7.1.6.4/1
The auto type-specifier signifies that the type of an object being declared shall be deduced from its initializer or specified explicitly at the end of a function declarator.

7.1.6.4/3
… The decl-specifier-seq shall be followed by one or more init-declarators, each of which shall have a non-empty initializer of either of the following forms:
= assignment-expression
( assignment-expression )