13 Function template argument deduction

13.1 Deduction from a function call [temp.deduct.call.scpp]

1. Except as modified by this subclause, [temp.deduct.call] applies unchanged to deduction of template arguments from a function call in a SCPP26 program.

2. A function parameter pack in a function parameter list shall appear only as the last parameter declaration in that list.

3. Deduction from a function call is performed over the full function parameter list. The compatibility of an earlier function parameter with its corresponding argument is not finally determined before deduction from every later non-defaulted function parameter has been considered.

4. If the type of a function parameter P contains a template parameter pack Args..., and that pack is first directly deduced from one or more later function parameters, the implementation shall:

(4.1) determine the final binding of Args... from those later deduction sources;

(4.2) substitute that binding into P; and

(4.3) then determine whether the corresponding argument satisfies P so substituted.

5. If the substituted type in (4.2) is not satisfied by the corresponding argument, the program is ill-formed.

6. This subclause does not authorize a non-final function parameter pack; (2) still applies when deduction proceeds as in (4).

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