new file mode 100644
--- /dev/null
+++ b/src/testdir/test_const.vim
@@ -0,0 +1,228 @@
+" Test for :const
+
+func s:noop()
+endfunc
+
+func Test_define_var_with_lock()
+    const i = 1
+    const f = 1.1
+    const s = 'vim'
+    const F = funcref('s:noop')
+    const l = [1, 2, 3]
+    const d = {'foo': 10}
+    if has('channel')
+      const j = test_null_job()
+      const c = test_null_channel()
+    endif
+    const b = v:true
+    const n = v:null
+    const bl = 0zC0FFEE
+    const here =<< trim EOS
+    hello
+    EOS
+
+    call assert_fails('let i = 1', 'E741:')
+    call assert_fails('let f = 1.1', 'E741:')
+    call assert_fails('let s = "vim"', 'E741:')
+    call assert_fails('let F = funcref("s:noop")', 'E741:')
+    call assert_fails('let l = [1, 2, 3]', 'E741:')
+    call assert_fails('let d = {"foo": 10}', 'E741:')
+    if has('channel')
+      call assert_fails('let j = test_null_job()', 'E741:')
+      call assert_fails('let c = test_null_channel()', 'E741:')
+    endif
+    call assert_fails('let b = v:true', 'E741:')
+    call assert_fails('let n = v:null', 'E741:')
+    call assert_fails('let bl = 0zC0FFEE', 'E741:')
+    call assert_fails('let here = "xxx"', 'E741:')
+
+    " Unlet
+    unlet i
+    unlet f
+    unlet s
+    unlet F
+    unlet l
+    unlet d
+    unlet j
+    unlet c
+    unlet b
+    unlet n
+    unlet bl
+    unlet here
+endfunc
+
+func Test_define_l_var_with_lock()
+    " With l: prefix
+    const l:i = 1
+    const l:f = 1.1
+    const l:s = 'vim'
+    const l:F = funcref('s:noop')
+    const l:l = [1, 2, 3]
+    const l:d = {'foo': 10}
+    if has('channel')
+      const l:j = test_null_job()
+      const l:c = test_null_channel()
+    endif
+    const l:b = v:true
+    const l:n = v:null
+    const l:bl = 0zC0FFEE
+    const l:here =<< trim EOS
+    hello
+    EOS
+
+    call assert_fails('let l:i = 1', 'E741:')
+    call assert_fails('let l:f = 1.1', 'E741:')
+    call assert_fails('let l:s = "vim"', 'E741:')
+    call assert_fails('let l:F = funcref("s:noop")', 'E741:')
+    call assert_fails('let l:l = [1, 2, 3]', 'E741:')
+    call assert_fails('let l:d = {"foo": 10}', 'E741:')
+    if has('channel')
+      call assert_fails('let l:j = test_null_job()', 'E741:')
+      call assert_fails('let l:c = test_null_channel()', 'E741:')
+    endif
+    call assert_fails('let l:b = v:true', 'E741:')
+    call assert_fails('let l:n = v:null', 'E741:')
+    call assert_fails('let l:bl = 0zC0FFEE', 'E741:')
+    call assert_fails('let l:here = "xxx"', 'E741:')
+
+    " Unlet
+    unlet l:i
+    unlet l:f
+    unlet l:s
+    unlet l:F
+    unlet l:l
+    unlet l:d
+    unlet l:j
+    unlet l:c
+    unlet l:b
+    unlet l:n
+    unlet l:bl
+    unlet l:here
+endfunc
+
+func Test_define_script_var_with_lock()
+    const s:x = 0
+    call assert_fails('let s:x = 1', 'E741:')
+    unlet s:x
+endfunc
+
+func Test_descructuring_with_lock()
+    const [a, b, c] = [1, 1.1, 'vim']
+
+    call assert_fails('let a = 1', 'E741:')
+    call assert_fails('let b = 1.1', 'E741:')
+    call assert_fails('let c = "vim"', 'E741:')
+
+    const [d; e] = [1, 1.1, 'vim']
+    call assert_fails('let d = 1', 'E741:')
+    call assert_fails('let e = [2.2, "a"]', 'E741:')
+endfunc
+
+func Test_cannot_modify_existing_variable()
+    let i = 1
+    let f = 1.1
+    let s = 'vim'
+    let F = funcref('s:noop')
+    let l = [1, 2, 3]
+    let d = {'foo': 10}
+    if has('channel')
+      let j = test_null_job()
+      let c = test_null_channel()
+    endif
+    let b = v:true
+    let n = v:null
+    let bl = 0zC0FFEE
+
+    call assert_fails('const i = 1', 'E995:')
+    call assert_fails('const f = 1.1', 'E995:')
+    call assert_fails('const s = "vim"', 'E995:')
+    call assert_fails('const F = funcref("s:noop")', 'E995:')
+    call assert_fails('const l = [1, 2, 3]', 'E995:')
+    call assert_fails('const d = {"foo": 10}', 'E995:')
+    if has('channel')
+      call assert_fails('const j = test_null_job()', 'E995:')
+      call assert_fails('const c = test_null_channel()', 'E995:')
+    endif
+    call assert_fails('const b = v:true', 'E995:')
+    call assert_fails('const n = v:null', 'E995:')
+    call assert_fails('const bl = 0zC0FFEE', 'E995:')
+    call assert_fails('const [i, f, s] = [1, 1.1, "vim"]', 'E995:')
+
+    const i2 = 1
+    const f2 = 1.1
+    const s2 = 'vim'
+    const F2 = funcref('s:noop')
+    const l2 = [1, 2, 3]
+    const d2 = {'foo': 10}
+    if has('channel')
+      const j2 = test_null_job()
+      const c2 = test_null_channel()
+    endif
+    const b2 = v:true
+    const n2 = v:null
+    const bl2 = 0zC0FFEE
+
+    call assert_fails('const i2 = 1', 'E995:')
+    call assert_fails('const f2 = 1.1', 'E995:')
+    call assert_fails('const s2 = "vim"', 'E995:')
+    call assert_fails('const F2 = funcref("s:noop")', 'E995:')
+    call assert_fails('const l2 = [1, 2, 3]', 'E995:')
+    call assert_fails('const d2 = {"foo": 10}', 'E995:')
+    if has('channel')
+      call assert_fails('const j2 = test_null_job()', 'E995:')
+      call assert_fails('const c2 = test_null_channel()', 'E995:')
+    endif
+    call assert_fails('const b2 = v:true', 'E995:')
+    call assert_fails('const n2 = v:null', 'E995:')
+    call assert_fails('const bl2 = 0zC0FFEE', 'E995:')
+    call assert_fails('const [i2, f2, s2] = [1, 1.1, "vim"]', 'E995:')
+endfunc
+
+func Test_const_with_index_access()
+    let l = [1, 2, 3]
+    call assert_fails('const l[0] = 4', 'E996:')
+    call assert_fails('const l[0:1] = [1, 2]', 'E996:')
+
+    let d = {'aaa': 0}
+    call assert_fails("const d['aaa'] = 4", 'E996:')
+    call assert_fails("const d.aaa = 4", 'E996:')
+endfunc
+
+func Test_const_with_compound_assign()
+    let i = 0
+    call assert_fails('const i += 4', 'E995:')
+    call assert_fails('const i -= 4', 'E995:')
+    call assert_fails('const i *= 4', 'E995:')
+    call assert_fails('const i /= 4', 'E995:')
+    call assert_fails('const i %= 4', 'E995:')
+
+    let s = 'a'
+    call assert_fails('const s .= "b"', 'E995:')
+
+    let [a, b, c] = [1, 2, 3]
+    call assert_fails('const [a, b, c] += [4, 5, 6]', 'E995:')
+
+    let [d; e] = [1, 2, 3]
+    call assert_fails('const [d; e] += [4, 5, 6]', 'E995:')
+endfunc
+
+func Test_const_with_special_variables()
+    call assert_fails('const $FOO = "hello"', 'E996:')
+    call assert_fails('const @a = "hello"', 'E996:')
+    call assert_fails('const &filetype = "vim"', 'E996:')
+    call assert_fails('const &l:filetype = "vim"', 'E996:')
+    call assert_fails('const &g:encoding = "utf-8"', 'E996:')
+endfunc
+
+func Test_lock_depth_is_1()
+    const l = [1, 2, 3]
+    const d = {'foo': 10}
+
+    " Modify list
+    call add(l, 4)
+    let l[0] = 42
+
+    " Modify dict
+    let d['bar'] = 'hello'
+    let d.foo = 44
+endfunc