custom variable (of a TFS build) can't be populated with expression

Question

In build definition (on Variables tab) I am trying to define a custom variable (Build.Repository.Clean) using simple expression $[not(false)]. But when I print variables during build -- regardless of expression used, Build.Repository.Clean value is always false.

Strangely enough, definining it with something like $(FullBuild) (where FullBuild is another custom variable) works just fine.

Am I missing something?

Notes:

• using TFS 2018

Backstory:

Trying to set Build.Repository.Clean variable depending on a custom variable QuickBuild (which can be set by user when kicking off a build). Tried specifying $[not(variables.QuickBuild)] (and other variations of same expression) -- no luck.

here is how it works right now (but I'd rather have QuickBuild instead of FullBuild -- just can't figure out how to negate a variable):

Answer 1

Expressions are not evaluated when they are used to initialize custom variables (on Variables tab). I.e. variable value ends up being a string with value equal to your expression (e.g. '$[not(<whatever>)]'). Later, when that variable gets used in context that expect boolean -- it still doesn't get evaluated, instead it gets type-casted and any non-empty string yields true.

On the other hand variable substitution happens -- i.e. value $(MyVar) gets replaced with value of MyVar.

Built-in variable seem to be special in sense that if you override them -- this process happens at the start and it's value gets immediately replaced with resulting value.

Note -- this may (or may not) be related to this.

Bottomline: you can't use expressions to override value of a built-in variable.