Compiler: fix conversion of array of array of structs

When finding the common types for array of array, we must recurse in the
inner type

Fixes: #3574

CHANGELOG.md

@@ -14,6 +14,7 @@ All notable changes to this project are documented in this file.
 
  - Added `Number`, `Decimal` variant to enum `InputType`
  - Added `spacing-horizontal` and `spacing-vertical` in `GridLayout`
+ - Fixed conversion in array of array of structs (#3574)
 
 ### Rust API
 

internal/compiler/passes/resolving.rs

@@ -1216,7 +1216,7 @@ impl Expression {
                                 ) => {
                                     *existing_field.get_mut() =
                                         Self::common_target_type_for_type_list(
-                                            [existing_field.get().clone(), elem_ty].iter().cloned(),
+                                            [existing_field.get().clone(), elem_ty].into_iter(),
                                         );
                                 }
                             }
@@ -1228,6 +1228,9 @@ impl Expression {
                             rust_attributes: rust_attributes.or(deriven),
                         }
                     }
+                    (Type::Array(lhs), Type::Array(rhs)) => Type::Array(
+                        Self::common_target_type_for_type_list([*lhs, *rhs].into_iter()).into(),
+                    ),
                     (Type::Color, Type::Brush) | (Type::Brush, Type::Color) => Type::Brush,
                     (target_type, expr_ty) => {
                         if expr_ty.can_convert(&target_type) {

internal/compiler/tests/syntax/lookup/conversion.slint

@@ -48,5 +48,6 @@ export X := Rectangle {
 
     property <[{a: [int]}]> ccc: [{a: []}, {}, {a: ["123"]}, {a: [123]}];
 //                               ^error{Cannot convert string to int}  (FIXME: error location)
+//                               ^^error{Cannot convert string to int}  (FIXME: duplicated)
 
 }

tests/cases/types/array_of_array.slint

@@ -0,0 +1,50 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.1 OR LicenseRef-Slint-commercial
+
+struct KeySlot { width-u: float, spacer: bool, background: color, font-size: length, }
+struct Row { key-slots: [KeySlot], }
+struct Layer { default-key-width-u: float, total-width-u: float, rows: [Row] }
+
+
+
+export component TestCase  {
+    // issue #3574
+    out property<[Layer]> layers: [ {
+           default-key-width-u: 1.0,
+           total-width-u: 10.0,
+           rows: [
+               { key-slots: [ { width-u: 0.5}, { spacer: true, width-u: 0.5}, ] },
+               { key-slots: [ { width-u: 2.5, background: #bbbbbb }, ] }
+           ],
+       } ];
+
+    property <[[{a: string, b: string, c: string}]]> array : [
+        [{a: "hello"}],
+        [],
+        [{a: "world"}, {a: "extra", b: "ok"} ],
+    ];
+    out property <bool> test: array[2][1].b == "ok" && layers[0].rows[0].key-slots[1].spacer;
+}
+
+
+
+/*
+
+```cpp
+auto handle = TestCase::create();
+const TestCase &instance = *handle;
+assert(instance.get_test());
+```
+
+
+```rust
+let instance = TestCase::new().unwrap();
+assert!(instance.get_test());
+```
+
+```js
+var instance = new slint.TestCase({});
+assert(instance.test);
+```
+
+*/