checker: fix generic option array arg passing to []T (fix #24423) (#24457)

vlib/v/checker/fn.v

@@ -1852,6 +1852,12 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 							c.handle_generic_lambda_arg(node, mut call_arg.expr)
 							continue
 						}
+						// passing []?T to []T
+						if !unwrap_typ.has_flag(.variadic) && unwrap_sym.kind == .array
+							&& c.table.final_sym(utyp).kind == .array
+							&& c.check_basic(c.table.value_type(utyp).clear_flag(.option), c.table.value_type(unwrap_typ)) {
+							continue
+						}
 						c.error('${err.msg()} in argument ${i + 1} to `${fn_name}`', call_arg.pos)
 					}
 				}
@@ -2607,6 +2613,12 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 					continue
 				}
 			}
+			// passing []?T to []T
+			if !exp_arg_typ.has_flag(.variadic) && param_typ_sym.kind == .array
+				&& c.table.final_sym(got_arg_typ).kind == .array
+				&& c.check_basic(c.table.value_type(got_arg_typ).clear_flag(.option), c.table.value_type(exp_arg_typ)) {
+				continue
+			}
 			c.error('${err.msg()} in argument ${i + 1} to `${left_sym.name}.${method_name}`',
 				arg.pos)
 		}

vlib/v/tests/generics/generic_array_pass_test.v

@@ -0,0 +1,31 @@
+struct Decoder {}
+
+pub fn decode[T](val string) !T {
+	mut decoder := Decoder{}
+	mut result := T{}
+	decoder.decode_value(mut result)!
+	return result
+}
+
+fn (mut decoder Decoder) decode_value[T](mut val T) ! {
+	$if T.unaliased_typ is $array {
+		// checking wrongly. `decode_array` think that `[]?int` is `[]int`
+		decoder.decode_array(mut val)!
+		return
+	} $else $if T.unaliased_typ is $struct {
+		$for field in T.fields {
+			decoder.decode_value(mut val.$(field.name))!
+		}
+	}
+}
+
+fn (mut decoder Decoder) decode_array[T](mut val []T) ! {}
+
+struct Foo {
+	int  []int
+	oint []?int
+}
+
+fn test_main() {
+	decode[Foo]('')!
+}