cgen: fix if expr with fn call result (#15702)

vlib/v/gen/c/if.v

@@ -50,6 +50,11 @@ fn (mut g Gen) need_tmp_var_in_expr(expr ast.Expr) bool {
 			if expr.or_block.kind != .absent {
 				return true
 			}
+			for arg in expr.args {
+				if g.need_tmp_var_in_expr(arg.expr) {
+					return true
+				}
+			}
 		}
 		ast.CastExpr {
 			return g.need_tmp_var_in_expr(expr.expr)

vlib/v/tests/if_expr_with_fn_call_result_test.v

@@ -0,0 +1,21 @@
+fn foo() !int {
+	return 0
+}
+
+fn bar(n int) bool {
+	return true
+}
+
+fn is_ok(b bool) !bool {
+	return if b {
+		bar(foo()!)
+	} else {
+		true
+	}
+}
+
+fn test_if_expr_with_fn_call_result() {
+	ret := is_ok(true) or { false }
+	println(ret)
+	assert ret
+}