text: """Functions that expose information about templates that might be
  interesting for introspection.
  """
  import typing as t
  
  from . import nodes
  from .compiler import CodeGenerator
  from .compiler import Frame
  
  if t.TYPE_CHECKING:
      from .environment import Environment
  
  
  class TrackingCodeGenerator(CodeGenerator):
      """We abuse the code generator for introspection."""
  
      def __init__(self, environment: "Environment") -> None:
          super().__init__(environment, "<introspection>", "<introspection>")
          self.undeclared_identifiers: t.Set[str] = set()
  
      def write(self, x: str) -> None:
          """Don't write."""
  
      def enter_frame(self, frame: Frame) -> None:
          """Remember all undeclared identifiers."""
          super().enter_frame(frame)
  
          for _, (action, param) in frame.symbols.loads.items():
              if action == "resolve" and param not in self.environment.globals:
                  self.undeclared_identifiers.add(param)
  
  
  def find_undeclared_variables(ast: nodes.Template) -> t.Set[str]:
      """Returns a set of all variables in the AST that will be looked up from
      the context at runtime.  Because at compile time it's not known which
      variables will be used depending on the path the execution takes at
      runtime, all variables are returned.
  
      >>> from jinja2 import Environment, meta
      >>> env = Environment()
      >>> ast = env.parse('{% set foo = 42 %}{{ bar + foo }}')
      >>> meta.find_undeclared_variables(ast) == {'bar'}
      True
  
      .. admonition:: Implementation
  
         Internally the code generator is used for finding undeclared variables.
         This is good to know because the code generator might raise a
         :exc:`TemplateAssertionError` during compilation and as a matter of
         fact this function can currently raise that exception as well.
      """
      codegen = TrackingCodeGenerator(ast.environment)  # type: ignore
      codegen.visit(ast)
      return codegen.undeclared_identifiers
  
  
  _ref_types = (nodes.Extends, nodes.FromImport, nodes.Import, nodes.Include)
  _RefType = t.Union[nodes.Extends, nodes.FromImport, nodes.Import, nodes.Include]
  
  
  def find_referenced_templates(ast: nodes.Template) -> t.Iterator[t.Optional[str]]:
      """Finds all the referenced templates from the AST.  This will return an
      iterator over all the hardcoded template extensions, inclusions and
      imports.  If dynamic inheritance or inclusion is used, `None` will be
      yielded.
  
      >>> from jinja2 import Environment, meta
      >>> env = Environment()
      >>> ast = env.parse('{% extends "layout.html" %}{% include helper %}')
      >>> list(meta.find_referenced_templates(ast))
      ['layout.html', None]
  
      This function is useful for dependency tracking.  For example if you want
      to rebuild parts of the website after a layout template has changed.
      """
      template_name: t.Any
  
      for node in ast.find_all(_ref_types):
          template: nodes.Expr = node.template  # type: ignore
  
          if not isinstance(template, nodes.Const):
              # a tuple with some non consts in there
              if isinstance(template, (nodes.Tuple, nodes.List)):
                  for template_name in template.items:
                      # something const, only yield the strings and ignore
                      # non-string consts that really just make no sense
                      if isinstance(template_name, nodes.Const):
                          if isinstance(template_name.value, str):
                              yield template_name.value
                      # something dynamic in there
                      else:
                          yield None
              # something dynamic we don't know about here
              else:
                  yield None
              continue
          # constant is a basestring, direct template name
          if isinstance(template.value, str):
              yield template.value
          # a tuple or list (latter *should* not happen) made of consts,
          # yield the consts that are strings.  We could warn here for
          # non string values
          elif isinstance(node, nodes.Include) and isinstance(
              template.value, (tuple, list)
          ):
              for template_name in template.value:
                  if isinstance(template_name, str):
                      yield template_name
          # something else we don't care about, we could warn here
          else:
              yield None
  