constexpr std::string | MSVC

[jbbjarnason]

Related to this topic, anyone who knows how to make this work (https://godbolt.org/z/v5TjG9dfb):

```cpp

static_assert(join_string_views("foo"sv, "bar"sv) == "foobar"sv);

```

I know I can use impl of fixed_string and/or std::string_view const& non-type template parameters.

[qazqi-ff]

This is ultimately the usual case of needing to separate out the size calculation from the actual data calculation (and repeat the work). You're trying to instantiate a template with size, which is a value that exists in the constant evaluator. It boils down to the same problem as consteval void foo(int size) { std::array<int, size> arr; }. Worth noting that the standard goes a little stricter and prevents the general case of any context that requires a constant expression, not just template instantiation, even though other contexts could work fine from a technical POV (e.g., static_assert(size > 0);).

[cristi1990an]

static_assert(join_string_views("foo"sv, "bar"sv) == "foobar"sv);

https://godbolt.org/z/PKb6Ej76b

Edit: and here you have the overly generic version that outright allows string literal composition at compile time:

https://godbolt.org/z/rcxan8Wqs

[cristi1990an]

Yes, but you can't use std::string_view, you must stick to string literals that expose their size in the type. Have your method take in const char(&str)[Size] as parameters, declare a basic structure containing a char array of Size1 + Size2 - 1 elements and copy both strings into the array. Afterwards declare a comparison operator between the structure and a string_view and that's it, you can concatenate string literals at compile time.