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Consider the following code:

template<typename ...Ts>
auto get_tuple(std::tuple<Ts...> &&tpl, int index)
{
  return std::get<index>(std::forward(tpl));
}

This is, of course, not legal. And for good reason: the return value of
this function would have to be able to *change* based on the parameter you
pass. That's very un-function-like behavior.

But we do have a way to do that. We instead make `index` a non-type
template parameter:

template<int index, typename ...Ts>
auto get_tuple(std::tuple<Ts...> &&tpl)
{
  return std::get<index>(std::forward(tpl));
}

However, this forces us to call the function via `get_tuple<index>(tpl)`.
And while this obviously works, it's sub-optimal visually.

So I suggest making the following possible:

template<typename ...Ts>
auto get_tuple(std::tuple<Ts...> &&tpl, constexpr int index)
{
  return std::get<index>(std::forward(tpl));
}

So, how does this work?

It works by two transformations. First, that declaration is implicitly
transformed (in a manner similar to Concepts TS's use of `auto` and Concept
parameter types) into adding a template parameter. However, on the surface,
it's still a function with two parameters.

Second, when the user calls the function, any arguments provided to a
`constexpr` parameter are implicitly transformed into template arguments
rather than function arguments. Function overloading and so forth behave as
normal (though `constexpr` parameters should probably score higher if the
argument provided is a constant expression. And obviously if you provide an
argument that is not a constant expression, and no other overload that
could work exists, you get a compiler error.

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<div dir=3D"ltr">Consider the following code:<br><br><div class=3D"prettypr=
int" style=3D"background-color: rgb(250, 250, 250); border-color: rgb(187, =
187, 187); border-style: solid; border-width: 1px; word-wrap: break-word;">=
<code class=3D"prettyprint"><div class=3D"subprettyprint"><span style=3D"co=
lor: #008;" class=3D"styled-by-prettify">template</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #00=
8;" class=3D"styled-by-prettify">typename</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">...</span><span style=3D"color: #606;" class=3D"sty=
led-by-prettify">Ts</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">&gt;</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y"><br></span><span style=3D"color: #008;" class=3D"styled-by-prettify">aut=
o</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> get_tupl=
e</span><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify">std</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify">tuple</span><span style=3D"color:=
 #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;=
" class=3D"styled-by-prettify">Ts</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">...&gt;</span><span style=3D"color: #000;" class=3D=
"styled-by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">&amp;&amp;</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify">tpl</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">,</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </s=
pan><span style=3D"color: #008;" class=3D"styled-by-prettify">int</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify"> index</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"><br>=C2=A0 </span><span style=3D"color: #008;" c=
lass=3D"styled-by-prettify">return</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> std</span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">::</span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">get</span><span style=3D"color: #080;" class=3D"styled-by-prettif=
y">&lt;index&gt;</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettify">std=
</span><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify">forward</span><spa=
n style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify">tpl</span><span style=3D"col=
or: #660;" class=3D"styled-by-prettify">));</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">}</span></div></code></div><br>This is, of cours=
e, not legal. And for good reason: the return value of this function would =
have to be able to <i>change</i> based on the parameter you pass. That&#39;=
s very un-function-like behavior.<br><br>But we do have a way to do that. W=
e instead make `index` a non-type template parameter:<br><br><div class=3D"=
prettyprint" style=3D"background-color: rgb(250, 250, 250); border-color: r=
gb(187, 187, 187); border-style: solid; border-width: 1px; word-wrap: break=
-word;"><code class=3D"prettyprint"><div class=3D"subprettyprint"><span sty=
le=3D"color: #008;" class=3D"styled-by-prettify">template</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"co=
lor: #008;" class=3D"styled-by-prettify">int</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify"> index</span><span style=3D"color: #660;"=
 class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=3D=
"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-b=
y-prettify">typename</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>...</span><span style=3D"color: #606;" class=3D"styled-by-prettify">Ts</sp=
an><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span sty=
le=3D"color: #008;" class=3D"styled-by-prettify">auto</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> get_tuple</span><span style=3D"=
color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify">std</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">::</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify">tuple</span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">&lt;</span><span style=3D"color: #606;" class=3D"styled-by-pr=
ettify">Ts</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
....&gt;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </=
span><span style=3D"color: #660;" class=3D"styled-by-prettify">&amp;&amp;</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify">tpl</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"><br>=C2=A0 </span><span style=3D"color: #00=
8;" class=3D"styled-by-prettify">return</span><span style=3D"color: #000;" =
class=3D"styled-by-prettify"> std</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">::</span><span style=3D"color: #008;" class=3D"styl=
ed-by-prettify">get</span><span style=3D"color: #080;" class=3D"styled-by-p=
rettify">&lt;index&gt;</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y">std</span><span style=3D"color: #660;" class=3D"styled-by-prettify">::</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify">forward</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify">tpl</span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">));</span><span style=3D"color:=
 #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">}</span></div></code></div><br>However, this=
 forces us to call the function via `get_tuple&lt;index&gt;(tpl)`. And whil=
e this obviously works, it&#39;s sub-optimal visually.<br><br>So I suggest =
making the following possible:<br><br><div class=3D"prettyprint" style=3D"b=
ackground-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); bord=
er-style: solid; border-width: 1px; word-wrap: break-word;"><code class=3D"=
prettyprint"><div class=3D"subprettyprint"><span style=3D"color: #008;" cla=
ss=3D"styled-by-prettify">template</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">&lt;</span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">typename</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-pre=
ttify">...</span><span style=3D"color: #606;" class=3D"styled-by-prettify">=
Ts</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><sp=
an style=3D"color: #008;" class=3D"styled-by-prettify">auto</span><span sty=
le=3D"color: #000;" class=3D"styled-by-prettify"> get_tuple</span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify">std</span><span style=3D"color: #66=
0;" class=3D"styled-by-prettify">::</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify">tuple</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">&lt;</span><span style=3D"color: #606;" class=3D"styled=
-by-prettify">Ts</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">...&gt;</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">&amp;&=
amp;</span><span style=3D"color: #000;" class=3D"styled-by-prettify">tpl</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">,</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D=
"color: #008;" class=3D"styled-by-prettify">constexpr</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #0=
08;" class=3D"styled-by-prettify">int</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"> index</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">)</span><span style=3D"color: #000;" class=3D"style=
d-by-prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">{</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
<br>=C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>return</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> st=
d</span><span style=3D"color: #660;" class=3D"styled-by-prettify">::</span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">get</span><span s=
tyle=3D"color: #080;" class=3D"styled-by-prettify">&lt;index&gt;</span><spa=
n style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify">std</span><span style=3D"col=
or: #660;" class=3D"styled-by-prettify">::</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify">forward</span><span style=3D"color: #660;" =
class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify">tpl</span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">));</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"><br></span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
}</span></div></code></div><br>So, how does this work?<br><br>It works by t=
wo transformations. First, that declaration is implicitly transformed (in a=
 manner similar to Concepts TS&#39;s use of `auto` and Concept parameter ty=
pes) into adding a template parameter. However, on the surface, it&#39;s st=
ill a function with two parameters.<br><br>Second, when the user calls the =
function, any arguments provided to a `constexpr` parameter are implicitly =
transformed into template arguments rather than function arguments. Functio=
n overloading and so forth behave as normal (though `constexpr` parameters =
should probably score higher if the argument provided is a constant express=
ion. And obviously if you provide an argument that is not a constant expres=
sion, and no other overload that could work exists, you get a compiler erro=
r.<br></div>

<p></p>

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