Leave std::float32_t/std::float64_t unmodified on invalid input (fixes #424)

The fixed-width wrappers parsed into a zero-initialized temporary and
copied it back unconditionally, so invalid input overwrote the
destination with zero. Copy back only when the result is not
invalid_argument, as required by [charconv.from.chars]/1.

include/fast_float/parse_number.h

@@ -158,7 +158,10 @@ template <> struct from_chars_caller<std::float32_t> {
     // float32_t
     float val = 0.0f;
     auto ret = from_chars_advanced(first, last, val, options);
-    value = val;
+    // If no characters match the pattern, value must be left unmodified.
+    if (ret.ec != std::errc::invalid_argument) {
+      value = val;
+    }
     return ret;
   }
 };
@@ -175,7 +178,10 @@ template <> struct from_chars_caller<std::float64_t> {
     // float64_t
     double val = 0.0;
     auto ret = from_chars_advanced(first, last, val, options);
-    value = val;
+    // If no characters match the pattern, value must be left unmodified.
+    if (ret.ec != std::errc::invalid_argument) {
+      value = val;
+    }
     return ret;
   }
 };

tests/fixedwidthtest.cpp

@@ -2,6 +2,8 @@
 #include <iostream>
 #include <vector>
 #include <cstring>
+#include <limits>
+#include <string_view>
 #include "fast_float/fast_float.h"
 #include <cstdint>
 
@@ -9,6 +11,39 @@
 #include <stdfloat>
 #endif
 
+// If no characters match the pattern, value must be left unmodified
+// (see https://github.com/fastfloat/fast_float/issues/424).
+template <typename T> bool check_invalid_input_preserves_value() {
+  std::vector<std::string_view> const invalid_inputs{"x", "", "-", ".", "e1",
+                                                     "-x", "na", "+"};
+  for (auto const &f : invalid_inputs) {
+    T result = T(42);
+    auto answer = fast_float::from_chars(f.data(), f.data() + f.size(), result);
+    if (answer.ec != std::errc::invalid_argument || answer.ptr != f.data()) {
+      std::cerr << "Expected invalid_argument for input: \"" << f << "\""
+                << std::endl;
+      return false;
+    }
+    if (result != T(42)) {
+      std::cerr << "Value modified on invalid input: \"" << f << "\" got "
+                << result << std::endl;
+      return false;
+    }
+  }
+  // Out-of-range input must still store the documented value.
+  std::string_view const too_large = "1e999";
+  T result = T(42);
+  auto answer = fast_float::from_chars(
+      too_large.data(), too_large.data() + too_large.size(), result);
+  if (answer.ec != std::errc::result_out_of_range ||
+      result != std::numeric_limits<T>::infinity()) {
+    std::cerr << "Expected infinity and result_out_of_range for \"1e999\""
+              << std::endl;
+    return false;
+  }
+  return true;
+}
+
 int main() {
   // Write some testcases for the parsing of floating point numbers in the
   // float32_t type. We use the from_chars function defined in this library.
@@ -33,6 +68,9 @@ int main() {
       return EXIT_FAILURE;
     }
   }
+  if (!check_invalid_input_preserves_value<std::float32_t>()) {
+    return EXIT_FAILURE;
+  }
 #else
   std::cout << "No std::float32_t type available." << std::endl;
 #endif
@@ -60,6 +98,9 @@ int main() {
       return EXIT_FAILURE;
     }
   }
+  if (!check_invalid_input_preserves_value<std::float64_t>()) {
+    return EXIT_FAILURE;
+  }
 #else
   std::cout << "No std::float64_t type available." << std::endl;
 #endif