Make "v:" prefix mandatory in scriptversion 2

"version" cannot be used as "v:version" in scriptversion 2 anymore.
It can be used as a normal variable.

runtime/doc/eval.txt

@@ -12713,6 +12713,10 @@ instead of failing in mysterious ways. >
 < 	String concatenation with "." is not supported, use ".." instead.
 	This avoids the ambiguity using "." for Dict member access and
 	floating point numbers.  Now ".5" means the number 0.5.
+
+	All |vim-variable|s must be prefixed by "v:".  E.g. "version" doesn't
+	work as |v:version| anymore.  It can be used as a normal variable.
+
 	Test for support with: >
 		has('vimscript-2')
 

src/eval.c

@@ -7672,10 +7672,13 @@ find_var_ht(char_u *name, char_u **varname)
 	    return NULL;
 	*varname = name;
 
-	/* "version" is "v:version" in all scopes */
-	hi = hash_find(&compat_hashtab, name);
-	if (!HASHITEM_EMPTY(hi))
-	    return &compat_hashtab;
+	// "version" is "v:version" in all scopes if scriptversion is 1.
+	if (current_sctx.sc_version < 2)
+	{
+	    hi = hash_find(&compat_hashtab, name);
+	    if (!HASHITEM_EMPTY(hi))
+		return &compat_hashtab;
+	}
 
 	ht = get_funccal_local_ht();
 	if (ht == NULL)

src/testdir/test_eval_stuff.vim

@@ -133,6 +133,10 @@ func Test_string_concat_scriptversion2()
   call assert_fails('let a .= b', 'E985:')
   call assert_fails('let vers = 1.2.3', 'E15:')
 
+  call assert_fails('echo version', 'E121:')
+  let version = 1
+  call assert_equal(1, version)
+
   if has('float')
     let f = .5
     call assert_equal(0.5, f)
@@ -149,6 +153,10 @@ func Test_string_concat_scriptversion1()
   let vers = 1.2.3
   call assert_equal('123', vers)
 
+  echo version
+  call assert_fails('let version = 1', 'E46:')
+  call assert_equal(v:version, version)
+
   if has('float')
     call assert_fails('let f = .5', 'E15:')
   endif