--- a/mercurial/extensions.py	Sun May 14 15:46:45 2017 +0900
+++ b/mercurial/extensions.py	Sat May 13 15:41:50 2017 +0900
@@ -118,6 +118,20 @@
     if ui.debugflag:
         ui.traceback()
 
+# attributes set by registrar.command
+_cmdfuncattrs = ('norepo', 'optionalrepo', 'inferrepo')
+
+def _validatecmdtable(cmdtable):
+    """Check if extension commands have required attributes"""
+    for c, e in cmdtable.iteritems():
+        f = e[0]
+        missing = [a for a in _cmdfuncattrs if not util.safehasattr(f, a)]
+        if not missing:
+            continue
+        raise error.ProgrammingError(
+            'missing attributes: %s' % ', '.join(missing),
+            hint="use @command decorator to register '%s'" % c)
+
 def load(ui, name, path):
     if name.startswith('hgext.') or name.startswith('hgext/'):
         shortname = name[6:]
@@ -139,6 +153,7 @@
         ui.warn(_('(third party extension %s requires version %s or newer '
                   'of Mercurial; disabling)\n') % (shortname, minver))
         return
+    _validatecmdtable(getattr(mod, 'cmdtable', {}))
 
     _extensions[shortname] = mod
     _order.append(shortname)

--- a/tests/test-extension.t	Sun May 14 15:46:45 2017 +0900
+++ b/tests/test-extension.t	Sat May 13 15:41:50 2017 +0900
@@ -1534,6 +1534,40 @@
 
   $ cd ..
 
+Prohibit registration of commands that don't use @command (issue5137)
+
+  $ hg init deprecated
+  $ cd deprecated
+
+  $ cat <<EOF > deprecatedcmd.py
+  > def deprecatedcmd(repo, ui):
+  >     pass
+  > cmdtable = {
+  >     'deprecatedcmd': (deprecatedcmd, [], ''),
+  > }
+  > EOF
+  $ cat <<EOF > .hg/hgrc
+  > [extensions]
+  > deprecatedcmd = `pwd`/deprecatedcmd.py
+  > mq = !
+  > hgext.mq = !
+  > hgext/mq = !
+  > EOF
+
+  $ hg deprecatedcmd > /dev/null
+  *** failed to import extension deprecatedcmd from $TESTTMP/deprecated/deprecatedcmd.py: missing attributes: norepo, optionalrepo, inferrepo
+  *** (use @command decorator to register 'deprecatedcmd')
+  hg: unknown command 'deprecatedcmd'
+  [255]
+
+ the extension shouldn't be loaded at all so the mq works:
+
+  $ hg qseries --config extensions.mq= > /dev/null
+  *** failed to import extension deprecatedcmd from $TESTTMP/deprecated/deprecatedcmd.py: missing attributes: norepo, optionalrepo, inferrepo
+  *** (use @command decorator to register 'deprecatedcmd')
+
+  $ cd ..
+
 Test synopsis and docstring extending
 
   $ hg init exthelp