Improve `switch-to-buffer' in strongly dedicated windows (Bug#20472)

* lisp/window.el (switch-to-buffer-in-dedicated-window): New option.
(switch-to-buffer): If the selected window is strongly dedicated
to its buffer, signal error before prompting for buffer name.  Handle
`switch-to-buffer-in-dedicated-window'.  (Bug#20472)
* doc/lispref/windows.texi (Switching Buffers): Document
`switch-to-buffer-in-dedicated-window'.

doc/lispref/windows.texi

@@ -2005,16 +2005,49 @@ list---both the global buffer list and the selected frame's buffer
 list (@pxref{Buffer List}).  However, this is not done if the
 optional argument @var{norecord} is non-@code{nil}.
 
-Sometimes, @code{switch-to-buffer} may be unable to display the buffer
-in the selected window.  This happens if the selected window is a
-minibuffer window, or if the selected window is strongly dedicated to
-its buffer (@pxref{Dedicated Windows}).  In that case, the command
-normally tries to display the buffer in some other window, by invoking
-@code{pop-to-buffer} (see below).  However, if the optional argument
-@var{force-same-window} is non-@code{nil}, it signals an error
-instead.
+Sometimes, the selected window may not be suitable for displaying the
+buffer.  This happens if the selected window is a minibuffer window, or
+if the selected window is strongly dedicated to its buffer
+(@pxref{Dedicated Windows}).  In such cases, the command normally tries
+to display the buffer in some other window, by invoking
+@code{pop-to-buffer} (see below).
+
+If the optional argument @var{force-same-window} is non-@code{nil} and
+the selected window is not suitable for displaying the buffer, this
+function always signals an error when called non-interactively.  In
+interactive use, if the selected window is a minibuffer window, this
+function will try to use some other window instead.  If the selected
+window is strongly dedicated to its buffer, the option
+@code{switch-to-buffer-in-dedicated-window} described next can be used
+to proceed.
 @end deffn
 
+@defopt switch-to-buffer-in-dedicated-window
+This option, if non-@code{nil}, allows @code{switch-to-buffer} to
+proceed when called interactively and the selected window is strongly
+dedicated to its buffer.
+
+The following values are respected:
+
+@table @code
+@item nil
+Disallows switching and signals an error as in non-interactive use.
+
+@item prompt
+Prompts the user whether to allow switching.
+
+@item pop
+Invokes @code{pop-to-buffer} to proceed.
+
+@item t
+Marks the selected window as non-dedicated and proceeds.
+@end table
+
+When called non-interactively, @code{switch-to-buffer} always signals an
+error when the selected window is dedicated to its buffer and
+@var{force-same-window} is non-@code{nil}.
+@end defopt
+
 By default, @code{switch-to-buffer} shows the buffer at its position of
 @code{point}.  This behavior can be tuned using the following option.
 

etc/NEWS

@@ -165,7 +165,7 @@ buffers to allow certain parts of the text to be writable.
 ** A new function `directory-files-recursively' returns all matching
 files (recursively) under a directory.
 
-** The new `directory-name-p' can be used to check whether a file
+** The new function `directory-name-p' can be used to check whether a file
 name (as returned from, for instance, `file-name-all-completions' is
 a directory file name.  It returns non-nil if the last character in
 the name is a forward slash.
@@ -585,8 +585,8 @@ nil to disable this.
 fitting for use in money calculations; factorial works with
 non-integer inputs.
 
-** HideIfDef mode now support full C/C++ expressions, argumented macro expansions
-, interactive macro evaluation and automatic scanning of #defined symbols.
+** HideIfDef mode now support full C/C++ expressions, argumented macro expansions,
+interactive macro evaluation and automatic scanning of #defined symbols.
 
 *** New custom variable `hide-ifdef-header-regexp' to define C/C++ header file
 name patterns. Default case-insensitive .h, .hh, .hpp and .hxx.
@@ -755,7 +755,7 @@ If you want the old behavior of calling the function in the buffer
 from which the minibuffer was entered, call it with the new argument
 `switch-buffer'.
 
-** window-configurations no longer record the buffers's marks.
+** window-configurations no longer record the buffers' marks.
 
 ** inhibit-modification-hooks now also inhibits lock-file checks, as well as
 active region handling.
@@ -960,6 +960,11 @@ fullwidth frames, the behavior may depend on the toolkit used.
 windows without "fixing" it.  It's supported by `fit-window-to-buffer',
 `temp-buffer-resize-mode' and `display-buffer'.
 
++++
+** New option `switch-to-buffer-in-dedicated-window' allows to customize
+how `switch-to-buffer' proceeds interactively when the selected window
+is strongly dedicated to its buffer.
+
 ** Tearoff menus and detachable toolbars for Gtk+ has been removed.
 Those features have been deprecated in Gtk+ for a long time.
 

lisp/window.el

@@ -6884,6 +6884,33 @@ the selected window or never appeared in it before, or if
   :group 'windows
   :version "24.3")
 
+(defcustom switch-to-buffer-in-dedicated-window nil
+  "Allow switching to buffer in strongly dedicated windows.
+If non-nil, allow `switch-to-buffer' to proceed when called
+interactively and the selected window is strongly dedicated to
+its buffer.
+
+The following values are recognized:
+
+nil - disallow switching; signal an error
+
+prompt - prompt user whether to allow switching
+
+pop - perform `pop-to-buffer' instead
+
+t - undedicate selected window and switch
+
+When called non-interactively, `switch-to-buffer' always signals
+an error when the selected window is dedicated to its buffer and
+FORCE-SAME-WINDOW is non-nil."
+  :type '(choice
+	  (const :tag "Disallow" nil)
+	  (const :tag "Prompt" prompt)
+	  (const :tag "Pop" pop)
+	  (const :tag "Allow" t))
+  :group 'windows
+  :version "25.1")
+
 (defun switch-to-buffer (buffer-or-name &optional norecord force-same-window)
   "Display buffer BUFFER-OR-NAME in the selected window.
 
@@ -6891,10 +6918,12 @@ WARNING: This is NOT the way to work on another buffer temporarily
 within a Lisp program!  Use `set-buffer' instead.  That avoids
 messing with the window-buffer correspondences.
 
-If the selected window cannot display the specified
-buffer (e.g. if it is a minibuffer window or strongly dedicated
-to another buffer), call `pop-to-buffer' to select the buffer in
-another window.
+If the selected window cannot display the specified buffer
+because it is a minibuffer window or strongly dedicated to
+another buffer, call `pop-to-buffer' to select the buffer in
+another window.  In interactive use, if the selected window is
+strongly dedicated to its buffer, the value of the option
+`switch-to-buffer-in-dedicated-window' specifies how to proceed.
 
 If called interactively, read the buffer name using the
 minibuffer.  The variable `confirm-nonexistent-file-or-buffer'
@@ -6911,8 +6940,9 @@ at the front of the buffer list, and do not make the window
 displaying it the most recently selected one.
 
 If optional argument FORCE-SAME-WINDOW is non-nil, the buffer
-must be displayed in the selected window; if that is impossible,
-signal an error rather than calling `pop-to-buffer'.
+must be displayed in the selected window when called
+non-interactively; if that is impossible, signal an error rather
+than calling `pop-to-buffer'.
 
 The option `switch-to-buffer-preserve-window-point' can be used
 to make the buffer appear at its last position in the selected
@@ -6920,7 +6950,25 @@ window.
 
 Return the buffer switched to."
   (interactive
-   (list (read-buffer-to-switch "Switch to buffer: ") nil 'force-same-window))
+   (let ((force-same-window
+          (cond
+           ((window-minibuffer-p) nil)
+           ((not (eq (window-dedicated-p) t)) 'force-same-window)
+           ((pcase switch-to-buffer-in-dedicated-window
+              (`nil (user-error
+                     "Cannot switch buffers in a dedicated window"))
+              (`prompt
+               (if (y-or-n-p
+                    (format "Window is dedicated to %s; undedicate it"
+                            (window-buffer)))
+                   (progn
+                     (set-window-dedicated-p nil nil)
+                     'force-same-window)
+                 (user-error
+                  "Cannot switch buffers in a dedicated window")))
+              (`pop nil)
+              (_ (set-window-dedicated-p nil nil) 'force-same-window))))))
+     (list (read-buffer-to-switch "Switch to buffer: ") nil force-same-window)))
   (let ((buffer (window-normalize-buffer-to-switch-to buffer-or-name)))
     (cond
      ;; Don't call set-window-buffer if it's not needed since it