[bug] MatrixType bug fix: Fix indexing support for custom vector types (

taichi-dev#6568)

Issue: taichi-dev#5819

### Brief Summary

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/taichi/lang/matrix.py

@@ -1771,6 +1771,11 @@ def __call__(self, *args):
                 entries += x
             elif isinstance(x, np.ndarray):
                 entries += list(x.ravel())
+            elif isinstance(x, impl.Expr) and x.ptr.is_tensor():
+                entries += [
+                    impl.Expr(e) for e in impl.get_runtime().prog.
+                    current_ast_builder().expand_expr([x.ptr])
+                ]
             elif isinstance(x, Matrix):
                 entries += x.entries
             else:
@@ -1797,6 +1802,10 @@ def cast(self, mat):
         if isinstance(mat, impl.Expr) and mat.ptr.is_tensor():
             return ops_mod.cast(mat, self.dtype)
 
+        if isinstance(mat, Matrix) and impl.current_cfg().real_matrix:
+            arr = mat.entries
+            return ops_mod.cast(make_matrix(arr), self.dtype)
+
         return mat.cast(self.dtype)
 
     def filled_with_scalar(self, value):
@@ -1863,6 +1872,11 @@ def __call__(self, *args):
                 entries += list(x.ravel())
             elif isinstance(x, Matrix):
                 entries += x.entries
+            elif isinstance(x, impl.Expr) and x.ptr.is_tensor():
+                entries += [
+                    impl.Expr(e) for e in impl.get_runtime().prog.
+                    current_ast_builder().expand_expr([x.ptr])
+                ]
             else:
                 entries.append(x)
 
@@ -1884,6 +1898,10 @@ def cast(self, vec):
         if isinstance(vec, impl.Expr) and vec.ptr.is_tensor():
             return ops_mod.cast(vec, self.dtype)
 
+        if isinstance(vec, Matrix) and impl.current_cfg().real_matrix:
+            arr = vec.entries
+            return ops_mod.cast(make_matrix(arr), self.dtype)
+
         return vec.cast(self.dtype)
 
     def filled_with_scalar(self, value):

tests/python/test_vector_swizzle.py

@@ -26,8 +26,7 @@ def test_vector_swizzle_python():
     assert all(z == w.xxxx)
 
 
-@test_utils.test(debug=True)
-def test_vector_swizzle_taichi():
+def _test_vector_swizzle_taichi():
     @ti.kernel
     def foo():
         v = ti.math.vec3(0)
@@ -50,6 +49,16 @@ def foo():
     foo()
 
 
+@test_utils.test(debug=True)
+def test_vector_swizzle_taichi():
+    _test_vector_swizzle_taichi()
+
+
+@test_utils.test(real_matrix=True, real_matrix_scalarize=True, debug=True)
+def test_vector_swizzle_taichi_matrix_scalarize():
+    _test_vector_swizzle_taichi()
+
+
 @test_utils.test(debug=True)
 def test_vector_swizzle2_taichi():
     @ti.kernel