checker: disallow option or result return type, for infix operato…

…r overloading (#20494)

vlib/v/checker/fn.v

@@ -346,6 +346,8 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 						sptype := c.table.type_to_str(param_type)
 						c.error('the receiver type `${srtype}` should be the same type as the operand `${sptype}`',
 							node.pos)
+					} else if node.return_type.has_option_or_result() {
+						c.error('return type cannot be Option or Result', node.return_type_pos)
 					}
 				}
 			}

vlib/v/checker/tests/operator_overloading_return_type_option_or_result.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/operator_overloading_return_type_option_or_result.vv:3:22: error: return type cannot be Option or Result
+    1 | type Vec = []int
+    2 | 
+    3 | fn (v Vec) + (u Vec) !Vec {
+      |                      ~~~~
+    4 |     if v.len != u.len {
+    5 |         return error("Operations require dim(v) == dim(u)")
+vlib/v/checker/tests/operator_overloading_return_type_option_or_result.vv:10:22: error: return type cannot be Option or Result
+    8 | }
+    9 | 
+   10 | fn (v Vec) - (u Vec) ?Vec {
+      |                      ~~~~
+   11 |     if v.len != u.len {
+   12 |         return none

vlib/v/checker/tests/operator_overloading_return_type_option_or_result.vv

@@ -0,0 +1,22 @@
+type Vec = []int
+
+fn (v Vec) + (u Vec) !Vec {
+	if v.len != u.len {
+		return error("Operations require dim(v) == dim(u)")
+	}
+	return Vec([v[0] + u[0], v[1] + u[1], v[2] + u[2]])
+}
+
+fn (v Vec) - (u Vec) ?Vec {
+	if v.len != u.len {
+		return none
+	}
+	return Vec([v[0] + u[0], v[1] + u[1], v[2] + u[2]])
+}
+
+
+
+fn main() {
+	vec := Vec([1, 2, 3])
+	dump(vec + vec)
+}