[basic.def.odr] p1 says

Each of the following is termed a definable item:

• [...]
• a default argument for a parameter (for a function in a given scope) ([dcl.fct.default]), or

Then [basic.def.odr] p14 says

For any definable item D with definitions in multiple translation units,

• [...]
• if the definitions in different translation units do not satisfy the following requirements,

This obviously implies that a default argument is considered as a definition(also a declaration). But [basic.def] says

Each entity declared by a declaration is also defined by that declaration unless:

• [...]

First, the italic declaration cross-references to subclause [dcl.pre], which didn't give a grammar meaning that is a declaration for the default argument, let alone how could it be a definition?

We lack a clarification for the default arguments that appear in non-template functions, which may have the similar manner as [temp.decls.general] p3 did

For purposes of name lookup and instantiation, default arguments, type-constraints, requires-clauses ([temp.pre]), and noexcept-specifiers of function templates and of member functions of class templates are considered definitions;

Might it be?

The default arguments of a function that is not templated are considered definitions.