checker: check error for sumtype in array (#19183)

vlib/v/checker/infix.v

@@ -190,6 +190,12 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 							for i, typ in node.right.expr_types {
 								c.ensure_type_exists(typ, node.right.exprs[i].pos())
 							}
+						} else {
+							elem_type := right_final_sym.array_info().elem_type
+							c.check_expected(left_type, elem_type) or {
+								c.error('left operand to `${node.op}` does not match the array element type: ${err.msg()}',
+									left_right_pos)
+							}
 						}
 					}
 				}

vlib/v/checker/tests/infix_sumtype_in_array_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/infix_sumtype_in_array_err.vv:15:7: error: left operand to `in` does not match the array element type: expected `rune`, not `RuneAliasOrOtherType`
+   13 |     match x {
+   14 |         RuneAlias {
+   15 |             if x in whitespace {
+      |                ~~~~~~~~~~~~~~~
+   16 |                 // doing `if x as RuneAlias in whitepsace` here
+   17 |                 // works but it should be doing that automatically

vlib/v/checker/tests/infix_sumtype_in_array_err.vv

@@ -0,0 +1,25 @@
+struct OtherType {
+	foo string
+	bar string
+}
+
+type RuneAlias = rune
+type RuneAliasOrOtherType = OtherType | RuneAlias
+
+const whitespace = [rune(0x0009), 0x000a, 0x000c, 0x000d, 0x0020]
+
+fn main() {
+	mut x := RuneAliasOrOtherType(RuneAlias(rune(`!`)))
+	match x {
+		RuneAlias {
+			if x in whitespace {
+				// doing `if x as RuneAlias in whitepsace` here
+				// works but it should be doing that automatically
+				// since I'm inside the RuneAlias match condition.
+			}
+		}
+		else {
+			// do something else
+		}
+	}
+}