bin: fix handling of -o '>>-' in the close/reopen path

* tests/core/serial.test: Add test case.
* bin/common_file.cc: Fix it.

bin/common_file.cc

@@ -47,16 +47,13 @@ output_file::output_file(const char* name, bool force_append)
 void
 output_file::reopen_for_append(const std::string& name)
 {
-  if (of_ && of_->is_open())     // nothing to do
+  if (os_ == &std::cout || of_->is_open())     // nothing to do
     return;
   const char* cname = name.c_str();
   if (cname[0] == '>' && cname[1] == '>')
     cname += 2;
-  if (name[0] == '-' && name[1] == 0)
-    {
-      os_ = &std::cout;
-      return;
-    }
+  // the name cannot be "-" at this point, otherwise os_ would be
+  // equal to std::cout.
   of_->open(cname, std::ios_base::app);
   if (!*of_)
     error(2, errno, "cannot reopen '%s'", cname);

tests/core/serial.test

@@ -91,3 +91,25 @@ cat >expected <<EOF
 5 pass2
 EOF
 diff out expected
+
+# The case where the output filename is a computed "-".
+ltl2tgba 1 --name '-' > naut-.hoa
+for i in 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15; do
+  ltl2tgba 1 --name "file$i" > naut$i.hoa
+done
+autfilt naut*.hoa naut*.hoa --output='>>%M' --format=%M > stdout
+cat >expected <<EOF
+-
+-
+EOF
+diff stdout expected
+cat >expected7 <<EOF
+file7
+file7
+EOF
+diff file7 expected7
+cat >expected15 <<EOF
+file15
+file15
+EOF
+diff file15 expected15