checker: disallow infix op on an optional type (#6406)

vlib/v/checker/check_types.v

@@ -182,6 +182,9 @@ pub fn (c &Checker) promote(left_type, right_type table.Type) table.Type {
 	}
 	if right_type.is_number() && left_type.is_number() {
 		return c.promote_num(left_type, right_type)
+	} else if left_type.has_flag(.optional) != right_type.has_flag(.optional) {
+		// incompatible
+		return table.void_type
 	} else {
 		return left_type // default to left if not automatic promotion possible
 	}

vlib/v/checker/checker.v

@@ -629,6 +629,9 @@ pub fn (mut c Checker) infix_expr(mut infix_expr ast.InfixExpr) table.Type {
 					left_name := c.table.type_to_str(left_type)
 					right_name := c.table.type_to_str(right_type)
 					c.error('mismatched types `$left_name` and `$right_name`', infix_expr.pos)
+				} else if promoted_type.has_flag(.optional) {
+					s := c.table.type_to_str(promoted_type)
+					c.error('`$infix_expr.op` cannot be used with `$s`', infix_expr.pos)
 				} else if promoted_type.is_float() {
 					if infix_expr.op in [.mod, .xor, .amp, .pipe] {
 						side := if left_type == promoted_type { 'left' } else { 'right' }

vlib/v/checker/tests/infix_err.out

@@ -0,0 +1,39 @@
+vlib/v/checker/tests/infix_err.vv:7:8: error: mismatched types `string` and `?string`
+    5 |     return none
+    6 | }
+    7 | _ = '' + f()
+      |        ^
+    8 | _ = f() + ''
+    9 | _ = f() + f()
+vlib/v/checker/tests/infix_err.vv:8:9: error: mismatched types `?string` and `string`
+    6 | }
+    7 | _ = '' + f()
+    8 | _ = f() + ''
+      |         ^
+    9 | _ = f() + f()
+   10 |
+vlib/v/checker/tests/infix_err.vv:9:9: error: `+` cannot be used with `?string`
+    7 | _ = '' + f()
+    8 | _ = f() + ''
+    9 | _ = f() + f()
+      |         ^
+   10 |
+   11 | _ = 4 + g()
+vlib/v/checker/tests/infix_err.vv:11:7: error: `+` cannot be used with `?int`
+    9 | _ = f() + f()
+   10 |
+   11 | _ = 4 + g()
+      |       ^
+   12 | _ = int(0) + g() // FIXME not detected
+   13 | _ = g() + int(3)
+vlib/v/checker/tests/infix_err.vv:13:9: error: `+` cannot be used with `?int`
+   11 | _ = 4 + g()
+   12 | _ = int(0) + g() // FIXME not detected
+   13 | _ = g() + int(3)
+      |         ^
+   14 | _ = g() + 3
+vlib/v/checker/tests/infix_err.vv:14:9: error: `+` cannot be used with `?int`
+   12 | _ = int(0) + g() // FIXME not detected
+   13 | _ = g() + int(3)
+   14 | _ = g() + 3
+      |         ^

vlib/v/checker/tests/infix_err.vv

@@ -0,0 +1,14 @@
+fn f() ?string {
+	return none
+}
+fn g() ?int {
+	return none
+}
+_ = '' + f()
+_ = f() + ''
+_ = f() + f()
+
+_ = 4 + g()
+_ = int(0) + g() // FIXME not detected
+_ = g() + int(3)
+_ = g() + 3