Fix code generation when the if branch has a value and there is no …

…`else` branch

Fixes #4942

internal/compiler/generator/rust.rs

@@ -2149,11 +2149,12 @@ fn compile_expression(expr: &Expression, ctx: &EvaluationContext) -> TokenStream
             let condition_code = compile_expression(condition, ctx);
             let true_code = compile_expression(true_expr, ctx);
             let false_code = compile_expression(false_expr, ctx);
+            let semi = if false_expr.ty(ctx) == Type::Void { quote!(;) } else { quote!(as _) };
             quote!(
                 if #condition_code {
-                    #true_code
+                    (#true_code) #semi
                 } else {
-                    (#false_code) as _
+                    #false_code
                 }
             )
         }

internal/compiler/llr/expression.rs

@@ -288,7 +288,7 @@ impl Expression {
             }
             Self::UnaryOp { sub, .. } => sub.ty(ctx),
             Self::ImageReference { .. } => Type::Image,
-            Self::Condition { true_expr, .. } => true_expr.ty(ctx),
+            Self::Condition { false_expr, .. } => false_expr.ty(ctx),
             Self::Array { element_ty, .. } => Type::Array(element_ty.clone().into()),
             Self::Struct { ty, .. } => ty.clone(),
             Self::EasingCurve(_) => Type::Easing,

tests/cases/issues/issue_4942_no_else_value.slint

@@ -0,0 +1,45 @@
+// 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 TestCase {
+
+    public pure function issue4942(cond : bool) -> int {
+        if (cond) {
+            45
+        }
+        12
+    }
+
+    out property <bool> test: issue4942(true) == 12 && issue4942(false) == 12;
+}
+
+
+/*
+
+FIXME: The C++ test currently fails with this warning
+(see also https://github.com/slint-ui/slint/issues/4954)
+//ignore: cpp
+
+(This is the warning, for reference)
+    statement has no effect [-Werror=unused-value]
+      |     return [&]{ [&]() -> void { if (arg_0) { 45; } else { ; }}();return 12; }();
+      |                                              ^~
+
+```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);
+```
+
+*/