Beamer: mark slide as [fragile] if header has fragile class.

Closes #2119.

README

@@ -3159,6 +3159,16 @@ To show the notes window, press `s` while viewing the presentation.
 Notes are not yet supported for other slide formats, but the notes
 will not appear on the slides themselves.
 
+Marking frames "fragile" in beamer
+----------------------------------
+
+Sometimes it is necessary to add the LaTeX `[fragile]` option to
+a frame in beamer (for example, when using the `minted` environment).
+This can be forced by adding the `fragile` class to the header
+introducing the slide:
+
+    # Fragile slide {.fragile}
+
 EPUB Metadata
 =============
 

changelog

@@ -96,6 +96,8 @@ pandoc (1.14)
       Harriott). Previously, pandoc hard-coded some commands to make
       tight lists in LaTeX.  Now we use a custom command instead,
       allowing the styling to be changed in a macro in the header.
+    + Beamer output: if the header introducing a slide has the
+      class `fragile`, add the `[fragile]` option to the slide (#2119).
 
   * MediaWiki writer:
 

src/Text/Pandoc/Writers/LaTeX.hs

@@ -274,10 +274,11 @@ elementToBeamer slideLevel  (Sec lvl _num (ident,classes,kvs) tit elts)
       let hasCode (Code _ _) = [True]
           hasCode _          = []
       opts <- gets stOptions
-      let fragile = not $ null $ query hasCodeBlock elts ++
+      let fragile = "fragile" `elem` classes ||
+                    not (null $ query hasCodeBlock elts ++
                                      if writerListings opts
                                         then query hasCode elts
-                                        else []
+                                        else [])
       let allowframebreaks = "allowframebreaks" `elem` classes
       let optionslist = ["fragile" | fragile] ++
                         ["allowframebreaks" | allowframebreaks]