From 8a000e3ecc330ff8a4953ebe8c7da9a54eca5c58 Mon Sep 17 00:00:00 2001
From: John MacFarlane <jgm@berkeley.edu>
Date: Mon, 12 Jun 2017 09:16:05 +0200
Subject: Markdown writer: don't allow soft break in header.

Closes #3736.
---
 src/Text/Pandoc/Writers/Markdown.hs |  7 +++++++
 test/command/3736.md                | 25 +++++++++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 test/command/3736.md

diff --git a/src/Text/Pandoc/Writers/Markdown.hs b/src/Text/Pandoc/Writers/Markdown.hs
index 69243a214..3ac677943 100644
--- a/src/Text/Pandoc/Writers/Markdown.hs
+++ b/src/Text/Pandoc/Writers/Markdown.hs
@@ -475,6 +475,8 @@ blockToMarkdown' opts (Header level attr inlines) = do
                                     space <> attrsToMarkdown attr
                      | otherwise -> empty
   contents <- inlineListToMarkdown opts $
+                 -- ensure no newlines; see #3736
+                 walk lineBreakToSpace $
                  if level == 1 && plain
                     then capitalize inlines
                     else inlines
@@ -1203,3 +1205,8 @@ toSubscript c
                  Just $ chr (0x2080 + (ord c - 48))
   | isSpace c = Just c
   | otherwise = Nothing
+
+lineBreakToSpace :: Inline -> Inline
+lineBreakToSpace LineBreak = Space
+lineBreakToSpace SoftBreak = Space
+lineBreakToSpace x         = x
diff --git a/test/command/3736.md b/test/command/3736.md
new file mode 100644
index 000000000..b66e0a359
--- /dev/null
+++ b/test/command/3736.md
@@ -0,0 +1,25 @@
+```
+% pandoc --wrap=preserve -f html -t markdown
+<h2>hi
+there</h2>
+^D
+hi there
+--------
+```
+
+```
+% pandoc --wrap=preserve -f html -t markdown
+<h2>hi <em>there
+again</em></h2>
+^D
+hi *there again*
+----------------
+```
+
+```
+% pandoc --wrap=preserve -f html -t markdown
+<h2>hi<br>there</h2>
+^D
+hi there
+--------
+```
-- 
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