Backport PR matplotlib#29209: FIX: pcolormesh with no x y args and ne…

…arest interp

lib/matplotlib/axes/_axes.py

@@ -5928,10 +5928,7 @@ def _pcolorargs(self, funcname, *args, shading='auto', **kwargs):
             else:
                 X, Y = np.meshgrid(np.arange(ncols + 1), np.arange(nrows + 1))
                 shading = 'flat'
-            C = cbook.safe_masked_invalid(C, copy=True)
-            return X, Y, C, shading
-
-        if len(args) == 3:
+        elif len(args) == 3:
             # Check x and y for bad data...
             C = np.asanyarray(args[2])
             # unit conversion allows e.g. datetime objects as axis values

lib/matplotlib/tests/test_axes.py

@@ -1499,6 +1499,20 @@ def test_pcolormesh_rgba(fig_test, fig_ref, dims, alpha):
     ax.pcolormesh(c[..., 0], cmap="gray", vmin=0, vmax=1, alpha=alpha)
 
 
+@check_figures_equal(extensions=["png"])
+def test_pcolormesh_nearest_noargs(fig_test, fig_ref):
+    x = np.arange(4)
+    y = np.arange(7)
+    X, Y = np.meshgrid(x, y)
+    C = X + Y
+
+    ax = fig_test.subplots()
+    ax.pcolormesh(C, shading="nearest")
+
+    ax = fig_ref.subplots()
+    ax.pcolormesh(x, y, C, shading="nearest")
+
+
 @image_comparison(['pcolormesh_datetime_axis.png'], style='mpl20')
 def test_pcolormesh_datetime_axis():
     # Remove this line when this test image is regenerated.