From 391bf996873721fdcb8d68003d96121b378d2654 Mon Sep 17 00:00:00 2001
From: dgp <dgp@users.sourceforge.net>
Date: Wed, 6 May 2020 19:22:25 +0000
Subject: Restore safe calls of Invalid().

---
 generic/tclUtf.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/generic/tclUtf.c b/generic/tclUtf.c
index 199ff83..24ec3d2 100644
--- a/generic/tclUtf.c
+++ b/generic/tclUtf.c
@@ -659,9 +659,6 @@ Tcl_UtfNext(
     int left;
     const char *next;
 
-    if (Invalid(src)) {
-	return src + 1;
-    }
     left = totalBytes[UCHAR(*src)];
     next = src + 1;
     while (--left) {
@@ -675,6 +672,16 @@ Tcl_UtfNext(
 	}
 	next++;
     }
+    /*
+     * Call Invalid() here only if required conditions are met:
+     *    src[0] is known a lead byte.
+     *    src[1] is known a trail byte.
+     * Especially important to prevent calls when src[0] == '\xF8' or '\xFC'
+     * See tests utf-6.37 through utf-6.43 through valgrind or similar tool.
+     */
+    if ((next == src + 1) || Invalid(src)) {
+	return src + 1;
+    }
     return next;
 }
 
-- 
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