foo(42); // the template specialization foo<int>(int, int) is not viable

In the context of the other question, what's relevant is whether or not type deduction for the function template succeeds (and substitution takes place):

template<class T>
struct has_no_nested_type {};

// I think you need some specialization for which the following class template
// `non_immediate_context` can be instantiated, otherwise the program is
// ill-formed, NDR
template<>
struct has_no_nested_type<double>
{ using type = double; };

// make the error appear NOT in the immediate context
template<class T>
struct non_immediate_context
{
    using type = typename has_no_nested_type<T>::type;
};


template<class T>
typename non_immediate_context<T>::type
foo(T, int) { return {}; }

template<class T>
bool foo(T) { return {}; }


int main()
{
    foo(42);      // well-formed? clang++3.5 and g++4.8.2 accept it
    foo<int>(42); // well-formed? clang++3.5 accepts it, but not g++4.8.2
}

When instantiating the first function template foo for T == int, the substitution produces an invalid type not in the immediate context of foo. This leads to a hard error (this is what the related question is about.)

However, when letting foo deduce its template-argument, g++ and clang++ agree that no instantiation takes place. As Johannes Schaub explains, this is because there is a "match error".

Question: What is a "match error", and where and how is it specified in the Standard?

Altenative question: Why is there a difference between foo(42) and foo<int>(42) for g++?

What I've found / tried so far:

[over.match.funcs]/7 and [temp.over] seem to describe the overload resolution specifics for function templates. The latter seem to mandate the substitution of template parameters for foo.

Interestingly, [over.match.funcs]/7 triggers the process described in [temp.over] before checking for viability of the function template (specialization). Similarly, type deduction does not to take into account, say, default function arguments (other than making them a non-deduced context). It seems not to be concerned with viability, as far as I can tell.

Another possibly important aspect is how type deduction is specified. It acts on single function parameters, but I don't see where the distinction is made between parameter types that contain / are dependent on template parameters (like T const&) and those which aren't (like int).

Yet, g++ makes a difference between explicitly specifying the template parameter (hard error) and letting them be deduced (deduction failure / SFINAE). Why?

Johannes Schaub - litb Johannes Schaub - litb · Accepted Answer · 2014-03-14T21:18:37

What I've summarized is the process described at 14.8.2.1p1

Template argument deduction is done by comparing each function template parameter type (call it P) with the type of the corresponding argument of the call (call it A) as described below.

In our case, we have for P (T, int) and for A, we have (int). For the first pair of P/A, which is T against int, we can match T to int (by the process described in 14.8.2.5). But for the second "pair", we have int but have no counterpart. Thus deduction cannot be made for this "pair".

Thereby, by 14.8.2.5p2, "If type deduction cannot be done for any P/A pair, ..., template argument deduction fails.".

You then won't ever come to the point where you substitute template arguments into the function template.

This can all probably described more precisely in the Standard (IMO), but I believe this is how one could implement things to match the actual behavior of Clang and GCC and it seems a reasonable interpretation of the Standardese.

Type deduction for non-viable function templates

1 Answers