add failing test for wide chars

tests/BUILD.bazel

@@ -88,6 +88,15 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "wide_char_test",
+    srcs = ["wide_char_test.cpp"],
+    deps = [
+        "//:fast_float",
+        "@doctest//doctest",
+    ],
+)
+
 cc_test(
     name = "string_test",
     srcs = ["string_test.cpp"],

tests/CMakeLists.txt

@@ -67,6 +67,7 @@ function(fast_float_add_cpp_test TEST_NAME)
 endfunction(fast_float_add_cpp_test)
 
 fast_float_add_cpp_test(rcppfastfloat_test)
+fast_float_add_cpp_test(wide_char_test)
 fast_float_add_cpp_test(example_test)
 fast_float_add_cpp_test(example_comma_test)
 fast_float_add_cpp_test(basictest)

tests/wide_char_test.cpp

@@ -0,0 +1,57 @@
+#include "fast_float/fast_float.h"
+#include <iostream>
+#include <string>
+#include <system_error>
+
+bool tester(std::string s, double expected,
+            fast_float::chars_format fmt = fast_float::chars_format::general) {
+  std::wstring input(s.begin(), s.end());
+  double result;
+  auto answer = fast_float::from_chars(
+      input.data(), input.data() + input.size(), result, fmt);
+  if (answer.ec != std::errc()) {
+    std::cerr << "parsing of \"" << s << "\" should succeed\n";
+    return false;
+  }
+  if (result != expected && !(std::isnan(result) && std::isnan(expected))) {
+    std::cerr << "parsing of \"" << s << "\" succeeded, expected " << expected
+              << " got " << result << "\n";
+    return false;
+  }
+  input[0] += 256;
+  answer = fast_float::from_chars(input.data(), input.data() + input.size(),
+                                  result, fmt);
+  if (answer.ec == std::errc()) {
+    std::cerr << "parsing of altered \"" << s << "\" should fail\n";
+    return false;
+  }
+  return true;
+}
+
+bool test_minus() { return tester("-42", -42); }
+
+bool test_plus() {
+  return tester("+42", 42,
+                fast_float::chars_format::general |
+                    fast_float::chars_format::allow_leading_plus);
+}
+
+bool test_space() {
+  return tester(" 42", 42,
+                fast_float::chars_format::general |
+                    fast_float::chars_format::skip_white_space);
+}
+
+bool test_nan() {
+  return tester("nan", std::numeric_limits<double>::quiet_NaN());
+}
+
+int main() {
+  if (test_minus() && test_plus() && test_space() && test_nan()) {
+    std::cout << "all ok" << std::endl;
+    return EXIT_SUCCESS;
+  }
+
+  std::cout << "test failure" << std::endl;
+  return EXIT_FAILURE;
+}