From fdf4b7c6eef91298ad2b24e4f622229eb10cc9b5 Mon Sep 17 00:00:00 2001
From: dgp <dgp@users.sourceforge.net>
Date: Fri, 16 Jun 2017 12:56:03 +0000
Subject: Extend cases where string rep generation can be prevented.

---
 generic/tclStringObj.c | 16 ++++++++++++++++
 tests/string.test      | 15 +++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/generic/tclStringObj.c b/generic/tclStringObj.c
index 0a38836..261e01f 100644
--- a/generic/tclStringObj.c
+++ b/generic/tclStringObj.c
@@ -2966,9 +2966,25 @@ TclStringCatObjv(
 
 		Tcl_GetStringFromObj(objPtr, &numBytes); /* PANIC? */
 		if (numBytes == 0) {
+		    if (pendingPtr && pendingPtr->bytes) {
+			/*
+			 * Generating string rep of objPtr also 
+			 * generated string rep of pendingPtr.
+			 */
+			if (pendingPtr->length) {
+			    /* Can this happen? */
+			    goto foo;
+			} else {
+			    /* string-29.14 */
+			    first = objc - 1;
+			    last = 0;
+			    pendingPtr = NULL;
+			}
+		    }
 		    continue;
 		}
 		last = objc - oc;
+foo:
 		if (pendingPtr) {
 		    Tcl_GetStringFromObj(pendingPtr, &length); /* PANIC? */
 		    pendingPtr = NULL;
diff --git a/tests/string.test b/tests/string.test
index 9c43f29..7b02928 100644
--- a/tests/string.test
+++ b/tests/string.test
@@ -2024,6 +2024,21 @@ test string-29.13 {string cat, efficiency} -body {
     tcl::unsupported::representation [string cat \
 	[encoding convertto utf-8 {}] [encoding convertto utf-8 {}] [list x]]
 } -match glob -result {*, string representation "x"}
+test string-29.14 {string cat, efficiency} -setup {
+    set e [encoding convertto utf-8 {}]
+} -cleanup {
+    unset e
+} -body {
+    tcl::unsupported::representation [string cat $e $e [list x]]
+} -match glob -result {*no string representation}
+test string-29.15 {string cat, efficiency} -setup {
+    set e [encoding convertto utf-8 {}]
+    set f [encoding convertto utf-8 {}]
+} -cleanup {
+    unset e f
+} -body {
+    tcl::unsupported::representation [string cat $e $f $e $f [list x]]
+} -match glob -result {*no string representation}
 
 
 
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