Before this commit, the command word was highlighted as "unknown-token"
not because alias loops are invalid, as a comment incorrectly claimed,
but because the command word «a» resolved to a «b» that was ineligible
for being expanded as an alias, and there was no function/builtin/etc.
called "b".

Add a function "b" to demonstrate that alias loops are valid.  I've also
filed issue #695 about the overloading of "unknown-token".

Fixes #501.

Fixes #616 (the original form; not the form in
test-data/alias-comment1.zsh which is now considered o be #677 (see
previous commit for details)).

Fixes a latent bug in test-data/always2.zsh.

No user-visible effect, and therefore, no changelog entry.

See https://github.com/zsh-users/zsh-syntax-highlighting/issues/677#issuecomment-599225740 for details.

(In particular, there's already another test that calls itself #616.)

Fixes #651.

Fixes the test added in the penultimate (grandparent) commit.

Used by the next commit.

- pipes (issue #651)
- semicolons
- literal newlines
  (also discussed on #651)

Fixes #646.

It is fixed in the next commit.

See 1a55dc8fc2 (commitcomment-37476021)

Fixes the bug the previous commit added a test for.

Suggested-by: @QBobWatson
(in https://github.com/zsh-users/zsh-syntax-highlighting/pull/682#issuecomment-588361771)

In «a="b=c"; $a», the '=' sign in the expansion of $a is not active.
Therefore, prevent the expansion of $a from being considered an
assignment.  Update test expectations accordingly.

Before the parent commit, they behaved as follows:

    ZSH_PATCHLEVEL=debian/5.7.1-1
    # parameter-value-contains-command-position1
    1..2
    ok 1 - [1,7] «$foobar» - # TODO "issue #670"
    not ok 2 - have 1 expectations and 6 region_highlight entries: «expected_region_highlight=( '1 7 assign "issue ♯670"' )» «region_highlight=( '0 7 assign' '2 7 default' '2 7 command-substitution-unquoted'
    zsh-syntax-highlighting: BUG: _zsh_highlight_highlighter_main_paint: start(2) >= end(2)
    Bail out! On './highlighters/main/test-data/parameter-value-contains-command-position2.zsh': output on stderr
    # parameter-value-contains-command-position2
    1..2
    ok 1 - [1,2] «$y» - # TODO "issue #670"
    ok 2 - cardinality check

Due to the the "BUG:" and "Bail out!" on the first one, they could not
be added as XFAIL tests before the parent commit.

Fixes #674.

Fixes #681.

Suggested-by: Matthew Martin
(in e2e97dde9c (r36941988))

Fixes #673.

See discussion on #664.

We cannot now for sure whether the flag is misspelled or simply unknown to us,
so err on the side of caution.  This fixes an unreleased regression.  Fixes #658.

Issue #641 was originally filed about this problem, but is left open to track
further enhancements.

Issue #202 is too overloaded.

$last_alias isn't needed; there's no reason to treat loops of length 2
(alias a=b b=a) differently to loops of length 1 (alias a=a), length 3
(alias a=b b=c c=a), or length N.

The «(( $+seen_alias[$arg] ))» check is redundant as of the last commit:
the enclosing condition ensures that $res is "alias", which implies that
«(( $+seen_alias[$arg] ))» is false.