Compiler: Don't panic when a Row is repeated in a GridLayout

Fixes #3729

internal/compiler/passes/lower_layout.rs

@@ -98,7 +98,20 @@ fn lower_element_layout(
         return;
     };
     match base_type.name.as_str() {
-        "Row" => panic!("Error caught at element lookup time"),
+        "Row" => {
+            // We shouldn't lower layout if we have a Row in there. Unless the Row is the root of a repeated item,
+            // in which case another error has been reported
+            assert!(
+                diag.has_error()
+                    && Rc::ptr_eq(&component.root_element, elem)
+                    && component
+                        .parent_element
+                        .upgrade()
+                        .map_or(false, |e| e.borrow().repeated.is_some()),
+                "Error should have been caught at element lookup time"
+            );
+            return;
+        }
         "GridLayout" => lower_grid_layout(component, elem, diag),
         "HorizontalLayout" => lower_box_layout(elem, diag, Orientation::Horizontal),
         "VerticalLayout" => lower_box_layout(elem, diag, Orientation::Vertical),

internal/compiler/tests/syntax/layout/if_in_grid_row.slint

@@ -0,0 +1,14 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.1 OR LicenseRef-Slint-commercial
+
+export component Test  {
+    property <bool> condition;
+
+    GridLayout {
+        // Issue #3729
+        if (condition) : r := Row {
+//                            ^error{'if' or 'for' expressions are not currently supported in grid layouts}
+            Rectangle {}
+        }
+    }
+}