cpplint: allow using-directive for a whitelist of namespaces (#67)

This will permit the use of std::chrono and other useful new literals in C++14, which are most conveniently brought in via "using namespace"

ament_cpplint/ament_cpplint/cpplint.py

@@ -5322,12 +5322,28 @@ def CheckLanguage(filename, clean_lines, linenum, file_extension,
           'Did you mean "memset(%s, 0, %s)"?'
           % (match.group(1), match.group(2)))
 
-  if Search(r'\busing namespace\b', line):
-    if Search(r'\bliterals\b', line):
-      error(filename, linenum, 'build/namespaces_literals', 5,
-            'Do not use namespace using-directives.  '
-            'Use using-declarations instead.')
-    else:
+  # Check for 'using namespace' which pollutes namespaces.
+  # This is tricky. Although in general 'using namespace' is a Bad Thing,
+  # an exception is made for certain standard namespaces, like std::*literals
+  # and std::placeholders, which are intended to be used in this fashion.
+  # This whitelist may grow over time as needed if/when shiny new libraries
+  # come along that are well-behaved in a 'using namespace' context.
+  # For example, 'using namespace std::chrono_literals;' is allowed, but
+  # 'using namespace foo;' is not allowed.
+  # Note that headers are not permitted to use this exception.
+  match = Search(r'\busing namespace\s+((\w|::)+)', line)
+  if match:
+    whitelist = [
+      'std::chrono_literals',
+      'std::complex_literals',
+      'std::literals',
+      'std::literals::chrono_literals',
+      'std::literals::complex_literals',
+      'std::literals::string_literals',
+      'std::placeholders',
+      'std::string_literals',
+    ]
+    if IsHeaderExtension(file_extension) or match.group(1) not in whitelist:
       error(filename, linenum, 'build/namespaces', 5,
             'Do not use namespace using-directives.  '
             'Use using-declarations instead.')