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There are a number of technical issues with the idea of constexpr argument
for now and it is my understanding that it's not going to change anytime
soon.

That's why I propose an alternative that, I believe would be simple to
implement while offering the same advantages.
A new specifier : inlined_constexpr.

An inlined_constexpr definition is a constexpr definition that is always
inlined (equivalent of __attribute__((alway_inline)) ).
This means no linking/mangling/operator== related issues on the
implementation side.
If the definition is a function/method/constructor its arguments are
implicitly constexpr.
If an argument is not constexpr whilst being used as such in the body of
the function an error will occur.
Otherwise non-constexpr arguments are allowed.

Example:
//in std::tuple
inlined_constexpr decltype(auto) operator[](size_t I)
{
     return std::get<I>(*this);
}

int main()
{
   std::tuple t = {...};
   t[0]; //ok, I is constexpr as requested in the body. As the return is
not required to be constexpr "*this" is ok (fallback on runtime version and
inline std::get<0>(t)).
   constexpr auto& x = t[0]; //error! "this" is being requested as a
constexpr value (*this) whilst not being constexpr.
   constexpr auto x1 = std::tuple{10, 42}[0]; //ok, "this" is constexpr as
requested.
}

The assembly of the above would be exactly the same as:
int main()
{
   std::tuple t = {...};
   std::get<0>(t);
   constexpr auto& x = std::get<0>(t); //error t is not constexpr
   constexpr auto x1 = std::get<0>(std::tuple{10, 42});
}

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<div dir=3D"ltr">There are a number of technical issues with the idea of co=
nstexpr argument for now and it is my understanding that it&#39;s not going=
 to change anytime soon.<div><br></div><div>That&#39;s why I propose an alt=
ernative that, I believe would be simple to implement while offering the sa=
me advantages.</div><div>A new specifier : inlined_constexpr.</div><div><br=
></div><div>An inlined_constexpr definition is a constexpr definition that =
is always inlined (equivalent of __attribute__((alway_inline)) ).</div><div=
>This means no linking/mangling/operator=3D=3D related issues on the implem=
entation side.</div><div>If the definition is a function/method/constructor=
 its arguments are implicitly constexpr.</div><div>If an argument is not co=
nstexpr whilst being used as such in the body of the function an error will=
 occur.</div><div>Otherwise non-constexpr arguments are allowed.</div><div>=
<br></div><div>Example:</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"prettypri=
nt"><div class=3D"subprettyprint"><span style=3D"color: #800;" class=3D"sty=
led-by-prettify">//in std::tuple</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br>inlined_constexpr </span><span style=3D"color: =
#008;" class=3D"styled-by-prettify">decltype</span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify">(</span><span style=3D"color: #008;" clas=
s=3D"styled-by-prettify">auto</span><span style=3D"color: #660;" class=3D"s=
tyled-by-prettify">)</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify"=
>operator</span><span style=3D"color: #660;" class=3D"styled-by-prettify">[=
](</span><span style=3D"color: #000;" class=3D"styled-by-prettify">size_t I=
</span><span style=3D"color: #660;" class=3D"styled-by-prettify">)</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 =C2=A0</span><sp=
an style=3D"color: #008;" class=3D"styled-by-prettify">return</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"> std</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">::</span><span style=3D"colo=
r: #008;" class=3D"styled-by-prettify">get</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify">I</span><span style=3D"color: #660;" class=3D"sty=
led-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-pre=
ttify">);</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><br></span><=
span style=3D"color: #008;" class=3D"styled-by-prettify">int</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> main</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">()</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #66=
0;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br>=C2=A0 =C2=A0std</span><span style=3D"color: #6=
60;" class=3D"styled-by-prettify">::</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify">tuple t </span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">=3D</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-pr=
ettify">{...};</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy"><br>=C2=A0 =C2=A0t</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">[</span><span style=3D"color: #066;" class=3D"styled-by-prettif=
y">0</span><span style=3D"color: #660;" class=3D"styled-by-prettify">];</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span =
style=3D"color: #800;" class=3D"styled-by-prettify">//ok, I is constexpr as=
 requested in the body. As the return is not required to be constexpr &quot=
;*this&quot; is ok (fallback on runtime version and inline std::get&lt;0&gt=
;(t)).</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-prettif=
y">constexpr</span><span style=3D"color: #000;" class=3D"styled-by-prettify=
"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">auto</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">&amp;</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> x </span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">=3D</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: #066;" cla=
ss=3D"styled-by-prettify">0</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">];</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> </span><span style=3D"color: #800;" class=3D"styled-by-prettify">=
//error! &quot;this&quot; is being requested as a constexpr value (*this) w=
hilst not being constexpr.</span><span style=3D"color: #000;" class=3D"styl=
ed-by-prettify"><br>=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"style=
d-by-prettify">auto</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> x1 </span><span style=3D"color: #660;" class=3D"styled-by-prettif=
y">=3D</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">tuple</span><span =
style=3D"color: #660;" class=3D"styled-by-prettify">{</span><span style=3D"=
color: #066;" class=3D"styled-by-prettify">10</span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">,</span><span style=3D"color: #000;" cla=
ss=3D"styled-by-prettify"> </span><span style=3D"color: #066;" class=3D"sty=
led-by-prettify">42</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">}[</span><span style=3D"color: #066;" class=3D"styled-by-prettify"=
>0</span><span style=3D"color: #660;" class=3D"styled-by-prettify">];</span=
><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span st=
yle=3D"color: #800;" class=3D"styled-by-prettify">//ok, &quot;this&quot; is=
 constexpr as requested.</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by-pr=
ettify">}</span></div></code></div><div><br>The assembly of the above would=
 be exactly the same as:</div><div><div class=3D"prettyprint" style=3D"back=
ground-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); border-=
style: solid; border-width: 1px; word-wrap: break-word;"><code class=3D"pre=
ttyprint"><div class=3D"subprettyprint"><span style=3D"color: #008;" class=
=3D"styled-by-prettify">int</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> main</span><span style=3D"color: #660;" class=3D"styled-b=
y-prettify">()</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy"><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 =
=C2=A0std</span><span style=3D"color: #660;" class=3D"styled-by-prettify">:=
:</span><span style=3D"color: #000;" class=3D"styled-by-prettify">tuple t <=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">{...};</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0std</span><spa=
n 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"col=
or: #660;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #0=
66;" class=3D"styled-by-prettify">0</span><span style=3D"color: #660;" clas=
s=3D"styled-by-prettify">&gt;(</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-prettif=
y"><br>=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-p=
rettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">=
auto</span><span style=3D"color: #660;" class=3D"styled-by-prettify">&amp;<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"> x </span><=
span style=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span st=
yle=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;" =
class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #066;" class=
=3D"styled-by-prettify">0</span><span style=3D"color: #660;" class=3D"style=
d-by-prettify">&gt;(</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"> </span=
><span style=3D"color: #800;" class=3D"styled-by-prettify">//error t is not=
 constexpr</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-pre=
ttify">constexpr</span><span style=3D"color: #000;" class=3D"styled-by-pret=
tify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">aut=
o</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> x1 </spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">=3D</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"colo=
r: #008;" class=3D"styled-by-prettify">get</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">&lt;</span><span style=3D"color: #066;" cla=
ss=3D"styled-by-prettify">0</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">std</span><span style=3D"color: #660;" class=3D"styled-by-prett=
ify">::</span><span style=3D"color: #000;" class=3D"styled-by-prettify">tup=
le</span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</span>=
<span style=3D"color: #066;" class=3D"styled-by-prettify">10</span><span st=
yle=3D"color: #660;" class=3D"styled-by-prettify">,</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #066=
;" class=3D"styled-by-prettify">42</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">});</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"><br></span><span style=3D"color: #660;" class=3D"styled-by=
-prettify">}</span></div></code></div></div></div>

<p></p>

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