llvm dev - Oct 2020 - Behavior of TableGen paste operator

I have just discovered a behavior of the TableGen paste operator (#) that I did
not expect. I'm wondering if this behavior was intended.

In a record name expression, the expression is parsed in "name mode"
so that the name of a defvar is treated verbatim.

def "foo_" # var { ...

Assume var is a defvar. Its value is not used, and the name is foo_var. This
allows fragments of record names to be specified without string quotes.

My surprise is that the same rule holds when the paste operator is used in a
normal expression.

def ...
  string foo = "foo_" # var;

The resulting string is "foo_var", not "foo_<value of
var>". This happens because normal expression parsing still forces
"name mode" when parsing the right-hand-side of a paste operator.

So, a # b is not identical to !strconcat(a, b) in a normal expression.

Thoughts?

@lattner
@nhaehnle