checker: fix index expr that left is if expr (fix #22654) (#22661)

vlib/v/checker/checker.v

@@ -1006,6 +1006,13 @@ fn (mut c Checker) fail_if_immutable(mut expr ast.Expr) (string, token.Pos) {
 		ast.InfixExpr {
 			return '', expr.pos
 		}
+		ast.IfExpr {
+			for mut branch in expr.branches {
+				mut last_expr := (branch.stmts.last() as ast.ExprStmt).expr
+				c.fail_if_immutable(mut last_expr)
+			}
+			return '', expr.pos
+		}
 		else {
 			if !expr.is_pure_literal() {
 				c.error('unexpected expression `${expr.type_name()}`', expr.pos())

vlib/v/checker/tests/immutable_array_in_if_expr_index.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/immutable_array_in_if_expr_index.vv:4:13: error: `array_a` is immutable, declare it with `mut` to make it mutable
+    2 |     array_a := [1, 2, 3]
+    3 |     array_b := [4, 5, 6]
+    4 |     (if true { array_a } else { array_b })[0] = 999
+      |                ~~~~~~~
+    5 |     println(array_a)
+    6 | }
+vlib/v/checker/tests/immutable_array_in_if_expr_index.vv:4:30: error: `array_b` is immutable, declare it with `mut` to make it mutable
+    2 |     array_a := [1, 2, 3]
+    3 |     array_b := [4, 5, 6]
+    4 |     (if true { array_a } else { array_b })[0] = 999
+      |                                 ~~~~~~~
+    5 |     println(array_a)
+    6 | }

vlib/v/checker/tests/immutable_array_in_if_expr_index.vv

@@ -0,0 +1,6 @@
+fn main() {
+	array_a := [1, 2, 3]
+	array_b := [4, 5, 6]
+	(if true { array_a } else { array_b })[0] = 999
+	println(array_a)
+}

vlib/v/gen/c/if.v

@@ -6,8 +6,8 @@ module c
 import v.ast
 
 fn (mut g Gen) need_tmp_var_in_if(node ast.IfExpr) bool {
-	if node.is_expr && g.inside_ternary == 0 {
-		if g.is_autofree || node.typ.has_option_or_result() || node.is_comptime {
+	if node.is_expr && (g.inside_ternary == 0 || g.is_assign_lhs) {
+		if g.is_autofree || node.typ.has_option_or_result() || node.is_comptime || g.is_assign_lhs {
 			return true
 		}
 		for branch in node.branches {

vlib/v/tests/indexexpr_with_if_expr_test.v

@@ -0,0 +1,12 @@
+fn test_indexexpr_with_if_expr() {
+	mut array_a := [1, 2, 3]
+	mut array_b := [4, 5, 6]
+
+	(if true {
+		array_a
+	} else {
+		array_b
+	})[0] = 999
+	println(array_a)
+	assert array_a == [999, 2, 3]
+}