[dcl.spec.auto.general] p2 is basically discussing the placeholder types of the parameters of a function or lambda. It simply says "auto type-specifier introducing a trailing-return-type" is not considered as a generic parameter type placeholder. I think they are not redundant and it is significant to clarify which placeholder types can be a generic parameter type placeholders.

But can the "auto type-specifier introducing a trailing-return-type" be "a decl-specifier of the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression"? It reads to me as if "the auto in 'auto f() -> T' appears in the declaration of a parameter". Could it be to account for a case like

auto f(auto(*)()->int) -> int;

"auto type-specifier introducing a trailing-return-type" be "a decl-specifier of the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression"

Yes, it is permitted by the grammar of the parameter-declaration defined at [dcl.fct] p3. Hence,

auto f(auto(*)()->int) -> int;

It is valid but the parameter is not considered to have a generic parameter type placeholder, it does not form the abbreviate function template.

Paragraph 2 can be read as

A placeholder-type-specifier of the form type-constraint _auto auto can be used as a decl-specifier in

• the decl-specifier-seq of a parameter-declaration of a function declaration, or
• the decl-specifier-seq of a parameter-declaration of a lambda-expression

if it is not the auto type-specifier introducing a trailing-return-type

• the placeholder type is a generic parameter type placeholder of the parameter of a function or lambda

Otherwise

• the placeholder type is not a generic parameter type placeholder

Thank you. So the intention is to prevent g in auto f(auto(*g)()->int) -> int; from being a generic parameter type placeholder.