Don't let ADL pick to_string_view

[tiagomacarios]

02bf4d1c ("Disable to_string_view ADL") qualified the is_string trait, and has_to_string_view and char_t are qualified for the same reason. Two call sites were missed:

• core.h — value's string constructor
• format.h — the write() overload for types with a string view conversion

Both sit inside fmt::detail, so an unqualified call adds the argument's own namespace to the overload set.

That splits the two halves of one decision. Both call sites are reached only through has_to_string_view or char_t, which are defined by the qualified expression — so ADL can never be needed to satisfy them, it can only add candidates the gate never considered. When the argument's namespace declares a to_string_view template, the two tie during partial ordering and the call is ambiguous:

core.h(2211): error C2668: 'to_string_view': ambiguous call to overloaded function
note: could be 'string_view N::to_string_view<T>(const T&)' [found using argument-dependent lookup]
note: or 'basic_string_view<char> fmt::detail::to_string_view<T,0>(const T&)'

Both are reachable:

• format("{}", x) stores the argument through value's constructor.
• to_string(x) passes it to detail::write unmapped, which lands on the write() overload — as do FMT_COMPILE named fields and nested_formatter::write_arg.

This turned up in Microsoft Office, which declares a constrained to_string_view template next to its own string types. It only breaks where those types are distinct classes; where they are std aliases the ADL set is namespace std and picks up nothing.

Tests

base_test.adl_to_string_view in core-test and format_test.adl_to_string_view in format-test. core-test cannot include format.h (it has an #error guard), so each call site needs its own test. Reverting either line alone fails the build of the test that covers it.

Full suite green at C++17 on MSVC 19.51 — 22/22 test binaries.

One behaviour change worth flagging

A non-template to_string_view in the argument's namespace previously won outright at these call sites. A type that satisfies is_std_string_like through find_first_of and data() but has no size() therefore used to format through ADL, and now fails to compile: the trait only checks that the qualified overload is viable, not that its body instantiates. That is the extension point removed in 02bf4d1 going away rather than a new restriction.

[vitaut]

Adding namespace qualification seems OK but the tests are an overkill, let's drop them.

[vitaut]

thank you