Re: [vim/vim] `expand()` and `expandcmd()` behave differently on `%:~` (Issue #20793)

h-east left a comment (vim/vim#20793)

@dkearns is right, `%:~` is not a special case. Here is the code for the two
steps.

`expandcmd()` is a thin wrapper around `expand_filename()` (see `f_expandcmd()`
in `src/evalfunc.c`), and `expand_filename()` in `src/ex_docmd.c` does this:

1. `eval_vars()` evaluates the cmdline-special item, so `%:~` becomes
   `~/file`.

2. The replacement text is then run through `expand_env_save()`:

```c
	// Wildcards won't be expanded below, the replacement is taken
	// literally.  But do expand "~/file", "~user/file" and "$HOME/file".
	if (vim_strchr(repl, '$') != NULL || vim_strchr(repl, '~') != NULL)
	{
	    char_u *l = repl;

	    repl = expand_env_save(repl);
	    vim_free(l);
	}
```

So the `~` produced by step 1 is expanded in step 2, just like it is when you
type the command. `expand()` stops after step 1, which is why it keeps the
`~`.

A literal `~/path` in the string is handled by the same mechanism at the end
of `expand_filename()`: `mch_has_wildcard()` returns TRUE for a `~` that is
followed by another character, and the argument is then passed to
`expand_env_esc()`, which expands `~` only at the start of a name.

So the behaviour is correct and I do not intend to change it. What is missing
is that the help never mentions the second step. I would rather document that
order than describe `%:~` as an exception, since it is not one:

```diff
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -3099,6 +3099,10 @@ expandcmd({string} [, {options}])			*expandcmd()*
 		like with |expand()|, and environment variables, anywhere in
 		{string}.  "~user" and "~/path" are only expanded at the
 		start.
+		The expansion is done in two steps: the special keywords are
+		evaluated first, then "~" and environment variables are
+		expanded in the result.  Thus `expand('%:~')` keeps the "~",
+		while `expandcmd('%:~')` returns the full path.
 
 		The following items are supported in the {options} Dict
 		argument:
```

The patch only writes down what @dkearns already described.

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