cgen: fix print fn + cycle const error message (#17355)

vlib/v/checker/checker.v

@@ -3118,7 +3118,7 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 					return c.expected_type
 				}
 			}
-			c.error('cycle in constant `${c.const_decl}`', node.pos)
+			c.error('cycle in constant `${c.const_var.name}`', node.pos)
 			return ast.void_type
 		}
 		c.const_deps << name

vlib/v/checker/tests/const_cycle_decl_err.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/const_cycle_decl_err.vv:1:11: error: cycle in constant `a`
+    1 | const a = a + 0
+      |           ^

vlib/v/checker/tests/const_cycle_decl_err.vv

@@ -0,0 +1 @@
+const a = a + 0

vlib/v/gen/c/auto_str_methods.v

@@ -989,7 +989,8 @@ fn struct_auto_str_func(sym &ast.TypeSymbol, _field_type ast.Type, fn_name strin
 		}
 		return 'indent_${fn_name}(${obj}, indent_count + 1)', true
 	} else if sym.kind == .function {
-		return '${fn_name}()', true
+		obj := '${deref}it.${c_name(field_name)}${sufix}'
+		return '${fn_name}(${obj})', true
 	} else if sym.kind == .chan {
 		return '${fn_name}(${deref}it.${c_name(field_name)}${sufix})', true
 	} else {

vlib/v/gen/c/str.v

@@ -153,6 +153,8 @@ fn (mut g Gen) gen_expr_to_string(expr ast.Expr, etype ast.Type) {
 				g.write('*')
 			}
 			g.expr_with_cast(expr, typ, typ)
+		} else {
+			g.expr_with_cast(expr, typ, typ)
 		}
 		g.write(')')
 	}

vlib/v/tests/print_anon_fn_test.v

@@ -0,0 +1,16 @@
+struct Foo {
+mut:
+	f ?fn (int)
+	g fn (int)
+}
+
+fn test_main() {
+	mut foo := ?Foo{}
+	assert foo == none
+
+	foo = Foo{}
+	assert foo != none
+
+	println(foo?.f)
+	println('${foo?.f}')
+}