cgen: allow sort with`<` and `>` op overloading (#8042)

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Array decompose: `[1, 2, 3]...` is now `...[1, 2, 3]`
 - Treating `enum` as `int` and operations on `enum` except `==` and `!=` are removed for strict type checking.
 - Support `[manualfree] fn f1(){}` and `[manualfree] module m1`, for functions doing their own memory management.
+- Allow usage of `<` and `>` operators for struct in `.sort` method for arrays, i.e. `arr.sort(a < b)`.
 
 ## V 0.2.1
 *30 Dec 2020*

vlib/v/gen/array.v

@@ -252,6 +252,12 @@ fn (mut g Gen) gen_array_sort(node ast.CallExpr) {
 			// Variables `a` and `b` are used in the `.sort(a < b)` syntax, so we can reuse them
 			// when generating the function as long as the args are named the same.
 			g.definitions.writeln('int $compare_fn ($styp* a, $styp* b) {')
+			sym := g.table.get_type_symbol(typ)
+			if sym.has_method('<') && infix_expr.left.str().len == 1 {
+				g.definitions.writeln('\tif (${styp}__lt(*a, *b)) { return -1; } else { return 1; }}')
+			} else if sym.has_method('>') && infix_expr.left.str().len == 1 {
+				g.definitions.writeln('\tif (${styp}__gt(*a, *b)) { return -1; } else { return 1; }}')
+			} else {
 				field_type := g.typ(infix_expr.left_type)
 				left_expr_str := g.write_expr_to_string(infix_expr.left).replace_once('.',
 					'->')
@@ -281,6 +287,7 @@ fn (mut g Gen) gen_array_sort(node ast.CallExpr) {
 				g.definitions.writeln('if ($op2) return 1; return 0; }\n')
 			}
 		}
+	}
 	if is_reverse && !compare_fn.ends_with('_reverse') {
 		compare_fn += '_reverse'
 	}

vlib/v/tests/sorting_by_different_criteria_test.v

@@ -7,6 +7,10 @@ struct Parent {
 	name  string
 }
 
+fn (p Parent) < (p1 Parent) bool {
+    return p.name < p1.name
+}
+
 fn test_sorting_by_different_criteria_in_same_function() {
 	mut arr := [
 		Parent{Child{0.2}, 'def'},
@@ -20,4 +24,6 @@ fn test_sorting_by_different_criteria_in_same_function() {
 	// println(arr)
 	arr.sort(a.child.f < b.child.f)
 	assert arr[0].name == 'xyz'
+	arr.sort(a < b)
+	assert arr[0].name == 'abc'
 }