Index exists API with wildcard returns true even when no index exists

[cdalexndr]

Elasticsearch version (bin/elasticsearch --version):
Version: 7.6.0, Build: default/docker/7f634e9f44834fbc12724506cc1da681b0c3b1e3/2020-02-06T00:09:00.449973Z, JVM: 13.0.2

Plugins installed: []

OS version (uname -a if on a Unix-like system):
Linux 406d050fe126 4.19.76-linuxkit #1 SMP Thu Oct 17 19:31:58 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior:

Checking if index exists with HEAD /twitter-* returns 200 - OK when no index exists.

Steps to reproduce:

1. No index exists.
2. HEAD /twitter-*

[cdalexndr]

After more playing around, it seems that adding ?allow_no_indices=false query param does what I need.

In Index exists API docs, there is no default value for allow_no_indices, so it's not clear what is the default behavior when calling without this param.

Even more, from the current doc info:
(Optional, boolean) If true, the request does not return an error if a wildcard expression or _all value retrieves only missing or closed indices.
one can imply that the default value is false because it only says the behavior when it is set to true (adding an optional param should change the default behavior), but actually it's default value is true.

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Indices APIs)

[matriv]

The default value of allow_no_indices is clarified in the docs with #52635.

Imho though, the current behaviour of HEAD /<index-pattern> seems weird, since a 200 status is returned even if no index is matched (since allow_no_indices is by default true for this API) and differs from the behaviour of the HEAD /<index-pattern>/_alias/<alias> API which returns 404 if no index is found. To clarify, the alias exists API still considers allow_no_indices with default value true, but when no aliases are found (since no indices are found) returns 404.