Biblatex authoryear dashed for entries with no year

Question

I'm using biblatex authoryear and have multiple entries of the same author that do not have a year. When I cite them there are letters shown to distinguish them:

But in the bibliography those letters are not shown and you can only distinguish them by the title. Is it possible to show the letters in the bibliography, too?

\documentclass[a4paper,12pt]{article}%
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{filecontents}
\begin{filecontents}{bib.bib}
@BOOK{bar1,
  title = {Text One},
  year = {},
  author = {Firstname Lastname},
}
@BOOK{bar2,
  title = {Text Two},
  year = {},
  author = {Firstname Lastname},
}
\end{filecontents}

\usepackage[backend=biber, style=authoryear-icomp, dashed=true]{biblatex}
\addbibresource{bib.bib}

\begin{document}

\cite{bar1} and \cite{bar2}

\printbibliography
\end{document}

Answer 1

By default biblatex does not print anything in the bibliography if the date is not set

\renewbibmacro*{date+extrayear}{%
  \iffieldundef{\thefield{datelabelsource}year}
    {}
    {\printtext[parens]{%
       \iffieldsequal{year}{\thefield{datelabelsource}year}
         {\printdateextralabel}%
         {\printfield{labelyear}%
          \printfield{extrayear}}}}}%

You can force biblatex to always print the year

\renewbibmacro*{date+extrayear}{%
  \printtext[parens]{%
    \iffieldsequal{year}{\thefield{datelabelsource}year}
      {\printdateextralabel}%
      {\printfield{labelyear}%
       \printfield{extrayear}}}}%

Or to only print the letter if no year is available

\renewbibmacro*{date+extrayear}{%
  \iffieldundef{\thefield{datelabelsource}year}
    {\printfield{extrayear}}
    {\printtext[parens]{%
       \iffieldsequal{year}{\thefield{datelabelsource}year}
         {\printdateextralabel}%
         {\printfield{labelyear}%
          \printfield{extrayear}}}}}%

This applies to the standard mergedate=true/mergdetae=compact setting, if you have any other mergdate option in place, you might have to modify the redefinitions above.

\documentclass[a4paper,12pt]{article}%
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@BOOK{bar1,
  title = {Text One},
  year = {},
  author = {Firstname Lastname},
}
@BOOK{bar2,
  title = {Text Two},
  year = {},
  author = {Firstname Lastname},
}
\end{filecontents}

\usepackage[backend=biber, style=authoryear, dashed=true]{biblatex}
\addbibresource{\jobname.bib}

\renewbibmacro*{date+extrayear}{%
  \printtext[parens]{%
    \iffieldsequal{year}{\thefield{datelabelsource}year}
      {\printdateextralabel}%
      {\printfield{labelyear}%
       \printfield{extrayear}}}}%

\begin{document}

\cite{bar1} and \cite{bar2}

\printbibliography
\end{document}