[libc++] Fix num_get base parsing (llvm#170460)

This fixes two bugs reported in llvm#121795 and adds regression tests.
Specifically, these bugs are in the base detection mechanism. The first
bug is that the out parameter isn't set when the stream only contains
zero and after that is the end of the stream. The second one is that we
don't consider `0` to be a number, and instead we only parse it as the
start of an octal literal.

Author: philnik777

libcxx/include/__locale_dir/num.h

@@ -436,6 +436,7 @@ class num_get : public locale::facet, private __num_get<_CharT> {
         ++__first;
         if (__first == __last) {
           __err |= ios_base::eofbit;
+          __v = 0;
           return __first;
         }
         // __c2 == 'x' || __c2 == 'X'
@@ -444,6 +445,7 @@ class num_get : public locale::facet, private __num_get<_CharT> {
           ++__first;
         } else {
           __base = 8;
+          __parsed_num = true; // We only swallowed '0', so we've started to parse a number
         }
       } else {
         __base = 10;

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long.pass.cpp

@@ -670,5 +670,101 @@ int main(int, char**)
       assert(v == std::numeric_limits<long>::min());
     }
 
+  { // Check that auto-detection of the base works properly
+    ios.flags(ios.flags() & ~std::ios::basefield);
+    { // zeroes
+      {
+        v                          = -1;
+        const char str[]           = "0";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 1), ios, err, v);
+        assert(base(iter) == str + 1);
+        assert(err == ios.eofbit);
+        assert(v == 0);
+      }
+      {
+        v                          = -1;
+        const char str[]           = "00";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 2), ios, err, v);
+        assert(base(iter) == str + 2);
+        assert(err == ios.eofbit);
+        assert(v == 0);
+      }
+      {
+        v                          = -1;
+        const char str[]           = "0x0";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 3), ios, err, v);
+        assert(base(iter) == str + 3);
+        assert(err == ios.eofbit);
+        assert(v == 0);
+      }
+      {
+        v                          = -1;
+        const char str[]           = "0X0";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 3), ios, err, v);
+        assert(base(iter) == str + 3);
+        assert(err == ios.eofbit);
+        assert(v == 0);
+      }
+    }
+    { // first character after base is out of range
+      {
+        v                          = -1;
+        const char str[]           = "08";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 2), ios, err, v);
+        assert(base(iter) == str + 1);
+        assert(err == ios.goodbit);
+        assert(v == 0);
+      }
+      {
+        v                          = -1;
+        const char str[]           = "1a";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 2), ios, err, v);
+        assert(base(iter) == str + 1);
+        assert(err == ios.goodbit);
+        assert(v == 1);
+      }
+      {
+        v                          = -1;
+        const char str[]           = "0xg";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 3), ios, err, v);
+        assert(base(iter) == str + 2);
+        assert(err == ios.failbit);
+        assert(v == 0);
+      }
+      {
+        v                          = -1;
+        const char str[]           = "0Xg";
+        std::ios_base::iostate err = ios.goodbit;
+
+        cpp17_input_iterator<const char*> iter =
+            f.get(cpp17_input_iterator<const char*>(str), cpp17_input_iterator<const char*>(str + 3), ios, err, v);
+        assert(base(iter) == str + 2);
+        assert(err == ios.failbit);
+        assert(v == 0);
+      }
+    }
+  }
+
   return 0;
 }