Fix indent filter ignoring blank=False for empty first line

CHANGES.rst

@@ -13,6 +13,8 @@ Unreleased
 -   Use modern packaging metadata with ``pyproject.toml`` instead of ``setup.cfg``.
     :pr:`1793`
 -   Use ``flit_core`` instead of ``setuptools`` as build backend.
+-   Fix ``indent`` filter not respecting ``blank=False`` for the first line
+    when ``first=True`` is set. :issue:`2176`
 
 
 Version 3.1.6

src/jinja2/filters.py

@@ -860,7 +860,7 @@ def do_indent(
                 indention + line if line else line for line in lines
             )
 
-    if first:
+    if first and (blank or rv.split("\n", 1)[0]):
         rv = indention + rv
 
     return rv

tests/test_filters.py

@@ -166,7 +166,7 @@ def _test_indent_multiline_template(env, markup=False):
         t = env.from_string("{{ foo|indent(2, false, true) }}")
         assert t.render(foo=text) == '\n  foo bar\n  "baz"\n  '
         t = env.from_string("{{ foo|indent(2, true, false) }}")
-        assert t.render(foo=text) == '  \n  foo bar\n  "baz"\n'
+        assert t.render(foo=text) == '\n  foo bar\n  "baz"\n'
         t = env.from_string("{{ foo|indent(2, true, true) }}")
         assert t.render(foo=text) == '  \n  foo bar\n  "baz"\n  '
 
@@ -179,6 +179,21 @@ def test_indent(self, env):
         t = env.from_string('{{ "jinja"|indent(blank=true) }}')
         assert t.render() == "jinja"
 
+    def test_indent_first_blank(self, env):
+        # ``blank=False`` (the default) must suppress indentation of the first
+        # line when it is empty, even when ``first=True``.  Regression for
+        # https://github.com/pallets/jinja/issues/2176
+        t = env.from_string("{% filter indent(4, first=true) %}{% endfilter %}")
+        assert t.render() == ""
+        # A leading blank line is not indented when blank=False.
+        t = env.from_string('{{ "\nhello"|indent(4, first=true) }}')
+        assert t.render() == "\n    hello"
+        # blank=True still indents the first line even when it is empty.
+        t = env.from_string(
+            "{% filter indent(4, first=true, blank=true) %}{% endfilter %}"
+        )
+        assert t.render() == "    "
+
     def test_indent_markup_input(self, env):
         """
         Tests cases where the filter input is a Markup type