How does concatenation work.

[BengtGustafsson]

As of now it is allowed to concatenate cooked and raw literals with the same encoding:

const char* raw = R"(raw)" "cooked";

It is also allowed to specify the encoding on any of concatenated literals, as long as there are no contradictions. The result follows the encoding given, if any.

const wchar_t* mixed = "normal" L"wide";

Example: godbolt

In keeping with this I think we should allow f or x to exist on any of a set of concatenated literals, but obviously only one of them (in case both are available). All expression-fields are collected and placed after the last literal. This combines nicely with the mixes already allowed as exemplified above, so we can write for instance this:

std::wstring str = "One " L" two" f"{1+2}" R"(åäö)" fR"(£€{2+3})";

Here the first literal does not specify anything, the second one specifies the encoding for the concatenated result, the third has an expression-field while the fourth and fifth are raw with the fifth being also a f-literal. The compiler would see:

std::wstring str = std::format(L"One two{}åäö£€{}", 1+2, 2+3);

But with the literal in an internal representation so that any wchar_t characters can be represented. This is done in phase 6.

[BengtGustafsson]

If prefix UDLs are used as the basis for this we can't just change the encoding with the L as in the second example above, so instead we define that the L, u, U and u8 are built in prefix UDL functions that don't extract any fields. These can't be combined with other prefix UDLs as the preprocessor is not able to know what kind of string literal these are defined for (and they can't be defined for more than one as there is nothing to resolve the resulting ambiguity when not concatenated with a string literal with one of the built in prefixes.