[libc++] Use the default initializer for char_type in std::num_get::do_get

This is to fix an error that occurs when the char type is a class type.
Thanks to Yichen Yan for the patch.

Differential Revision: https://reviews.llvm.org/D100005

Author: ldionne

libcxx/include/locale

@@ -1085,7 +1085,7 @@ num_get<_CharT, _InputIterator>::do_get(iter_type __b, iter_type __e,
     int __base = 16;
     // Stage 2
     char_type __atoms[26];
-    char_type __thousands_sep = 0;
+    char_type __thousands_sep = char_type();
     string __grouping;
     std::use_facet<ctype<_CharT> >(__iob.getloc()).widen(__num_get_base::__src,
                                                          __num_get_base::__src + 26, __atoms);

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/user_defined_char_type.pass.cpp

@@ -0,0 +1,116 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <locale>
+//
+// Make sure that num_get works with a user-defined char_type that has a
+// constructor making initialization from bare `int` invalid.
+
+#include <cstddef>
+#include <cstdint>
+#include <locale>
+#include <string>
+
+#include "test_macros.h"
+
+struct Char {
+  Char() = default;
+  Char(char c) : underlying_(c) {}
+  Char(unsigned i) : underlying_(i) {}
+  explicit Char(std::int32_t i) : underlying_(i) {}
+  operator std::int32_t() const { return underlying_; }
+
+  char underlying_;
+};
+
+namespace std {
+template <>
+struct char_traits<Char> {
+  using char_type  = Char;
+  using int_type   = int;
+  using off_type   = streamoff;
+  using pos_type   = streampos;
+  using state_type = mbstate_t;
+
+  static void assign(char_type& a, const char_type& b) { a = b; }
+  static bool eq(char_type a, char_type b) { return a.underlying_ == b.underlying_; }
+  static bool lt(char_type a, char_type b) { return a.underlying_ < b.underlying_; }
+
+  static int compare(const char_type* s1, const char_type* s2, std::size_t n) {
+    return char_traits<char>::compare(reinterpret_cast<const char*>(s1), reinterpret_cast<const char*>(s2), n);
+  }
+  static std::size_t length(const char_type* s) { return char_traits<char>::length(reinterpret_cast<const char*>(s)); }
+  static const char_type* find(const char_type* p, std::size_t n, const char_type& c) {
+    for (size_t i = 0; i != n; ++i) {
+      if (static_cast<int32_t>(p[i]) == static_cast<int32_t>(c)) {
+        return p + n;
+      }
+    }
+    return nullptr;
+  }
+  static char_type* move(char_type* dest, const char_type* source, std::size_t count) {
+    char_traits<char>::move(reinterpret_cast<char*>(dest), reinterpret_cast<const char*>(source), count);
+    return dest;
+  }
+  static char_type* copy(char_type* dest, const char_type* source, std::size_t count) {
+    char_traits<char>::copy(reinterpret_cast<char*>(dest), reinterpret_cast<const char*>(source), count);
+    return dest;
+  }
+  static char_type* assign(char_type* dest, std::size_t n, char_type c) {
+    char_traits<char>::assign(reinterpret_cast<char*>(dest), n, c.underlying_);
+    return dest;
+  }
+
+  static int_type not_eof(int_type i) { return char_traits<char>::not_eof(i); }
+  static char_type to_char_type(int_type i) { return Char(char_traits<char>::to_char_type(i)); }
+  static int_type to_int_type(char_type c) { return char_traits<char>::to_int_type(c.underlying_); }
+  static bool eq_int_type(int_type i, int_type j) { return i == j; }
+  static int_type eof() { return char_traits<char>::eof(); }
+};
+
+template <>
+class ctype<Char> : public locale::facet {
+public:
+  static locale::id id;
+  Char toupper(Char c) const { return Char(std::toupper(c.underlying_)); }
+  const char* widen(const char* first, const char* last, Char* dst) const {
+    for (; first != last;)
+      *dst++ = Char(*first++);
+    return last;
+  }
+};
+
+locale::id ctype<Char>::id;
+
+template <>
+class numpunct<Char> : public locale::facet {
+public:
+  typedef basic_string<Char> string_type;
+  static locale::id id;
+  Char decimal_point() const { return Char('.'); }
+  Char thousands_sep() const { return Char(','); }
+  string grouping() const { return ""; }
+  string_type truename() const {
+    static Char yes[] = {Char('t')};
+    return string_type(yes, 1);
+  }
+  string_type falsename() const {
+    static Char no[] = {Char('f')};
+    return string_type(no, 1);
+  }
+};
+
+locale::id numpunct<Char>::id;
+
+} // namespace std
+
+int main(int, char**) {
+  std::locale l(std::locale::classic(), new std::num_get<Char>);
+
+  return 0;
+}