Treat unmapped ALT/META as ESC+c in all modes

In #8226 <A-x> and <M-x> were changed to behave like <Esc>x in insert
mode when no mapping exists. This commit backs out that change and
replaces it with a more general one that makes unmapped ALT and META
keypresses as <Esc>+char in all modes. This fixes an unnecessary and
confusing inconsistency between modes.

runtime/doc/cmdline.txt

@@ -224,6 +224,10 @@ CTRL-[						*c_CTRL-[* *c_<Esc>* *c_Esc*
 		present in 'cpoptions', start entered command.
 		Note: If your <Esc> key is hard to hit on your keyboard, train
 		yourself to use CTRL-[.
+						*c_META* *c_ALT*
+		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		For example <A-x> acts like <Esc>x if <A-x> does not have a
+		command-line mode mapping.
 							*c_CTRL-C*
 CTRL-C		quit command-line without executing
 

runtime/doc/intro.txt

@@ -382,6 +382,8 @@ Note:
   <k1>, ..., <k9> and <kPoint> will not work.
 - Nvim supports mapping multibyte chars with modifiers such as `<M-ä>`. Which
   combinations actually work depends on the the UI or host terminal.
+- When a key is pressed using a meta or alt modifier and no mapping exists
+  for that keypress, Nvim behaves as though <Esc> was pressed before the key.
 
 								*<>*
 Examples are often given in the <> notation.  Sometimes this is just to make

runtime/doc/vim_diff.txt

@@ -195,7 +195,7 @@ Input/Mappings:
   <M-1>, <M-BS>, <M-Del>, <M-Ins>, <M-/>, <M-\>, <M-Space>, <M-Enter>, etc.
   Case-sensitive: <M-a> and <M-A> are two different keycodes.
 
-  ALT in insert-mode behaves like <Esc> if not mapped. |i_ALT|
+  ALT behaves like <Esc> if not mapped. |i_ALT| |v_ALT| |c_ALT|
 
 Normal commands:
   |g<Tab>| goes to the last-accessed tabpage.

runtime/doc/visual.txt

@@ -159,7 +159,10 @@ If you want to highlight exactly the same area as the last time, you can use
 
 							*v_<Esc>*
 <Esc>			In Visual mode: Stop Visual mode.
-
+						*v_META* *v_ALT*
+		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		For example <A-x> acts like <Esc>x if <A-x> does not have a
+		visual-mode mapping.
 							*v_CTRL-C*
 CTRL-C			In Visual mode: Stop Visual mode.  When insert mode is
 			pending (the mode message shows

src/nvim/edit.c

@@ -1254,14 +1254,6 @@ check_pum:
 normalchar:
     // Insert a normal character.
 
-    if (mod_mask == MOD_MASK_ALT || mod_mask == MOD_MASK_META) {
-      // Unmapped ALT/META chord behaves like ESC+c. #8213
-      stuffcharReadbuff(ESC);
-      stuffcharReadbuff(s->c);
-      u_sync(false);
-      break;
-    }
-
     if (!p_paste) {
       // Trigger InsertCharPre.
       char_u *str = do_insert_char_pre(s->c);

src/nvim/getchar.c

@@ -1528,6 +1528,17 @@ int vgetc(void)
         c = utf_ptr2char(buf);
       }
 
+      // If mappings are enabled (i.e., not Ctrl-v) and the user directly typed
+      // something with a meta- or alt- modifier that was not mapped, interpret
+      // <M-x> as <Esc>x rather than as an unbound meta keypress. #8213
+      if (!no_mapping && KeyTyped
+          && (mod_mask == MOD_MASK_ALT || mod_mask == MOD_MASK_META)) {
+        mod_mask = 0;
+        stuffcharReadbuff(c);
+        u_sync(false);
+        c = ESC;
+      }
+
       break;
     }
   }

test/functional/normal/meta_key_spec.lua

@@ -0,0 +1,22 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, feed, insert = helpers.clear, helpers.feed, helpers.insert
+local command = helpers.command
+local expect = helpers.expect
+
+describe('meta-keys-in-normal-mode', function()
+  before_each(function()
+    clear()
+  end)
+
+  it('ALT/META', function()
+    -- Unmapped ALT-chords behave as Esc+c
+    insert('hello')
+    feed('0<A-x><M-x>')
+    expect('llo')
+    -- Mapped ALT-chord behaves as mapped.
+    command('nnoremap <M-l> Ameta-l<Esc>')
+    command('nnoremap <A-j> Aalt-j<Esc>')
+    feed('<A-j><M-l>')
+    expect('lloalt-jmeta-l')
+  end)
+end)

test/functional/visual/meta_key_spec.lua

@@ -0,0 +1,22 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, feed, insert = helpers.clear, helpers.feed, helpers.insert
+local command = helpers.command
+local expect = helpers.expect
+
+describe('meta-keys-in-visual-mode', function()
+  before_each(function()
+    clear()
+  end)
+
+  it('ALT/META', function()
+    -- Unmapped ALT-chords behave as Esc+c
+    insert('peaches')
+    feed('viw<A-x>viw<M-x>')
+    expect('peach')
+    -- Mapped ALT-chord behaves as mapped.
+    command('vnoremap <M-l> Ameta-l<Esc>')
+    command('vnoremap <A-j> Aalt-j<Esc>')
+    feed('viw<A-j>viw<M-l>')
+    expect('peachalt-jmeta-l')
+  end)
+end)