/s/ → /ʃ/
The fricative alveolar /s/ becomes a fricative post-alveolar /ʃ/ in two different cases:
1. When followed by a post-alveolar /ʃ/.
2. When followed by a palatal /j/, which is another case of coalescence.
/s/ (followed by /ʃ/ or /j/) becomes /ʃ/
Since she’s going to ask you, why not make a clean breast of it?
In case you want to come with us, there’s still room in the car.
When the second phoneme is a /j/, that /j/ sound can be retained or not, depending on the degree of the assimilation.
In case you want… /s/ + /j/ → /ʃ/ |ɪn ˈkeɪ ʃu ˈwɒnt|
In case you want… /s/ + /j/ → /ʃ j/ |ɪn ˈkeɪʃ ju ˈwɒnt|
And here’s an interesting array of naturally occurring examples:
Maybe wearing a nice shirt and tie and a nice pairs of cufflinks, you know, is important to you impress your clients (from Cambridge Proficiency Test).
Once you can find something of yourself in that character, or once you feel that that character represents something about you (Howard Jacobson, OpenLearn).
Certainly a certain type of playwright does take great pleasure in going to an office and listening to what the buzzwords are in offices this year, because that changes from year to year, management speak, the kind of words that politicians are using this year, the kind of words that school kids are using this year (Mark Ravenhill, OpenLearn).
Now notice the difference between keeping the /j/ sound or not that I mentioned before:
Once you can find something of yourself in that character. /s/ + /j/ → /ʃ/. The sound /j/ is not heard (or barely heard).
is important to you impress your clients. /s/ + /j/ → /j ʃ/. The sound /j/ is still clearly heard.
So, both are correct. It’s a subtle, interesting nuance, and you can choose to do it whichever way you like.
And finally, listen to this recording in which Melvyn Bragg assimilates /s/ and /j/ within a single word:
Is it possible for us to look back and assume that Gatsby’s world was normal? What do you think about the setting that Fitzgerald gives him (Melvyn Bragg, BBC4)? /əˈsjuːm/ → əˈʃuːm/