13 Templates [temp]

13.10 Function template specializations [temp.fct.spec]

13.10.3 Template argument deduction [temp.deduct] Deducing conversion function template arguments [temp.deduct.conv]

Template argument deduction is done by comparing the return type of the conversion function template (call it P) with the type specified by the conversion-type-id of the conversion-function-id being looked up (call it A) as described in [temp.deduct.type].
If the conversion-function-id is constructed during overload resolution ([over.match.funcs]), the following transformations apply.
If P is a reference type, the type referred to by P is used in place of P for type deduction and for any further references to or transformations of P in the remainder of this subclause.
If A is not a reference type:
If A is a cv-qualified type, the top-level cv-qualifiers of A's type are ignored for type deduction.
If A is a reference type, the type referred to by A is used for type deduction.
In general, the deduction process attempts to find template argument values that will make the deduced A identical to A.
However, certain attributes of A may be ignored:
  • If the original A is a reference type, any cv-qualifiers of A (i.e., the type referred to by the reference).
  • If the original A is a function pointer or pointer-to-member-function type, its noexcept.
  • Any cv-qualifiers in A that can be restored by a qualification conversion.
These attributes are ignored only if type deduction would otherwise fail.
If ignoring them allows more than one possible deduced A, the type deduction fails.