rn-placeholder | 🏖️ Display some placeholder stuff | Frontend Framework library

_{All standard references below refers to N4861: March 2020 post-Prague working draft/C++20 DIS..}

From [basic.link]/11 [emphasis mine]:

After all adjustments of types (during which typedefs are replaced by their definitions), the types specified by all declarations referring to a given variable or function shall be identical, except that declarations for an array object can specify array types that differ by the presence or absence of a major array bound ([dcl.array]). A violation of this rule on type identity does not require a diagnostic.

[dcl.spec.auto]/3 covers that a placeholder type can appear with a function declarator, and if this declarator does not include a trailing-return-type (as is the case of OP's example)

[...] Otherwise [no trailing-return-type], the function declarator shall declare a function.

where

[...] the return type of the function is deduced from non-discarded return statements, if any, in the body of the function ([stmt.if]).

[dcl.fct]/1 covers function declarators that do not include a trailing-return-type [emphasis mine, removing opt parts of the grammar that do not apply in this particular example]:

In a declaration T D where D has the form [...] the type of the declarator-id in D is “derived-declarator-type-list function of parameter-type-list returning T” [...]

Thus, the two declarations