[temp.deduct.guide]

# 13 Templates [[temp]](./#temp)

## 13.7 Template declarations [[temp.decls]](temp.decls#temp.deduct.guide)

### 13.7.2 Class templates [[temp.class]](temp.class#temp.deduct.guide)

#### 13.7.2.3 Deduction guides [temp.deduct.guide]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2823)

Deduction guides are used
when a [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") or [*splice-type-specifier*](dcl.type.splice#nt:splice-type-specifier "9.2.9.9 Type splicing [dcl.type.splice]") appears as a type specifier
for a deduced class type ([[dcl.type.class.deduct]](dcl.type.class.deduct "9.2.9.8 Deduced class template specialization types"))[.](#1.sentence-1)

Deduction guides are not found by name lookup[.](#1.sentence-2)

Instead, when performing class template argument deduction ([[over.match.class.deduct]](over.match.class.deduct "12.2.2.9 Class template argument deduction")),
all reachable deduction guides declared for the class template are considered[.](#1.sentence-3)

[deduction-guide:](#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]")  
[*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]")opt [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") ( [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") ) -> [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]") [*requires-clause*](temp.pre#nt:requires-clause "13.1 Preamble [temp.pre]")opt ;

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2837)

[*Example [1](#example-1)*: template<class T, class D = int>struct S { T data;};template<class U> S(U) -> S<typename U::type>;

struct A {using type = short; operator type();};
S x{A()}; // x is of type S<short, int> — *end example*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2855)

The same restrictions apply
to the [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") of a deduction guide
as in a function declaration ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")),
except that a generic parameter type placeholder ([[dcl.spec.auto]](dcl.spec.auto "9.2.9.7 Placeholder type specifiers"))
shall not appear in the [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") of
a deduction guide[.](#3.sentence-1)

The [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]") shall name a class template specialization[.](#3.sentence-2)

The [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") shall be the same [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") as the [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") of the [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]")[.](#3.sentence-3)

A [*deduction-guide*](#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]") shall inhabit the scope
to which the corresponding class template belongs
and, for a member class template, have the same access[.](#3.sentence-4)

Two deduction guide declarations
for the same class template
shall not have equivalent [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]")*s* if either is reachable from the other[.](#3.sentence-5)