When resolving a placeholder for a deduced class type ([dcl.type.class.deduct]
where the template-name
names a primary class template C
a set of functions and function templates, called the guides of C
is formed comprising:
for each constructor of C
a function template with the following properties:
The template parameters are the template parameters of C
by the template parameters (including default template arguments) of the constructor,
The types of the function parameters are those of the constructor.
The return type is the class template specialization
designated by C
and template arguments
corresponding to the template parameters of C.
is not defined or does not declare any constructors,
an additional function template derived as above
from a hypothetical constructor C().
For each deduction-guide
a function or function template
with the following properties:
In addition, if C
and its definition satisfies the conditions for
an aggregate class ([dcl.init.aggr]
with the assumption that any dependent base class has
no virtual functions and no virtual base classes, and
the initializer is a non-empty braced-init-list
there are no deduction-guides
the set contains an additional function template,
called the aggregate deduction candidate
, defined as follows.
be the corresponding aggregate element
or of one of its (possibly recursive) subaggregates
that would be initialized by
- brace elision is not considered for any aggregate element
that has a dependent non-array type or
an array type with a value-dependent bound, and
- each non-trailing aggregate element that is a pack expansion
is assumed to correspond to no elements of the initializer list, and
- a trailing aggregate element that is a pack expansion is assumed to correspond
to all remaining elements of the initializer list (if any).
If there is no such aggregate element
the aggregate deduction candidate is not added to the set.
The aggregate deduction candidate is derived as above
from a hypothetical constructor
except that additional parameter packs of the form
are inserted into the parameter list in their original aggregate element position corresponding to each non-trailing aggregate element of type
that was skipped because it was a parameter pack, and
the trailing sequence of parameters corresponding
to a trailing aggregate element that is a pack expansion (if any)
is replaced by a single parameter of the form .