Controlled error for unknown font series

Fixes #856.

Author: josephwright

CHANGELOG.md

@@ -8,6 +8,11 @@ Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to
 
 ## [Unreleased]
 
+### Fixed
+
+- Issue controlled warning for unknown font series (see
+  issue [\#856](https://github.com/josephwright/siunitx/issues/856))
+
 ## [v3.5.0] - 2026-03-14
 
 ### Added

siunitx-print.dtx

@@ -623,7 +623,11 @@
       }
   }
 \cs_new:Npn \@@_convert_series:n #1
-  { \tl_use:c { l_@@_version_ #1 _tl } }
+  {
+    \tl_if_exist:cTF { l_@@_version_ #1 _tl }
+      { \tl_use:c { l_@@_version_ #1 _tl } }
+      { \msg_expandable_error:nnn { siunitx } { unknown-series } {#1} }
+  }
 \cs_generate_variant:Nn \@@_convert_series:n { v }
 \cs_new_protected:Npn \@@_math_auxi:n #1
   {
@@ -1113,6 +1117,13 @@
 % \end{macro}
 % \end{macro}
 %
+% \subsection{Messages}
+%
+%    \begin{macrocode}
+\msg_new:nnn { siunitx } { unknown-series }
+  { Unknown~font~series~"#1"! }
+%    \end{macrocode}
+%
 % \subsection{Standard settings for module options}
 %
 % Some of these follow naturally from the point of definition

testfiles/siunitx-print.luatex.tlg

@@ -3438,4 +3438,60 @@ l. ...  }
 <argument> \l_tmpa_box 
 l. ...  }
 ============================================================
+============================================================
+TEST 26: Unknown series
+============================================================
+! Use of \??? doesn't match its definition.
+<argument> \???  
+                 ! Package siunitx Error: Unknown font series "foo"!
+l. ...  }
+If you say, e.g., `\def\a1{...}', then you must always
+put `1' after `\a', since control sequence names are
+made up of letters only. The macro here has not been
+followed by the required stuff, so I'm ignoring it.
+> \box...=
+\hbox(4.30554+0.0)x8.78014, direction TLT
+.\mathon
+.\OML/cmm/m/it/10 m
+.\mathoff
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+! Use of \??? doesn't match its definition.
+<argument> \???  
+                 ! Package siunitx Error: Unknown font series "foo"!
+l. ...  }
+If you say, e.g., `\def\a1{...}', then you must always
+put `1' after `\a', since control sequence names are
+made up of letters only. The macro here has not been
+followed by the required stuff, so I'm ignoring it.
+> \box...=
+\hbox(4.30554+0.0)x8.78014, direction TLT
+.\mathon
+.\OML/cmm/m/it/10 m
+.\mathoff
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+LaTeX Font Warning: Font shape `TU/lmr/foo/n' undefined
+(Font)              using `TU/lmr/m/n' instead on input line ....
+> \box...=
+\hbox(4.42+0.0)x8.33, direction TLT
+.\hbox(4.42+0.0)x8.33, direction TLT
+..\TU/lmr/m/n/10 m
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+> \box...=
+\hbox(4.42+0.0)x8.33, direction TLT
+.\mathon
+.\hbox(4.42+0.0)x8.33, direction TLT
+..\hbox(4.42+0.0)x8.33, direction TLT
+...\TU/lmr/foo/n/10 m
+.\mathoff
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+============================================================
 (siunitx-print.aux)
+LaTeX Font Warning: Some font shapes were not available, defaults substituted.

testfiles/siunitx-print.lvt

@@ -332,4 +332,12 @@
     \test:nn { unit } { \ensuremath { \times } }
   }
 
+\TEST { Unknown~series }
+  {
+    \keys_set:nn { siunitx } 
+      { reset-text-series = false, text-series-to-math = true }
+    \cs_set_nopar:cpn { f@series } { foo }
+    \test:nn { unit } { m }
+  }
+
 \end{document}

testfiles/siunitx-print.tlg

@@ -3402,4 +3402,60 @@ l. ...  }
 <argument> \l_tmpa_box 
 l. ...  }
 ============================================================
+============================================================
+TEST 26: Unknown series
+============================================================
+! Use of \??? doesn't match its definition.
+<argument> \???  
+                 ! Package siunitx Error: Unknown font series "foo"!
+l. ...  }
+If you say, e.g., `\def\a1{...}', then you must always
+put `1' after `\a', since control sequence names are
+made up of letters only. The macro here has not been
+followed by the required stuff, so I'm ignoring it.
+> \box...=
+\hbox(4.30554+0.0)x8.78014
+.\mathon
+.\OML/cmm/m/it/10 m
+.\mathoff
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+! Use of \??? doesn't match its definition.
+<argument> \???  
+                 ! Package siunitx Error: Unknown font series "foo"!
+l. ...  }
+If you say, e.g., `\def\a1{...}', then you must always
+put `1' after `\a', since control sequence names are
+made up of letters only. The macro here has not been
+followed by the required stuff, so I'm ignoring it.
+> \box...=
+\hbox(4.30554+0.0)x8.78014
+.\mathon
+.\OML/cmm/m/it/10 m
+.\mathoff
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+LaTeX Font Warning: Font shape `OT1/cmr/foo/n' undefined
+(Font)              using `OT1/cmr/m/n' instead on input line ....
+> \box...=
+\hbox(4.30554+0.0)x8.33336
+.\hbox(4.30554+0.0)x8.33336
+..\OT1/cmr/m/n/10 m
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+> \box...=
+\hbox(4.30554+0.0)x8.33336
+.\mathon
+.\hbox(4.30554+0.0)x8.33336
+..\hbox(4.30554+0.0)x8.33336
+...\OT1/cmr/foo/n/10 m
+.\mathoff
+! OK.
+<argument> \l_tmpa_box 
+l. ...  }
+============================================================
 (siunitx-print.aux)
+LaTeX Font Warning: Some font shapes were not available, defaults substituted.