13 Templates [temp]

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 rules in the remainder of this subclause 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:

• (3.1)
  If P is an array type, the pointer type produced by the array-to-pointer standard conversion is used in place of P for type deduction; otherwise,
• (3.2)
  If P is a function type, the pointer type produced by the function-to-pointer standard conversion is used in place of P for type deduction; otherwise,
• (3.3)
  If P is a cv-qualified type, the top-level cv-qualifiers of P's type are ignored for type deduction.

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:

• (5.1)
  If the original A is a reference type, any cv-qualifiers of A (i.e., the type referred to by the reference).
• (5.2)
  If the original A is a function pointer or pointer-to-member-function type with a potentially-throwing exception specification ([except.spec]), the exception specification.
• (5.3)
  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.