quote nesting bug

[Matt Zyzik]

bug-bash,

I am noticing a behavior that appears to be a bug. Additionally, what I
am expecting to happen is the actual behavior in both Zsh and Ksh.

This is the command (ignore leading whitespace):
                echo "${fpbnoinervzkjeh:-'good   morning}"
This is the output in both Zsh and Ksh (ignore leading whitespace):
                'good   morning

In Bash, there is no output, only a prompt that appears. However, the
following command (ignore leading whitespace):
                echo "${fpbnoinervzkjeh:-''good   morning}"
produces the following output in all 3 shells (ignore leading whitespace):
                ''good   morning

Can I request comment? Thank you.

--Matt Zyzik