Detect if lconv contains thousands_sep

(cherry picked from commit 96c28f74)

fmt/format.h

@@ -910,23 +910,6 @@ class ThousandsSep {
   }
 };
 
-// Returns the thousands separator for the current locale.
-// On android the lconv structure is stubbed using an empty structure.
-// The test is for the size only, not for the presense of
-// thousands_sep in std::lconv, because if one would add thousands_sep
-// at some point, the size of structure would be at least sizeof(char*).
-template<typename Lconv, bool=(sizeof(Lconv) >= sizeof(char*))>
-struct Locale {
-  static fmt::StringRef thousands_sep() { return ""; }
-};
-
-template<typename Lconv>
-struct Locale<Lconv, true> {
-  static fmt::StringRef thousands_sep() {
-    return static_cast<Lconv*>(std::localeconv())->thousands_sep;
-  }
-};
-
 // Formats a decimal unsigned integer value writing into buffer.
 // thousands_sep is a functor that is called after writing each char to
 // add a thousands separator if necessary.
@@ -1138,6 +1121,21 @@ struct Not { enum { value = 0 }; };
 template<>
 struct Not<false> { enum { value = 1 }; };
 
+template<typename T, T> struct LConvCheck {
+  LConvCheck(int) {}
+};
+
+// Returns the thousands separator for the current locale.
+// We check if ``lconv`` contains ``thousands_sep`` because on Android
+// ``lconv`` is stubbed as an empty struct.
+template <typename LConv>
+inline StringRef thousands_sep(
+    LConv *lc, LConvCheck<char *LConv::*, &LConv::thousands_sep> = 0) {
+  return lc->thousands_sep;
+}
+
+inline fmt::StringRef thousands_sep(...) { return ""; }
+
 // Makes an Arg object from any type.
 template <typename Formatter>
 class MakeValue : public Arg {
@@ -2786,7 +2784,7 @@ void BasicWriter<Char>::write_int(T value, Spec spec) {
   }
   case 'n': {
     unsigned num_digits = internal::count_digits(abs_value);
-    fmt::StringRef sep = internal::Locale<lconv>::thousands_sep();
+    fmt::StringRef sep = internal::thousands_sep(std::localeconv());
     unsigned size = static_cast<unsigned>(
           num_digits + sep.size() * (num_digits - 1) / 3);
     CharPtr p = prepare_int_buffer(size, spec, prefix, prefix_size) + 1;

test/util-test.cc

@@ -956,3 +956,17 @@ TEST(UtilTest, Conditional) {
   fmt::internal::Conditional<false, int, char>::type *pc = &c;
   (void)pc;
 }
+
+struct TestLConv {
+  char *thousands_sep;
+};
+
+struct EmptyLConv {};
+
+TEST(UtilTest, ThousandsSep) {
+  char foo[] = "foo";
+  TestLConv lc = {foo};
+  EXPECT_EQ("foo", fmt::internal::thousands_sep(&lc).to_string());
+  EmptyLConv empty_lc;
+  EXPECT_EQ("", fmt::internal::thousands_sep(&empty_lc));
+}