>I am still trying to understand this part. Let me slightly rephrase my
example code:
int^ x = {1,2.5,"C++"}.select(runtime_index); // runtime_index happens to
be 2
x = 2;
How is this compiled? Are there exceptions involved? What should happen if
the runtime index does not fit the type T^ like in the example?
****answer****
It will throw an expression since its a runtime_index, hence the
instantiation code runs at runtime, but if it was a compile time index then
it would be a compiler error, since the instansitation code could be run at
compile time. A way to implement it would be to provide two blocks of code
in the definition of select() or operator[]():
one runs if std::is_constant_evaluated() is true, and the other if false.
see this:https://en.cppreference.com/w/cpp/types/is_constant_evaluated.html
.

I think the syntax of defining using {1,2.5,"C++"}.select was bad, and I
used it for demonstration. a better one would be the same syntax for
defining tuples:
std::heterogeneous_llist<Args_t...>{Args_T... args} . especially since the
container itself is a normal lvalue.

Thanks for your feedback, apreciate it.

On Fri, Apr 10, 2026 at 3:16 PM Simon Schröder via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
> On Wed, Apr 8, 2026 at 6:52 PM Simon Schröder <
> dr.simon.schroeder_at_[hidden]> wrote:
>
>>
>>
>> > On Apr 8, 2026, at 9:47 AM, Muneem <itfllow123_at_[hidden]> wrote:
>> >
>> > int^ x = {1,2.5,”C++”}.select(2); This would throw a huge error at
>> compile time and is exactly why we need a new expression type T^. Not using
>> T^ would give too much freedom that could backfire as shown in the example.
>> In fact for T^, you cant even assign float^ to an int^.
>>
>> May bad, I wasn’t clear enough with my example. I do understand that with
>> index=2 the compiler should use the compile time version and return the
>> correct type. What my question actually is what would happen if the index
>> is a runtime index that happens to be 2. How would the code be compiled to
>> handle this case? What would happen to following source code lines that use
>> ‘x’? Or would you just throw an exception?
>>
>>
>> I am still trying to understand this part. Let me slightly rephrase my
> example code:
> int^ x = {1,2.5,"C++"}.select(runtime_index); // runtime_index happens to
> be 2
> x = 2;
>
> How is this compiled? Are there exceptions involved? What should happen if
> the runtime index does not fit the type T^ like in the example?
> --
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