[dcl.attr.assume] says:

The expression is contextually converted to bool. The expression is not evaluated. If the converted expression would evaluate to true at the point where the assumption appears, the assumption has no effect.
Otherwise, the behavior is undefined.

This wording is mildly contradictory because the first sentence says that contextual conversion takes place. This might even invoke user-defined conversion operators, so the reader should assume that something is being evaluated here.

However, this is then contradicted by saying that the expression is not evaluated. If it's not evaluated, then obviously contextual conversion can't take place.

I think it would be much clearer to write:

-The expression is contextually converted to bool.
+The expression shall be contextually convertible to bool.
 The expression is not evaluated.
 If the converted expression would evaluate to true at the point where the assumption appears,
 the assumption has no effect.
 Otherwise, the behavior is undefined.

I am not sure if this issue is so severe that it's non-editorial.