From 94397f4949511881dfc35fbab76cb237f6911424 Mon Sep 17 00:00:00 2001
From: j1642 <60148902+j1642@users.noreply.github.com>
Date: Sun, 20 Nov 2022 22:54:03 -0500
Subject: [PATCH] BUG: Warn when an existing layout manager changes to tight
 layout

---
 lib/matplotlib/figure.py            |  4 ++++
 lib/matplotlib/tests/test_figure.py | 11 +++++++++++
 2 files changed, 15 insertions(+)

diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
index 147500dd5451..5a3a5df8d18b 100644
--- a/lib/matplotlib/figure.py
+++ b/lib/matplotlib/figure.py
@@ -3430,8 +3430,12 @@ def tight_layout(self, *, pad=1.08, h_pad=None, w_pad=None, rect=None):
         engine = TightLayoutEngine(pad=pad, h_pad=h_pad, w_pad=w_pad,
                                    rect=rect)
         try:
+            previous_engine = self.get_layout_engine()
             self.set_layout_engine(engine)
             engine.execute(self)
+            if not isinstance(previous_engine, TightLayoutEngine) \
+                    and previous_engine is not None:
+                _api.warn_external('The figure layout has changed to tight')
         finally:
             self.set_layout_engine(None)
 
diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
index e38a772fe81e..9c78f151c99c 100644
--- a/lib/matplotlib/tests/test_figure.py
+++ b/lib/matplotlib/tests/test_figure.py
@@ -598,6 +598,17 @@ def test_invalid_layouts():
         fig.set_layout_engine("constrained")
 
 
+@pytest.mark.parametrize('layout', ['constrained', 'compressed'])
+def test_layout_change_warning(layout):
+    """
+    Raise a warning when a previously assigned layout changes to tight using
+    plt.tight_layout().
+    """
+    fig, ax = plt.subplots(layout=layout)
+    with pytest.warns(UserWarning, match='The figure layout has changed to'):
+        plt.tight_layout()
+
+
 @check_figures_equal(extensions=["png", "pdf"])
 def test_add_artist(fig_test, fig_ref):
     fig_test.dpi = 100