Inferior without argument prints detail of current inferior.

This patch makes the inferior command display information about the
current inferior when called with no argument.  This behavior is similar
to the one of the thread command.

Before patch:

    (gdb) info inferior
      Num  Description       Connection           Executable
    * 1    process 19221     1 (native)           /home/lsix/tmp/a.out
      2    process 19239     1 (native)           /home/lsix/tmp/a.out
    (gdb) inferior 2
    [Switching to inferior 2 [process 19239] (/home/lsix/tmp/a.out)]
    [Switching to thread 2.1 (process 19239)]
    #0  0x0000000000401146 in main ()
    (gdb) inferior
    Argument required (expression to compute).

After patch:

    (gdb) info inferior
      Num  Description       Connection           Executable
    * 1    process 18699     1 (native)           /home/lsix/tmp/a.out
      2    process 18705     1 (native)           /home/lsix/tmp/a.out
    (gdb) inferior 2
    [Switching to inferior 2 [process 18705] (/home/lsix/tmp/a.out)]
    [Switching to thread 2.1 (process 18705)]
    #0  0x0000000000401146 in main ()
    (gdb) inferior
    [Current inferior is 2 [process 18705] (/home/lsix/tmp/a.out)]

gdb/doc/ChangeLog:

	* gdb.texinfo (Inferiors Connections and Programs): Document the
	inferior command when used without argument.

gdb/ChangeLog:

	* NEWS: Add entry for the behavior change of the inferior command.
	* inferior.c (inferior_command): When no argument is given to the
	inferior command, display info about the currently selected
	inferior.

gdb/testsuite/ChangeLog:

	* gdb.base/inferior-noarg.c: New test.
	* gdb.base/inferior-noarg.exp: New test.

2 files changed, 58 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/inferior-noarg.c b/gdb/testsuite/gdb.base/inferior-noarg.c
new file mode 100644
index 0000000..bfe52c0
--- /dev/null
+++ b/gdb/testsuite/gdb.base/inferior-noarg.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/inferior-noarg.exp b/gdb/testsuite/gdb.base/inferior-noarg.exp
new file mode 100644
index 0000000..65d23fe
--- /dev/null
+++ b/gdb/testsuite/gdb.base/inferior-noarg.exp
@@ -0,0 +1,36 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This test case checks that the 'inferior' command, when given no
+# argument, displays information about the inferior currently active.
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" \
+	${testfile} ${srcfile}] } {
+    return
+}
+
+gdb_test "inferior" "\[Current inferior is 1 \[<null>\] (.*)\]" "inferior not running"
+
+if { ![runto_main] } {
+    untested "could not run to main"
+    return
+}
+
+gdb_test "inferior" {\[Current inferior is 1 \[.*\] (.*)\]} "inferior running"
+gdb_test "clone-inferior" "Added inferior 2.*" "create new inferior"
+gdb_test "inferior 2" "\[Switching to inferior 2 \[<null>\] (.*)]" "change inferior"
+gdb_test "inferior" "\[Current inferior is 2 \[<null>\] (.*)\]" "show new inferior"