gen: implement type_name method for sum type instances (#6802)

doc/docs.md

@@ -1634,8 +1634,11 @@ struct Venus {}
 type World = Moon | Mars | Venus
 
 sum := World(Moon{})
+assert sum.type_name() == 'Moon'
 println(sum)
 ```
+The built-in method `type_name` returns the name of the currently held 
+type.
 
 #### Dynamic casts
 

vlib/v/ast/ast.v

@@ -294,7 +294,7 @@ pub:
 	left               Expr // `user` in `user.register()`
 	mod                string
 pub mut:
-	name               string
+	name               string // left.name()
 	is_method          bool
 	is_field           bool // temp hack, remove ASAP when re-impl CallExpr / Selector (joe)
 	args               []CallArg

vlib/v/checker/checker.v

@@ -1097,6 +1097,9 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 		c.error('optional type cannot be called directly', call_expr.left.position())
 		return table.void_type
 	}
+	if left_type_sym.kind == .sum_type && method_name == 'type_name' {
+		return table.string_type
+	}
 	// TODO: remove this for actual methods, use only for compiler magic
 	// FIXME: Argument count != 1 will break these
 	if left_type_sym.kind == .array &&
@@ -4424,6 +4427,9 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 		if sym.kind == .interface_ {
 			c.error('interfaces cannot be used as method receiver', node.receiver_pos)
 		}
+		if sym.kind == .sum_type && node.name == 'type_name' {
+			c.error('method overrides built-in sum type method', node.pos)
+		}
 		// if sym.has_method(node.name) {
 		// c.warn('duplicate method `$node.name`', node.pos)
 		// }

vlib/v/checker/tests/incorrect_name_sum_type.out

@@ -1,3 +1,12 @@
 vlib/v/checker/tests/incorrect_name_sum_type.vv:1:1: error: sum type `integer` must begin with capital letter
     1 | type integer = i8 | i16 | int | i64
-      | ~~~~~~~~~~~~
+      | ~~~~~~~~~~~~
+    2 | type Integer = i8 | i16 | int | i64
+    3 |
+vlib/v/checker/tests/incorrect_name_sum_type.vv:4:1: error: method overrides built-in sum type method
+    2 | type Integer = i8 | i16 | int | i64
+    3 | 
+    4 | fn (i Integer) type_name() {
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~
+    5 | }
+    6 |

vlib/v/checker/tests/incorrect_name_sum_type.vv

@@ -1 +1,6 @@
 type integer = i8 | i16 | int | i64
+type Integer = i8 | i16 | int | i64
+
+fn (i Integer) type_name() {
+}
+

vlib/v/gen/fn.v

@@ -377,6 +377,12 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 			else {}
 		}
 	}
+	if left_sym.kind == .sum_type && node.name == 'type_name' {
+		g.write('tos3( /* $left_sym.name */ v_typeof_sumtype_${node.receiver_type}( (')
+		g.expr(node.left)
+		g.write(').typ ))')
+		return
+	}
 	if node.name == 'str' {
 		mut styp := g.typ(node.receiver_type)
 		if node.receiver_type.is_ptr() {

vlib/v/tests/typeof_test.v

@@ -65,6 +65,9 @@ fn test_typeof_on_sumtypes() {
 	assert typeof(a) == 'int'
 	assert typeof(b) == 'f32'
 	assert typeof(c) == 'FooBar'
+	assert a.type_name() == 'int'
+	assert b.type_name() == 'f32'
+	assert c.type_name() == 'FooBar'
 	// typeof should be known at compile-time for all types
 	assert typeof(a).name == 'MySumType'
 	assert typeof(b).name == 'MySumType'
@@ -96,6 +99,10 @@ fn test_typeof_on_sumtypes_of_structs() {
 	assert typeof(b) == 'BinExpr'
 	assert typeof(c) == 'BoolExpr'
 	assert typeof(d) == 'UnaryExpr'
+	assert a.type_name() == 'UnaryExpr'
+	assert b.type_name() == 'BinExpr'
+	assert c.type_name() == 'BoolExpr'
+	assert d.type_name() == 'UnaryExpr'
 }
 
 fn myfn(i int) int {