Test for behavior of negative underflow formatting.

dlls/oleaut32/tests/vartype.c

@@ -4586,6 +4586,7 @@ static void test_VarBoolChangeTypeEx(void)
 static void test_VarBstrFromR4(void)
 {
   static const WCHAR szNative[] = { '6','5','4','3','2','2','.','3','\0' };
+  static const WCHAR szZero[] = {'0', '\0'};
   LCID lcid;
   HRESULT hres;
   BSTR bstr = NULL;
@@ -4608,6 +4609,14 @@ static void test_VarBstrFromR4(void)
     ok(memcmp(bstr, szNative, sizeof(szNative)) == 0, "string different\n");
     }
   }
+
+  f = -1e-400;    /* deliberately cause underflow */
+  hres = pVarBstrFromR4(f, lcid, 0, &bstr);
+  ok(hres == S_OK, "got hres 0x%08lx\n", hres);
+  if (bstr)
+  {
+    todo_wine ok(memcmp(bstr, szZero, sizeof(szZero)) == 0, "negative zero (got %s)\n", wtoascii(bstr));
+  }
 }
 
 #define BSTR_DATE(dt,str) SysFreeString(bstr); bstr = NULL; \