checker: fix checking mismatch of the fn array decompose argument (#15934)

vlib/v/checker/fn.v

@@ -901,24 +901,27 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 		}
 		if func.is_variadic && i >= func.params.len - 1 {
 			param_sym := c.table.sym(param.typ)
+			mut expected_type := param.typ
 			if param_sym.kind == .array {
 				info := param_sym.array_info()
-				c.expected_type = info.elem_type
+				expected_type = info.elem_type
+				c.expected_type = expected_type
 			}
 			typ := c.expr(call_arg.expr)
 			if i == node.args.len - 1 {
 				if c.table.sym(typ).kind == .array && call_arg.expr !is ast.ArrayDecompose
-					&& !param.typ.has_flag(.generic) && c.expected_type != typ {
+					&& !param.typ.has_flag(.generic) && expected_type != typ {
 					styp := c.table.type_to_str(typ)
-					elem_styp := c.table.type_to_str(c.expected_type)
+					elem_styp := c.table.type_to_str(expected_type)
 					c.error('to pass `$call_arg.expr` ($styp) to `$func.name` (which accepts type `...$elem_styp`), use `...$call_arg.expr`',
 						node.pos)
 				} else if call_arg.expr is ast.ArrayDecompose
-					&& c.table.sym(c.expected_type).kind == .sum_type && c.expected_type != typ {
-					expected_type := c.table.type_to_str(c.expected_type)
-					got_type := c.table.type_to_str(typ)
-					c.error('cannot use `...$got_type` as `...$expected_type` in argument ${i + 1} to `$fn_name`',
-						call_arg.pos)
+					&& c.table.sym(expected_type).kind == .sum_type
+					&& expected_type.idx() != typ.idx() {
+					expected_type_str := c.table.type_to_str(expected_type)
+					got_type_str := c.table.type_to_str(typ)
+					c.error('cannot use `...$got_type_str` as `...$expected_type_str` in argument ${
+						i + 1} to `$fn_name`', call_arg.pos)
 				}
 			}
 		} else {

vlib/v/checker/tests/fn_array_decompose_arg_mismatch_err_a.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/fn_array_decompose_arg_mismatch_err.vv:12:7: error: cannot use `...string` as `...Any` in argument 1 to `test`
+vlib/v/checker/tests/fn_array_decompose_arg_mismatch_err_a.vv:12:7: error: cannot use `...string` as `...Any` in argument 1 to `test`
    10 |     mut args := []string{cap: 3}
    11 |     args << ['test', 'test1', 'test2']
    12 |     test(...args)

vlib/v/checker/tests/fn_array_decompose_arg_mismatch_err_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/fn_array_decompose_arg_mismatch_err_b.vv:14:4: error: cannot use `...Foo1` as `...Foo` in argument 1 to `f`
+   12 |
+   13 | fn main() {
+   14 |     f(...[Foo1{}, Foo1{}])
+      |       ~~~~~~~~~~~~~~~~~~~
+   15 | }

vlib/v/checker/tests/fn_array_decompose_arg_mismatch_err_b.vv

@@ -0,0 +1,15 @@
+struct Foo1 {
+}
+
+struct Foo2 {
+}
+
+type Foo = Foo1 | Foo2
+
+fn f(v ...Foo) {
+	println(v)
+}
+
+fn main() {
+	f(...[Foo1{}, Foo1{}])
+}