Grunt less won't replace css file

Question

I use grunt-contrib-less with grunt-contrib-watch to compile my less files automatically upon change. When the css is not present, grunt compiles it ok. When a css already exists and watch sees the less file changed, the css file is not modified. I have to remove it every time and let grunt recreate it with the modifications.

less config:

less: {
    options: {
        banner: '<%= meta.banner %>'
    },
    dev: {
        options: {
            sourceMap: true,
            sourceMapFileInline: true,
            compress: true
        },
        src: '<%= meta.dev.less %>/main.less',
        dest: '<%= meta.prod.css %>/main.css'
    },
    prod: {
        options: {
            plugins: [
                new( require( 'less-plugin-clean-css' ) )( {
                    'advanced': true,
                    'compatibility': 'ie9'
                } )
            ],
        },
        src: '<%= meta.dev.less %>/main.less',
        dest: '<%= meta.prod.css %>/main.css'
    }
},

I'm under Windows 10 and every user have the rights to modifiy/delete the files in my dist folder. How can I let grunt modify the css file?

EDIT

watch config

watch: {
    options: {
        livereload: 6325
    },
    js: {
        files: [ '<%= meta.dev.js %>/main.js', '<%= meta.dev.js %>/plugins/*.js' ],
        tasks: [ 'newer:concat' ]
    },
    images: {
        files: '<%= meta.dev.img %>/**/*.{png,jpg,gif,svg}',
        tasks: [ 'newer:imagemin' ]
    },
    css: {
        files: '<%= meta.dev.less %>/**/*.less',
        tasks: [ 'newer:less:dev' ]
    }
}

Registration

grunt.registerTask( 'default', [ 'less:dev', 'concat', 'imagemin', 'copy', 'watch' ] );

Grunt output (verbose)

>> File "dev\less\elements\menu.less" changed.
Initializing
Command-line options: --verbose

Reading "Gruntfile.js" Gruntfile...OK

Registering Gruntfile tasks.
Reading package.json...OK
Parsing package.json...OK
Initializing config...OK
Loading "Gruntfile.js" tasks...OK
+ default, prod

Running tasks: newer:less:dev

Loading "grunt-newer" plugin

Registering "D:\[...]\static\node_modules\grunt-newer\tasks" tasks.
Loading "newer.js" tasks...OK
+ any-newer, newer, newer-clean, newer-postrun

Running "newer:less:dev" (newer) task
Options: cache="D:\\[...]\\static\\node_modules\\grunt-newer\\.cache", override=undefined
Files: dev/less/main.less -> dist/css/main.css
No newer files to process.

Answer 1

Here's what happens:

• you modify menu.less
• watch detects that and runs newer:less:dev
• less:dev uses file main.less as only source (not menu.less)
• that file hasn't changed, so newer concludes it doesn't need to run the task again

I suppose main.less includes menu.less, but newer doesn't know it. So my suggested fix would be to get rid of the newer part.