checker: require .sort() body for some types (#10550)

vlib/v/checker/checker.v

@@ -2196,6 +2196,10 @@ fn (mut c Checker) array_builtin_method_call(mut call_expr ast.CallExpr, left_ty
 					'`.sort()` requires a `<` or `>` comparison as the first and only argument' +
 					'\ne.g. `users.sort(a.id < b.id)`', call_expr.pos)
 			}
+		} else if !(c.table.get_type_symbol(elem_typ).has_method('<')
+			|| c.table.unalias_num_type(elem_typ) in [ast.int_type, ast.int_type.to_ptr(), ast.string_type, ast.string_type.to_ptr(), ast.i8_type, ast.i16_type, ast.i64_type, ast.byte_type, ast.rune_type, ast.u16_type, ast.u32_type, ast.u64_type, ast.f32_type, ast.f64_type, ast.char_type, ast.bool_type, ast.float_literal_type, ast.int_literal_type, ast.size_t_type_idx]) {
+			c.error('custom sorting condition must be supplied for type `${c.table.type_to_str(elem_typ)}`',
+				call_expr.pos)
 		}
 	} else if method_name == 'wait' {
 		elem_sym := c.table.get_type_symbol(elem_typ)

vlib/v/checker/tests/array_sort_struct_no_body_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/array_sort_struct_no_body_err.vv:6:5: error: custom sorting condition must be supplied for type `Foo`
+    4 | 
+    5 | mut arr := []Foo{}
+    6 | arr.sort()
+      |     ~~~~~~

vlib/v/checker/tests/array_sort_struct_no_body_err.vv

@@ -0,0 +1,6 @@
+struct Foo {
+	bar int
+}
+
+mut arr := []Foo{}
+arr.sort()

vlib/v/tests/array_sort_lt_overload_test.v

@@ -0,0 +1,22 @@
+struct Num {
+	value int
+}
+
+fn (a Num) < (b Num) bool {
+	return a.value < b.value
+}
+
+fn test_sort_lt_overloaded_struct_array() {
+	mut arr := []Num{}
+	arr << {
+		value: 10
+	}
+	arr << {
+		value: 5
+	}
+	arr << {
+		value: 7
+	}
+	arr.sort()
+	assert arr[0].value == 5
+}