Fix NLOHMANN_JSON_SERIALIZE_ENUM_STRICT's from_json message (#5698)

from_json built its out_of_range.410 message with "..." + j.dump(). If the
unmatched value is (or contains) a string with invalid UTF-8, that dump()
itself throws type_error.316, so the caller got type_error.316 instead of
the documented out_of_range.410; such strings can reach get<Enum>()
unvalidated, e.g. from from_cbor()/from_msgpack(). With a custom string_t,
j.dump() returns that type, and "const char*" + string_t does not compile
unless the type happens to provide operator+, so the macro failed to
compile for such types.

Build the message with detail::concat(), which appends any type exposing
data()/size() and always yields a std::string, and dump with
error_handler_t::replace so building the message itself cannot throw.

Added regression tests: an invalid-UTF-8 case in the existing strict-enum
test in unit-conversions.cpp, and a strict-enum use with alt_string (the
custom string_t from unit-alt-string.cpp) to cover the compile failure.

Fixes #5667.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-alt-string.cpp

@@ -174,6 +174,15 @@ bool operator<(const char* op1, const alt_string& op2) noexcept
     return op1 < op2.str_impl;
 }
 
+enum class alt_color { red, green };
+
+// NOLINTNEXTLINE(misc-use-internal-linkage,misc-const-correctness,cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays) - false positive
+NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(alt_color,
+{
+    {alt_color::red, "red"},
+    {alt_color::green, "green"},
+})
+
 TEST_CASE("alternative string type")
 {
     SECTION("binary formats")
@@ -374,4 +383,19 @@ TEST_CASE("alternative string type")
         const auto j2 = j.flatten();
         CHECK(j2.dump() == R"({"/foo/0":"bar","/foo/1":"baz"})");
     }
+
+    SECTION("strict enum")
+    {
+        // regression test for #5667: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT's from_json
+        // built its exception message with "..." + j.dump(), which does not compile
+        // when j.dump() returns a custom string_t (here alt_string) instead of
+        // std::string
+        alt_json doc;
+        doc = "red";
+        CHECK(doc.get<alt_color>() == alt_color::red);
+
+        alt_json _;
+        doc = "blue";
+        CHECK_THROWS_WITH_AS(_ = doc.get<alt_color>(), "[json.exception.out_of_range.410] enum value out of range for alt_color: \"blue\"", alt_json::out_of_range&);
+    }
 }