checker: check mismatch of return result type (#15413)

vlib/v/checker/return.v

@@ -104,6 +104,12 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 			c.error('cannot use `${c.table.type_to_str(got_typ)}` as type `${c.table.type_to_str(exp_type)}` in return argument',
 				pos)
 		}
+		if got_typ.has_flag(.result) && (!exp_type.has_flag(.result)
+			|| c.table.type_to_str(got_typ) != c.table.type_to_str(exp_type)) {
+			pos := node.exprs[expr_idxs[i]].pos()
+			c.error('cannot use `${c.table.type_to_str(got_typ)}` as type `${c.table.type_to_str(exp_type)}` in return argument',
+				pos)
+		}
 		if node.exprs[expr_idxs[i]] !is ast.ComptimeCall && !c.check_types(got_typ, exp_type) {
 			got_typ_sym := c.table.sym(got_typ)
 			mut exp_typ_sym := c.table.sym(exp_type)

vlib/v/checker/tests/return_result_type_mismatch.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/return_result_type_mismatch.vv:8:9: error: cannot use `!int` as type `?int` in return argument
+    6 | // Option
+    7 | fn bar(a int) ?int {
+    8 |     return foo(a)
+      |            ~~~~~~
+    9 | }
+   10 |

vlib/v/checker/tests/return_result_type_mismatch.vv

@@ -0,0 +1,14 @@
+// Result
+fn foo(a int) !int {
+	return a + 1
+}
+
+// Option
+fn bar(a int) ?int {
+	return foo(a)
+}
+
+fn main() {
+	x := bar(1)?
+	println('$x')
+}