Function cpp_candidate_argument_applicability 

pub fn cpp_candidate_argument_applicability(
    candidate: &CodeUnit,
    arg_types: &[Option<CppArgType>],
    parameter_types: &dyn Fn(&CodeUnit) -> Option<Vec<String>>,
    is_template: &dyn Fn(&CodeUnit) -> bool,
    resolve_type: &dyn Fn(&str) -> Option<CodeUnit>,
    assignable: &dyn Fn(&CodeUnit, &CodeUnit) -> bool,
) -> CppArgumentApplicability

Classify one candidate using the same structured parameter/type comparison as cpp_filter_candidates_by_args_with_parameter_types. This is a deliberately narrower question than overload selection: an exact written type match is applicable, a proven pointer-shape mismatch is inapplicable, and every conversion or incomplete fact stays unknown.