In reply to @hmenke:matrix.org
This of course does not work for stuff that has to be expandable like \pgfutil@ifnextchar or so. Most of the time I don't care how it figures out which branch to take but only which branch is taken.

In reply to @hmenke:matrix.org
I don't think it's non-trivial. When TeX sees \pgfutil@ifnextchar it reads #1#2#3 from the input and starts expansion. As soon as TeX reads the next token from the input it means that \pgfutil@ifnextchar has finished.

hmenke: To solve https://github.com/pgf-tikz/pgf/issues/1058, which choice do you prefer?

• make sloped always taking current transformation matrix into account, as demonstrated in this comment, or
• provide a new option, maybe transform sloped and remain the current behavior of sloped

--- a/tex/generic/pgf/basiclayer/pgfcoretransformations.code.tex
+++ b/tex/generic/pgf/basiclayer/pgfcoretransformations.code.tex
@@ -466,6 +466,7 @@
 % }
 %
 \def\pgftransformlineattime#1#2#3{%
+  \pgfgettransformentries\pgf@tempaa\pgf@tempab\pgf@tempba\pgf@tempbb\pgf@notimportantx\pgf@notimportanty
   \pgf@process{#2}%
   \pgf@xb=\pgf@x% xb/yb = start point
   \pgf@yb=\pgf@y%
@@ -479,6 +480,18 @@
   \ifpgfslopedattime%
     \advance\pgf@xc by-\pgf@xb%
     \advance\pgf@yc by-\pgf@yb%
+    % Now that we get the tangent vector without transformation
+    % It suffices to apply the non-shift part of the transformation
+    {%
+      \ifpgfresetnontranslationattime
+        \pgfsettransformentries\pgf@tempaa\pgf@tempab\pgf@tempba\pgf@tempbb{0pt}{0pt}%
+        \pgf@pos@transform{\pgf@xc}{\pgf@yc}%
+        \global\pgf@xc\pgf@xc
+        \global\pgf@yc\pgf@yc
+      \fi
+    }%
+    % OK, now xc and yc are correctly tangent to the line
+    % Continue the usual routine
     \ifpgfallowupsidedownattime%
     \else%
       \ifdim\pgf@xc<0pt%