checker: check if guard returning non-propagate option or result (fix #17742) (#17794)

vlib/v/checker/if.v

@@ -71,6 +71,11 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 			}
 		}
 		if mut branch.cond is ast.IfGuardExpr {
+			if branch.cond.expr_type.clear_flag(.option).clear_flag(.result) == ast.void_type
+				&& !(branch.cond.vars.len == 1 && branch.cond.vars[0].name == '_') {
+				c.error('if guard expects non-propagate option or result', branch.pos)
+				continue
+			}
 			sym := c.table.sym(branch.cond.expr_type)
 			if sym.kind == .multi_return {
 				mr_info := sym.info as ast.MultiReturn

vlib/v/checker/tests/if_guard_expr_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/if_guard_expr_err.vv:6:7: error: if guard expects non-propagate option or result
+    4 |
+    5 | fn main() {
+    6 |     a := if r := foo() {
+      |          ~~~~~~~~~~~~~
+    7 |         println(r)
+    8 |         true

vlib/v/checker/tests/if_guard_expr_err.vv

@@ -0,0 +1,14 @@
+fn foo() ! {
+	return error("error")
+}
+
+fn main() {
+	a := if r := foo() {
+		println(r)
+		true
+	} else {
+		false
+	}
+	
+	println(a)
+}