bug#50538: [PATCH v3] 28.0.50; electric-pair-mode fails to pair double quotes in some cases in CC mode

[Jim Porter <jporterbugs@gmail.com>]

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Ok, I've attached an updated patch that should apply cleanly on top of 
the patches for bug#49518. The tests still pass locally for me. If 
there's anything else that needs fixed/adjusted, just let me know.

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From c4d069c010d5e526fa85d5800ff8de5bb6a52b7e Mon Sep 17 00:00:00 2001
From: Jim Porter <jporterbugs@gmail.com>
Date: Thu, 16 Sep 2021 14:32:17 -0700
Subject: [PATCH] Improve behavior of 'electric-pair-mode' in 'cc-mode'

This ensures that quotes are paired correctly within comments, allows for
insertion of quote pairs immediately before another quote, and allows
inserting quote pairs within a string (thus splitting the string in two).

* lisp/progmodes/cc-mode.el (c-electric-pair-inhibit-predicate):
Inhibit insertion of paired quote in fewer cases.
* test/lisp/electric-tests.el (define-electric-pair-test):
Add 'c-mode' to list of modes to test by default.
---
 lisp/progmodes/cc-mode.el   | 22 +++++++++++++++-------
 test/lisp/electric-tests.el |  5 ++++-
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
index 8b30241449..80137590c7 100644
--- a/lisp/progmodes/cc-mode.el
+++ b/lisp/progmodes/cc-mode.el
@@ -2549,18 +2549,26 @@ c-electric-pair-inhibit-predicate
 
 At the time of call, point is just after the newly inserted CHAR.
 
-When CHAR is \", t will be returned unless the \" is marked with
-a string fence syntax-table text property.  For other characters,
-the default value of `electric-pair-inhibit-predicate' is called
-and its value returned.
+When CHAR is \" and not within a comment, t will be returned if
+the quotes on the current line are already balanced (i.e. if the
+last \" is not marked with a string fence syntax-table text
+property).  For other cases, the default value of
+`electric-pair-inhibit-predicate' is called and its value
+returned.
 
 This function is the appropriate value of
 `electric-pair-inhibit-predicate' for CC Mode modes, which mark
 invalid strings with such a syntax table text property on the
 opening \" and the next unescaped end of line."
-  (if (eq char ?\")
-      (not (equal (get-text-property (1- (point)) 'c-fl-syn-tab) '(15)))
-    (funcall (default-value 'electric-pair-inhibit-predicate) char)))
+  (or (and (eq char ?\")
+	   (not (memq (cadr (c-semi-pp-to-literal (1- (point)))) '(c c++)))
+	   (let ((last-quote (save-match-data
+			       (save-excursion
+				 (goto-char (c-point 'eoll))
+				 (search-backward "\"")))))
+	     (not (equal (c-get-char-property last-quote 'c-fl-syn-tab)
+			 '(15)))))
+      (funcall (default-value 'electric-pair-inhibit-predicate) char)))
 
 \f
 ;; Support for C
diff --git a/test/lisp/electric-tests.el b/test/lisp/electric-tests.el
index 46bcbfce30..4b3a39b85f 100644
--- a/test/lisp/electric-tests.el
+++ b/test/lisp/electric-tests.el
@@ -32,6 +32,9 @@
 (require 'elec-pair)
 (require 'cl-lib)
 
+;; When running tests in c-mode, use single-line comments (//).
+(add-hook 'c-mode-hook (lambda () (c-toggle-comment-style -1)))
+
 (defun call-with-saved-electric-modes (fn)
   (let ((saved-electric (if electric-pair-mode 1 -1))
         (saved-layout (if electric-layout-mode 1 -1))
@@ -174,7 +177,7 @@ define-electric-pair-test
           expected-string
           expected-point
           bindings
-          (modes '(quote (ruby-mode js-mode python-mode)))
+          (modes '(quote (ruby-mode js-mode python-mode c-mode)))
           (test-in-comments t)
           (test-in-strings t)
           (test-in-code t)
-- 
2.25.1