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Hi,

So we know that constexpr argument would be hard to implement because of
the the mangling and linking of function with such a parameter.
That being said a deduction guide don't have those issues as they only
exist at compile-time.
This means that constexpr arguments could be allowed in deduction guides.
This would allow through type-value semantics to simulate constexpr
arguments and generalized non-type template parameter.
Here's an example:

namespace std
{
   template<class T>
   integral_constant(constexpr T&& x) -> integral_constant<T, x>;

   template<class... Args>
   struct tuple
   {
      //...
      template<size_t I>
      constexpr decltype(auto) operator[](integral_constant<I>) const {
return std::get<I>(*this); }
      //...
   };
}

So that takes care of currently allowed non-type template parameter but
what about the rest ? With lambdas!

template<auto Fn>
struct constexpr_arg
{
   template<class T> constexpr constexpr_arg(T&&) {}
   static constexpr decltype(auto) value = Fn();
};

template<class T>
constexpr_arg(constexpr T&& x) -> constexpr_arg<+[] -> decltype(auto) {
return x;}>; //lambdas in unevaluated context now allowed

using runtime_arg = constexpr_arg<nullptr>;

template<class T>
constexpr_arg(T&&) -> runtime_arg

template<auto Fn>
void func(constexpr_arg<Fn>) //a bit ugly but no longer forces the caller
to write ugly code
{
   using type = constexpr_arg<Fn>;
   static_assert(type::value == 3.14);
}

void func(runtime_arg) //runtime specialization
{
}

int main()
{
   func(3.14);
   func(new int); //calls the runtime_arg version
}

This would also allow to simulate generalized non-type template parameters:

template<class T>
struct A
{
  static_assert(T::value == "hello"sv);
};

int main()
{
   A<decltype(constexpr_arg{"hello"sv})> tmp;
}


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<div dir=3D"ltr">Hi,<div><br></div><div>So we know that constexpr argument =
would be hard to implement because of the the mangling and linking of funct=
ion with such a parameter.</div><div>That being said a deduction guide don&=
#39;t have those issues as they only exist at compile-time.</div><div>This =
means that constexpr arguments could be allowed in deduction guides.</div><=
div>This would allow through type-value semantics to simulate constexpr arg=
uments and generalized non-type template parameter.</div><div>Here&#39;s an=
 example:</div><div><br></div><div class=3D"prettyprint" style=3D"backgroun=
d-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); border-style=
: solid; border-width: 1px; word-wrap: break-word;"><code class=3D"prettypr=
int"><div class=3D"subprettyprint"><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">namespace</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> std<br></span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">{</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"><br>=C2=A0 =C2=A0</span><span style=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"color: #008;" class=3D"styled-by-prett=
ify">class</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
 T</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>=C2=A0 =C2=
=A0integral_constant</span><span style=3D"color: #660;" 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">=
 T</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&amp;&am=
p;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> x</span=
><span style=3D"color: #660;" class=3D"styled-by-prettify">)</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">-&gt;</span><span style=3D"color: =
#000;" class=3D"styled-by-prettify"> integral_constant</span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify">T</span><span style=3D"color: #660;" =
class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"> x</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">&gt;;</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"><br><br>=C2=A0 =C2=A0</span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">template</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">&lt;</span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">class</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">...</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> <=
/span><span style=3D"color: #606;" class=3D"styled-by-prettify">Args</span>=
<span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><span =
style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0</span=
><span style=3D"color: #008;" class=3D"styled-by-prettify">struct</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify"> tuple<br>=C2=A0 =C2=
=A0</span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=
=A0 =C2=A0 </span><span style=3D"color: #800;" class=3D"styled-by-prettify"=
>//...</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=
=C2=A0 =C2=A0 =C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by=
-prettify">template</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">&lt;</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y">size_t I</span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>&gt;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=
=C2=A0 =C2=A0 =C2=A0 </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: #008;" class=3D"styled-by-prettify"=
>decltype</span><span style=3D"color: #660;" class=3D"styled-by-prettify">(=
</span><span style=3D"color: #008;" class=3D"styled-by-prettify">auto</span=
><span style=3D"color: #660;" class=3D"styled-by-prettify">)</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"co=
lor: #008;" class=3D"styled-by-prettify">operator</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">[](</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify">integral_constant</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify">I</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">&gt;)</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">const</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span=
 style=3D"color: #008;" class=3D"styled-by-prettify">return</span><span sty=
le=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"styled-by-prettify">get</span><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify">&lt;</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify">I</span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">&gt;(*</span><span style=3D"color: #008;" class=3D"styled-by=
-prettify">this</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">);</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </=
span><span style=3D"color: #660;" class=3D"styled-by-prettify">}</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 =C2=
=A0 </span><span style=3D"color: #800;" class=3D"styled-by-prettify">//...<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =
=C2=A0</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><span styl=
e=3D"color: #000;" class=3D"styled-by-prettify"><br></span></div></code></d=
iv><br><div>So that takes care of currently allowed non-type template param=
eter but what about the rest ? With lambdas!</div><div><br></div><div class=
=3D"prettyprint" style=3D"background-color: rgb(250, 250, 250); border-colo=
r: rgb(187, 187, 187); border-style: solid; border-width: 1px; word-wrap: b=
reak-word;"><code class=3D"prettyprint"><div class=3D"subprettyprint"><span=
 style=3D"color: #008;" class=3D"styled-by-prettify">template</span><span s=
tyle=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><span style=
=3D"color: #008;" class=3D"styled-by-prettify">auto</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #606=
;" class=3D"styled-by-prettify">Fn</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">&gt;</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"><br></span><span style=3D"color: #008;" class=3D"styled-b=
y-prettify">struct</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"> constexpr_arg<br></span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">{</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"><br>=C2=A0 =C2=A0</span><span style=3D"color: #008;" class=3D"styled=
-by-prettify">template</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">&lt;</span><span style=3D"color: #008;" class=3D"styled-by-pret=
tify">class</span><span style=3D"color: #000;" class=3D"styled-by-prettify"=
> T</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</s=
pan><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"> constexpr_arg</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify">T</span><span style=3D"color=
: #660;" class=3D"styled-by-prettify">&amp;&amp;)</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;"=
 class=3D"styled-by-prettify">{}</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br>=C2=A0 =C2=A0</span><span style=3D"color: #008;=
" class=3D"styled-by-prettify">static</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"> </span><span style=3D"color: #008;" class=3D"st=
yled-by-prettify">constexpr</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-pr=
ettify">decltype</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">(</span><span style=3D"color: #008;" class=3D"styled-by-prettify">aut=
o</span><span style=3D"color: #660;" class=3D"styled-by-prettify">)</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> value </span><spa=
n style=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color=
: #606;" class=3D"styled-by-prettify">Fn</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"st=
yled-by-prettify">};</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"><br><br></span><span style=3D"color: #008;" class=3D"styled-by-pr=
ettify">template</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">&lt;</span><span style=3D"color: #008;" class=3D"styled-by-prettify">=
class</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> T</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"><br>constexpr_arg</=
span><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span><spa=
n style=3D"color: #008;" class=3D"styled-by-prettify">constexpr</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">&amp;&amp;</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> x</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">)</span><span style=3D"color: #000;"=
 class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">-&gt;</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"> constexpr_arg</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">&lt;+[]</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-pr=
ettify">-&gt;</span><span style=3D"color: #000;" class=3D"styled-by-prettif=
y"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">declty=
pe</span><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">auto</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: #6=
60;" class=3D"styled-by-prettify">{</span><span style=3D"color: #008;" clas=
s=3D"styled-by-prettify">return</span><span style=3D"color: #000;" class=3D=
"styled-by-prettify"> x</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: #800;" class=3D"styled-by-prettify"=
>//lambdas in unevaluated context now allowed</span><span style=3D"color: #=
000;" class=3D"styled-by-prettify"><br><br></span><span style=3D"color: #00=
8;" class=3D"styled-by-prettify">using</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"> runtime_arg </span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">=3D</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> constexpr_arg</span><span style=3D"color: #080;" c=
lass=3D"styled-by-prettify">&lt;nullptr&gt;</span><span style=3D"color: #66=
0;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br><br></span><span style=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"color: #008;" class=3D"st=
yled-by-prettify">class</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> T</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">&gt;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><=
br>constexpr_arg</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettify">T</=
span><span style=3D"color: #660;" class=3D"styled-by-prettify">&amp;&amp;)<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">-&gt;</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> runtime_arg<br><br></spa=
n><span style=3D"color: #008;" class=3D"styled-by-prettify">template</span>=
<span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span><font =
color=3D"#000088"><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>auto</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </sp=
an><span style=3D"color: #606;" class=3D"styled-by-prettify">Fn</span></fon=
t><span style=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><spa=
n style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span styl=
e=3D"color: #008;" class=3D"styled-by-prettify">void</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify"> func</span><span style=3D"color:=
 #660;" class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">constexpr_arg</span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;" clas=
s=3D"styled-by-prettify">Fn</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">&gt;)</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"> </span><span style=3D"color: #800;" class=3D"styled-by-prettif=
y">//a bit ugly but no longer forces the caller to write ugly code</span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">{</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0</span><span styl=
e=3D"color: #008;" class=3D"styled-by-prettify">using</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> type </span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">=3D</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"> constexpr_arg</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #606;"=
 class=3D"styled-by-prettify">Fn</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">&gt;;</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify"><br>=C2=A0 =C2=A0</span><span style=3D"color: #008;" cla=
ss=3D"styled-by-prettify">static_assert</span><span style=3D"color: #660;" =
class=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify">type</span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">::</span><font color=3D"#000000"><span style=3D"color: #000;"=
 class=3D"styled-by-prettify">value </span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">=3D=3D</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> </span><span style=3D"color: #066;" class=3D"style=
d-by-prettify">3.14</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">);</span></font><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br><br=
></span><span style=3D"color: #008;" class=3D"styled-by-prettify">void</spa=
n><span style=3D"color: #000;" class=3D"styled-by-prettify"> func</span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify">runtime_arg</span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #800;"=
 class=3D"styled-by-prettify">//runtime specialization</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"><br></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><span style=3D"color: #000;" class=3D"style=
d-by-prettify"><br><br></span><span style=3D"color: #008;" class=3D"styled-=
by-prettify">int</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"> main</span><span style=3D"color: #660;" class=3D"styled-by-prettify"=
>()</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0func<=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span><sp=
an style=3D"color: #066;" class=3D"styled-by-prettify">3.14</span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">);</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0func</span><span =
style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=3D"=
color: #008;" class=3D"styled-by-prettify">new</span><span style=3D"color: =
#000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" cl=
ass=3D"styled-by-prettify">int</span><span style=3D"color: #660;" class=3D"=
styled-by-prettify">);</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"> </span><span style=3D"color: #800;" class=3D"styled-by-prettif=
y">//calls the runtime_arg version</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">}</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> </span></div></code></div><div><br></div><div>This would also all=
ow to simulate generalized non-type template parameters:</div><div><br></di=
v><div><div class=3D"prettyprint" 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"subp=
rettyprint"><span style=3D"color: #008;" class=3D"styled-by-prettify">templ=
ate</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</s=
pan><span style=3D"color: #008;" class=3D"styled-by-prettify">class</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color:=
 #008;" class=3D"styled-by-prettify">struct</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"> A<br></span><span style=3D"color: #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: #008;" class=3D=
"styled-by-prettify">static_assert</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"style=
d-by-prettify">T</span><span style=3D"color: #660;" class=3D"styled-by-pret=
tify">::</span><span style=3D"color: #000;" class=3D"styled-by-prettify">va=
lue </span><span style=3D"color: #660;" class=3D"styled-by-prettify">=3D=3D=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><s=
pan style=3D"color: #080;" class=3D"styled-by-prettify">&quot;hello&quot;</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify">sv</span><sp=
an 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: #00=
0;" class=3D"styled-by-prettify"><br><br></span><span style=3D"color: #008;=
" class=3D"styled-by-prettify">int</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> main</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-by=
-prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">{</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=
=C2=A0 =C2=A0A</span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">&lt;</span><span style=3D"color: #008;" class=3D"styled-by-prettify">de=
cltype</span><span style=3D"color: #660;" class=3D"styled-by-prettify">(</s=
pan><span style=3D"color: #000;" class=3D"styled-by-prettify">constexpr_arg=
</span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span><s=
pan style=3D"color: #080;" class=3D"styled-by-prettify">&quot;hello&quot;</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify">sv</span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">})&gt;</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> tmp</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></div><div>=C2=
=A0<br></div></div>

<p></p>

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