Can C Functions Utilize \"auto\" as an Argument Type?

Passing Auto as an Argument in C

There exists a method to utilize the "auto" keyword as an argument when passing it to another function. That is to say, you can define a function like this:

int function(auto data)
{
    // Does something
}

C 20's Introduction of Auto as a Function Parameter Type

C 20 enables the usage of "auto" as a function parameter type. Consider the following code snippet as an example:

int function(auto data) {
   // Do something, there are no constraints on data
}

Abbreviated Function Templates

Additionally, "auto" can be employed as an abbreviated function template. This is a specialized instance of a non-constraining type constraint, or, in other words, an "unconstrained auto parameter." A "constrained auto parameter," on the other hand, would employ concepts and appear as follows:

void function(const Sortable auto& data) {
    // Do something that requires data to be Sortable
    // Assuming there is a concept named Sortable
}

Explanation from the C Specification

The C specification defines "placeholder type specifiers" as follows:

type-constraint<sub>opt</sub> auto

type-constraint<sub>opt</sub> decltype ( auto )

Such placeholder-type-specifiers designate a placeholder type that is subject to later replacement through deduction from an initializer. When used in the "decl-specifier-seq" of a function declaration or lambda expression, a placeholder-type-specifier of the form "type-constraint_opt auto" indicates that the function is an abbreviated function template (as per 9.3.3.5).

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