The Player's Basic Rules and Player's Handbook describe the Thieves' Cant class feature as:
... a secret mix of dialect, jargon and code allows you to hide messages in seemingly normal conversation. Only another creature that knows thieves' cant understands such messages. It takes four times longer to convey such a message than it does to speak the same idea plainly.
In addition, you understand a set of secret signs and symbols used to convey short, simple messages, such as whether an area is dangerous or the territory of a thieves' guild, whether loot is nearby, or whether the people in an area are easy marks or will provide a safe house for thieves on the run. [Emphasis added] (PBR, p. 27; PHB, p. 96)
Thieves' Cant and Druidic are described as secret languages that can be taken with the DM's permission (PBR, p. 34; PHB, p. 123), but there is no other mention (other than being a rogue) of how to understand Thieves' Cant, whereas Druidic and the written ciphers of the Linguist feat specifically state they can be magically deciphered. (PHB, pp. 66 & 167) Since the PHB does not explicitly say so:
Is there any magical way a non-rogue could understand Thieves' Cant?
The spell Comprehend Languages allows the caster to "understand the literal meaning of any spoken language ... [and] any written language [while] touching the surface on which the words are written. ... [but] This spell doesn't decode secret messages in a text or a glyph, such as an arcane sigil, that isn't part of a written language" [Emphasis added] (PBR, p. 86; PHB p. 224) while the spell Tongues grants "the ability to understand any spoken language ..." (PHB, p. 283).
The warlock's Eyes of the Runekeeper eldritch invocation simply states, in its entirety, "You can read all writing." (PHB, p. 111)
Are the "hidden messages" and "secret signs and symbols" of Thieves' Cant considered a "language" or "writing" per the above spells or invocation?
On re-reading the PHB, I stumbled across the following under "Languages" in Chapter 4, "With your DM's permission, you can instead choose a language from the Exotic Language table or a secret language, such as thieves' cant or the tongue of the druids." (PHB, p. 123) So it appears that 1) you can gain understanding without being a rogue, and 2) it is considered a language in general; but, is it for magical deciphering purposes?