checker: add error for unwrapped result in infix expr (#16175)

vlib/v/checker/infix.v

@@ -613,12 +613,18 @@ pub fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 	// TODO move this to symmetric_check? Right now it would break `return 0` for `fn()?int `
 	left_is_optional := left_type.has_flag(.optional)
 	right_is_optional := right_type.has_flag(.optional)
-	if left_is_optional && right_is_optional {
-		c.error('unwrapped optionals cannot be used in an infix expression', left_right_pos)
-	} else if left_is_optional || right_is_optional {
+	if left_is_optional || right_is_optional {
 		opt_infix_pos := if left_is_optional { left_pos } else { right_pos }
 		c.error('unwrapped optional cannot be used in an infix expression', opt_infix_pos)
 	}
+
+	left_is_result := left_type.has_flag(.result)
+	right_is_result := right_type.has_flag(.result)
+	if left_is_result || right_is_result {
+		opt_infix_pos := if left_is_result { left_pos } else { right_pos }
+		c.error('unwrapped result cannot be used in an infix expression', opt_infix_pos)
+	}
+
 	// Dual sides check (compatibility check)
 	if node.left !is ast.ComptimeCall && node.right !is ast.ComptimeCall {
 		if !(c.symmetric_check(left_type, right_type) && c.symmetric_check(right_type, left_type))

vlib/v/checker/tests/unwrapped_result_infix_err.out

@@ -0,0 +1,8 @@
+vlib/v/checker/tests/unwrapped_result_infix_err.vv:7:9: error: unwrapped result cannot be used in an infix expression
+    5 | fn g() ! {
+    6 |     assert f('1')! == true
+    7 |     assert f('1') == true
+      |            ~~~~~~
+    8 | }
+    9 |
+

vlib/v/checker/tests/unwrapped_result_infix_err.vv

@@ -0,0 +1,12 @@
+fn f(k string) !bool {
+	return true
+}
+
+fn g() ! {
+	assert f('1')! == true
+	assert f('1') == true
+}
+
+fn main() {
+	g()!
+}