Does a placeholder in a trailing-return-type override an initial placeholder?

Question

g++ appears to accept any combination of auto and decltype(auto) as initial and trailing return types:

int a;
auto f() { return (a); }                             // int
auto g() -> auto { return (a); }                     // int
auto h() -> decltype(auto) { return (a); }           // int&
decltype(auto) i() { return (a); }                   // int&
decltype(auto) j() -> auto { return (a); }           // int
decltype(auto) k() -> decltype(auto) { return (a); } // int&

However, clang rejects j and k, saying: error: function with trailing return type must specify return type 'auto', not 'decltype(auto)' (demonstration).

Which compiler is correct? Which rule (auto or decltype(auto)) should be used in each case? And does it make any sense to use a placeholder type in a trailing-return-type?

Answer 1

auto is required when introducing a trailing-return-type.

§8.3.5 [dcl.fct] /2

In a declaration T D where D has the form

D1 ( parameter-declaration-clause ) cv-qualifier-seq_optref-qualifier_optexception-specification_optattribute-specifier-seq_opttrailing-return-type

and the type of the contained declarator-id in the declaration T D1 is “derived-declarator-type-list T”,

T shall be the single type-specifier auto. [...]

See also Core Issue 1852 for the apparent contradiction with [dcl.spec.auto]/1.