<s>
Concepts	O
are	O
an	O
extension	O
to	O
the	O
templates	B-Application
feature	O
provided	O
by	O
the	O
C++	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Concepts	O
are	O
named	O
Boolean	O
predicates	O
on	O
template	B-Application
parameters	O
,	O
evaluated	O
at	O
compile	B-Application
time	I-Application
.	O
</s>
<s>
A	O
concept	O
may	O
be	O
associated	O
with	O
a	O
template	B-Application
(	O
class	B-Application
template	B-Application
,	O
function	O
template	B-Application
,	O
member	O
function	O
of	O
a	O
class	B-Application
template	B-Application
,	O
variable	O
template	B-Application
,	O
or	O
alias	O
template	B-Application
)	O
,	O
in	O
which	O
case	O
it	O
serves	O
as	O
a	O
constraint	O
:	O
it	O
limits	O
the	O
set	O
of	O
arguments	O
that	O
are	O
accepted	O
as	O
template	B-Application
parameters	O
.	O
</s>
<s>
Originally	O
dating	O
back	O
to	O
suggestions	O
for	O
C++11	B-Language
,	O
the	O
original	O
concepts	O
specification	O
has	O
been	O
revised	O
multiple	O
times	O
before	O
formally	O
being	O
a	O
required	O
part	O
of	O
C++20	B-Language
.	O
</s>
<s>
There	O
are	O
five	O
different	O
places	O
in	O
a	O
function	O
template	B-Application
signature	O
where	O
a	O
constraint	O
can	O
be	O
used	O
(	O
labeled	O
below	O
as	O
C1	O
to	O
C5	O
)	O
:	O
</s>
<s>
This	O
kind	O
replaces	O
class	B-Application
or	O
typename	B-Language
for	O
declaring	O
a	O
type	B-Language
template	I-Language
parameter	I-Language
.	O
</s>
<s>
C++20	B-Language
added	O
abbreviated	O
function	O
templates	B-Application
which	O
use	O
auto	O
as	O
a	O
placeholder	O
type	O
in	O
the	O
parameter	O
declaration	O
.	O
</s>
<s>
A	O
trailing	O
requires-clause	O
can	O
be	O
applied	O
to	O
a	O
function	O
in	O
a	O
class	B-Application
template	B-Application
.	O
</s>
<s>
This	O
allows	O
the	O
function	O
to	O
remain	O
a	O
regular	O
,	O
template-free	O
function	O
,	O
which	O
can	O
be	O
enabled	O
or	O
disabled	O
depending	O
on	O
the	O
functions	O
trailing	O
requires-clause	O
.	O
</s>
<s>
The	O
constraint	O
forms	O
C1	O
and	O
C2	O
can	O
be	O
used	O
in	O
all	O
kinds	O
of	O
templates	B-Application
.	O
</s>
<s>
The	O
following	O
is	O
a	O
declaration	O
of	O
the	O
concept	O
"	O
equality_comparable	O
"	O
from	O
the	O
<concepts>	O
header	O
of	O
a	O
C++20	B-Language
standard	O
library	O
.	O
</s>
<s>
A	O
function	O
template	B-Application
constrained	O
on	O
this	O
concept	O
may	O
be	O
declared	O
as	O
follows	O
:	O
</s>
<s>
If	O
a	O
programmer	O
attempts	O
to	O
use	O
a	O
template	B-Application
argument	O
that	O
does	O
not	O
satisfy	O
the	O
requirements	O
of	O
the	O
template	B-Application
,	O
the	O
compiler	O
will	O
generate	O
an	O
error	O
.	O
</s>
<s>
Concepts	O
can	O
be	O
used	O
to	O
choose	O
function	O
template	B-Application
overloads	O
and	O
class	B-Application
template	B-Application
specializations	O
based	O
on	O
properties	O
of	O
their	O
template	B-Application
arguments	O
,	O
as	O
an	O
alternative	O
to	O
SFINAE	B-Language
and	O
tag	O
dispatching	O
.	O
</s>
<s>
Concepts	O
TS	O
,	O
as	O
specified	O
in	O
ISO/IEC	O
TS	O
19217:2015	O
,	O
are	O
implemented	O
as	O
an	O
experimental	O
feature	O
in	O
GCC	B-Application
6	I-Application
.	O
</s>
<s>
C++20	B-Language
concepts	O
are	O
fully	O
implemented	O
in	O
GCC	B-Application
10	I-Application
,	O
MSVC	B-Application
19.30	I-Application
,	O
and	O
Clang	B-Application
10	I-Application
.	O
</s>
<s>
A	O
different	O
form	O
of	O
Concepts	O
,	O
popularly	O
known	O
as	O
"	O
C++0x	B-Language
Concepts	O
"	O
,	O
was	O
temporarily	O
accepted	O
into	O
the	O
working	O
paper	O
for	O
C++11	B-Language
but	O
was	O
removed	O
in	O
2009	O
.	O
</s>
<s>
In	O
addition	O
to	O
concepts	O
themselves	O
,	O
"	O
C++0x	B-Language
Concepts	O
"	O
included	O
concept	O
maps	O
(	O
a	O
feature	O
that	O
could	O
make	O
it	O
possible	O
,	O
for	O
example	O
,	O
for	O
the	O
concept	O
"	O
Stack	O
"	O
to	O
accept	O
,	O
automatically	O
mapping	O
"	O
Stack	O
"	O
operations	O
such	O
as	O
to	O
differently	O
named	O
operations	O
on	O
,	O
such	O
as	O
)	O
and	O
axioms	O
(	O
a	O
facility	O
to	O
specify	O
semantic	O
properties	O
such	O
as	O
associativity	O
or	O
commutativity	O
,	O
allowing	O
the	O
compiler	O
to	O
take	O
advantage	O
of	O
these	O
properties	O
without	O
proof	O
)	O
.	O
</s>
<s>
In	O
contrast	O
to	O
this	O
abandoned	O
proposal	O
,	O
the	O
C++20	B-Language
version	O
of	O
Concepts	O
is	O
sometimes	O
referred	O
to	O
as	O
"	O
Concepts	O
Lite	O
"	O
.	O
</s>
<s>
During	O
the	O
C++	B-Language
standards	O
committee	O
meeting	O
in	O
March	O
2016	O
,	O
the	O
evolution	O
working	O
group	O
moved	O
to	O
merge	O
Concepts	O
into	O
the	O
mainline	O
C++17	B-Language
standard	O
,	O
but	O
the	O
motion	O
was	O
defeated	O
in	O
full	O
committee	O
.	O
</s>
<s>
Concepts	O
v1	O
was	O
merged	O
into	O
the	O
C++20	B-Language
draft	O
.	O
</s>
<s>
"	O
The	O
One	O
Range	O
"	O
version	O
of	O
Range	O
feature	O
that	O
depend	O
on	O
concepts	O
was	O
also	O
merged	O
into	O
C++20	B-Language
.	O
</s>
