libcxx: Fix for basic_stringbuf::seekoff() after r320604.

[pcc]

Summary

As a result of this change, the basic_stringbuf constructor that
takes a mode ends up leaving hm_ set to 0, causing the comparison
"hm_ - str_.data() < noff" in seekoff() to succeed, which caused
the function to incorrectly return -1. The fix is to account for the
possibility of hm_ being 0 when computing the distance from hm_
to the start of the string.

[mclow.lists]

libcxx/include/sstream

580

This can be const.

[pcc]

• Make __hm const

[mclow.lists]

LGTM. I'm going to sprinkle const throughout this file later, but that is a drive-by thing.
All the lines that start out ptrdiff_t __hm = will soon be const ptrdiff_t __hm = ,
but that's not necessary for this bug fix.

libcxx/include/sstream

 {
     if (__hm_ < this->pptr())
         __hm_ = this->pptr();
     if ((__wch & (ios_base::in | ios_base::out)) == 0)
         return pos_type(-1);
     if ((__wch & (ios_base::in | ios_base::out)) == (ios_base::in | ios_base::out)
         && __way == ios_base::cur)
         return pos_type(-1);
+    const ptrdiff_t __hm = __hm_ == nullptr ? 0 : __hm_ - __str_.data();
     off_type __noff;
     switch (__way)
     {
     case ios_base::beg:
         __noff = 0;
         break;
     case ios_base::cur:
         if (__wch & ios_base::in)
             __noff = this->gptr() - this->eback();
         else
             __noff = this->pptr() - this->pbase();
         break;
     case ios_base::end:
-        __noff = __hm_ - __str_.data();
+        __noff = __hm;
         break;
     default:
         return pos_type(-1);
     }
     __noff += __off;
-    if (__noff < 0 || __hm_ - __str_.data() < __noff)
+    if (__noff < 0 || __hm < __noff)
         return pos_type(-1);
     if (__noff != 0)
     {
         if ((__wch & ios_base::in) && this->gptr() == 0)
             return pos_type(-1);
         if ((__wch & ios_base::out) && this->pptr() == 0)
             return pos_type(-1);
     }

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.virtuals/seekoff.pass.cpp

 //                  ios_base::openmode which = ios_base::in | ios_base::out);
 
 #include <sstream>
 #include <cassert>
 
 int main()
 {
     {
+        std::stringbuf sb(std::ios_base::in);
+        assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::out) == -1);
+        assert(sb.pubseekoff(3, std::ios_base::cur, std::ios_base::out) == -1);
+        assert(sb.pubseekoff(-3, std::ios_base::end, std::ios_base::out) == -1);
+        assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::in | std::ios_base::out) == -1);
+        assert(sb.pubseekoff(3, std::ios_base::cur, std::ios_base::in | std::ios_base::out) == -1);
+        assert(sb.pubseekoff(-3, std::ios_base::end, std::ios_base::in | std::ios_base::out) == -1);
+        assert(sb.pubseekoff(0, std::ios_base::beg, std::ios_base::in) == 0);
+        assert(sb.pubseekoff(0, std::ios_base::cur, std::ios_base::in) == 0);
+        assert(sb.pubseekoff(0, std::ios_base::end, std::ios_base::in) == 0);
+    }
+    {
+        std::stringbuf sb(std::ios_base::out);
+        assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::in) == -1);
+        assert(sb.pubseekoff(3, std::ios_base::cur, std::ios_base::in) == -1);
+        assert(sb.pubseekoff(-3, std::ios_base::end, std::ios_base::in) == -1);
+        assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::in | std::ios_base::out) == -1);
+        assert(sb.pubseekoff(3, std::ios_base::cur, std::ios_base::in | std::ios_base::out) == -1);
+        assert(sb.pubseekoff(-3, std::ios_base::end, std::ios_base::in | std::ios_base::out) == -1);
+        assert(sb.pubseekoff(0, std::ios_base::beg, std::ios_base::out) == 0);
+        assert(sb.pubseekoff(0, std::ios_base::cur, std::ios_base::out) == 0);
+        assert(sb.pubseekoff(0, std::ios_base::end, std::ios_base::out) == 0);
+    }
+    {
         std::stringbuf sb("0123456789", std::ios_base::in);
         assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::out) == -1);
         assert(sb.pubseekoff(3, std::ios_base::cur, std::ios_base::out) == -1);
         assert(sb.pubseekoff(-3, std::ios_base::end, std::ios_base::out) == -1);
         assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::in | std::ios_base::out) == -1);
         assert(sb.pubseekoff(3, std::ios_base::cur, std::ios_base::in | std::ios_base::out) == -1);
         assert(sb.pubseekoff(-3, std::ios_base::end, std::ios_base::in | std::ios_base::out) == -1);
         assert(sb.pubseekoff(3, std::ios_base::beg, std::ios_base::in) == 3);