cgen, checker: fix indexexpr with sumtype of array types (#18515)

vlib/v/checker/checker.v

@@ -4035,8 +4035,8 @@ fn (mut c Checker) type_error_for_operator(op_label string, types_label string,
 }
 
 fn (mut c Checker) check_index(typ_sym &ast.TypeSymbol, index ast.Expr, index_type ast.Type, pos token.Pos, range_index bool, is_gated bool) {
-	index_type_sym := c.table.sym(index_type)
 	if typ_sym.kind in [.array, .array_fixed, .string] {
+		index_type_sym := c.table.sym(index_type)
 		if !(index_type.is_int() || index_type_sym.kind == .enum_
 			|| (index_type_sym.kind == .alias
 			&& (index_type_sym.info as ast.Alias).parent_type.is_int())
@@ -4102,10 +4102,17 @@ fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 		}
 		else {}
 	}
+	is_aggregate_arr := typ_sym.kind == .aggregate
+		&& (typ_sym.info as ast.Aggregate).types.filter(c.table.type_kind(it) !in [.array, .array_fixed, .string, .map]).len == 0
 	if typ_sym.kind !in [.array, .array_fixed, .string, .map] && !typ.is_ptr()
-		&& typ !in [ast.byteptr_type, ast.charptr_type] && !typ.has_flag(.variadic) {
+		&& typ !in [ast.byteptr_type, ast.charptr_type] && !typ.has_flag(.variadic)
+		&& !is_aggregate_arr {
 		c.error('type `${typ_sym.name}` does not support indexing', node.pos)
 	}
+	if is_aggregate_arr {
+		// treating indexexpr of sumtype of array types
+		typ = (typ_sym.info as ast.Aggregate).types[0]
+	}
 	if typ.has_flag(.option) {
 		if node.left is ast.Ident && (node.left as ast.Ident).or_expr.kind == .absent {
 			c.error('type `?${typ_sym.name}` is an Option, it must be unwrapped first; use `var?[]` to do it',

vlib/v/gen/c/index.v

@@ -48,6 +48,12 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 					g.write(')')
 				}
 			}
+		} else {
+			if sym.kind == .aggregate
+				&& (sym.info as ast.Aggregate).types.filter(g.table.type_kind(it) !in [.array, .array_fixed, .string, .map]).len == 0 {
+				// treating sumtype of array types
+				unwrapped_got_type := (sym.info as ast.Aggregate).types[g.aggregate_type_idx]
+				g.index_expr(ast.IndexExpr{ ...node, left_type: unwrapped_got_type })
 			} else {
 				g.expr(node.left)
 				g.write('[')
@@ -56,6 +62,7 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 			}
 		}
 	}
+}
 
 fn (mut g Gen) index_range_expr(node ast.IndexExpr, range ast.RangeExpr) {
 	sym := g.table.final_sym(node.left_type)

vlib/v/tests/match_sumtype_arr_test.v

@@ -0,0 +1,43 @@
+type MySumType = []MyStructA | []MyStructB
+type MySumTypePtr = []&MyStructA | []&MyStructB
+
+struct ParentStruct {
+	parent_field string
+}
+
+struct MyStructA {
+	ParentStruct
+}
+
+struct MyStructB {
+	ParentStruct
+}
+
+fn check(mut t MySumType) {
+	match mut t {
+		[]MyStructA, []MyStructB {
+			println(t[0].parent_field)
+		}
+	}
+}
+
+fn check2(mut t MySumTypePtr) {
+	match mut t {
+		[]&MyStructA, []&MyStructB {
+			println(t[0].parent_field)
+		}
+	}
+}
+
+fn test_main() {
+	s := MyStructA{
+		parent_field: 'common'
+	}
+
+	mut t := MySumType([s])
+	mut t2 := MySumTypePtr([&s])
+
+	check(mut t)
+	check2(mut t2)
+	assert true
+}