insert-mode: interpret unmapped META as ESC

closes #2454
closes #8213
ref #7972

test/functional/insert/insert_spec.lua

@@ -0,0 +1,41 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, feed, insert = helpers.clear, helpers.feed, helpers.insert
+local command = helpers.command
+local eq = helpers.eq
+local expect = helpers.expect
+local funcs = helpers.funcs
+
+describe('insert-mode', function()
+  before_each(function()
+    clear()
+  end)
+
+  it('CTRL-@', function()
+    -- Inserts last-inserted text, leaves insert-mode.
+    insert('hello')
+    feed('i<C-@>x')
+    expect('hellhello')
+
+    -- C-Space is the same as C-@.
+    -- CTRL-SPC inserts last-inserted text, leaves insert-mode.
+    feed('i<C-Space>x')
+    expect('hellhellhello')
+
+    -- CTRL-A inserts last inserted text
+    feed('i<C-A>x')
+    expect('hellhellhellhelloxo')
+  end)
+
+  it('ALT/META #8213', function()
+    -- Mapped ALT-chord behaves as mapped.
+    command('inoremap <M-l> meta-l')
+    command('inoremap <A-j> alt-j')
+    feed('i<M-l> xxx <A-j><M-h>a<A-h>')
+    expect('meta-l xxx alt-j')
+    eq({ 0, 1, 14, 0, }, funcs.getpos('.'))
+    -- Unmapped ALT-chord behaves as ESC+c.
+    command('iunmap <M-l>')
+    feed('0i<M-l>')
+    eq({ 0, 1, 2, 0, }, funcs.getpos('.'))
+  end)
+end)