ast, checker, cgen: fix error for go anon fn variable call (#13776)

vlib/v/ast/ast.v

@@ -557,6 +557,7 @@ pub mut:
 	name               string // left.name()
 	is_method          bool
 	is_field           bool // temp hack, remove ASAP when re-impl CallExpr / Selector (joe)
+	is_fn_var          bool // fn variable
 	is_keep_alive      bool // GC must not free arguments before fn returns
 	is_noreturn        bool // whether the function/method is marked as [noreturn]
 	is_ctor_new        bool // if JS ctor calls requires `new` before call, marked as `[use_new]` in V
@@ -568,6 +569,7 @@ pub mut:
 	left_type          Type // type of `user`
 	receiver_type      Type // User
 	return_type        Type
+	fn_var_type        Type   // fn variable type
 	should_be_skipped  bool   // true for calls to `[if someflag?]` functions, when there is no `-d someflag`
 	concrete_types     []Type // concrete types, e.g. <int, string>
 	concrete_list_pos  token.Pos

vlib/v/checker/fn.v

@@ -600,9 +600,13 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 			match obj {
 				ast.GlobalField {
 					typ = obj.typ
+					node.is_fn_var = true
+					node.fn_var_type = typ
 				}
 				ast.Var {
 					typ = if obj.smartcasts.len != 0 { obj.smartcasts.last() } else { obj.typ }
+					node.is_fn_var = true
+					node.fn_var_type = typ
 				}
 				else {}
 			}

vlib/v/gen/c/cgen.v

@@ -5106,6 +5106,8 @@ fn (mut g Gen) go_expr(node ast.GoExpr) {
 	} else if mut expr.left is ast.AnonFn {
 		g.gen_anon_fn_decl(mut expr.left)
 		name = expr.left.decl.name
+	} else if expr.is_fn_var {
+		name = g.table.sym(expr.fn_var_type).name
 	}
 	name = util.no_dots(name)
 	if g.pref.obfuscate && g.cur_mod.name == 'main' && name.starts_with('main__') {

vlib/v/tests/go_anon_fn_variable_call_test.v

@@ -0,0 +1,18 @@
+fn sum(a int, b int) int {
+	// Simply proxy the function into an anonymous function for demo purposes
+	sum_func := fn (a int, b int) int {
+		return a + b
+	}
+
+	// and run it concurrently
+	g := go sum_func(a, b)
+
+	result := g.wait()
+	return result
+}
+
+fn test_go_anon_fn_variable_call() {
+	ret := sum(22, 33)
+	println(ret)
+	assert ret == 55
+}