From b68d03cba45806254b0eb7cafabb507fa38b9044 Mon Sep 17 00:00:00 2001
From: Perry Fraser <perry@frasers.org>
Date: Tue, 27 Sep 2022 10:45:51 -0400
Subject: [PATCH] Don't error when transposing an empty buffer

---
 stdlib/REPL/src/LineEdit.jl  | 2 +-
 stdlib/REPL/test/lineedit.jl | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/stdlib/REPL/src/LineEdit.jl b/stdlib/REPL/src/LineEdit.jl
index 047411c21d12f..348defe79d197 100644
--- a/stdlib/REPL/src/LineEdit.jl
+++ b/stdlib/REPL/src/LineEdit.jl
@@ -1101,7 +1101,7 @@ end
 
 function edit_transpose_chars(buf::IOBuffer)
     # Moving left but not transpoing anything is intentional, and matches Emacs's behavior
-    eof(buf) && char_move_left(buf)
+    eof(buf) && position(buf) !== 0 && char_move_left(buf)
     position(buf) == 0 && return false
     char_move_left(buf)
     pos = position(buf)
diff --git a/stdlib/REPL/test/lineedit.jl b/stdlib/REPL/test/lineedit.jl
index 90badda189348..3d68ad1316e02 100644
--- a/stdlib/REPL/test/lineedit.jl
+++ b/stdlib/REPL/test/lineedit.jl
@@ -376,6 +376,8 @@ let buf = IOBuffer()
     LineEdit.edit_transpose_chars(buf)
     @test content(buf) == "βγαδε"
 
+
+    # Transposing a one-char buffer should behave like Emacs
     seek(buf, 0)
     @inferred(LineEdit.edit_clear(buf))
     edit_insert(buf, "a")
@@ -385,6 +387,13 @@ let buf = IOBuffer()
     LineEdit.edit_transpose_chars(buf)
     @test content(buf) == "a"
     @test position(buf) == 0
+
+    # Transposing an empty buffer shouldn't implode
+    seek(buf, 0)
+    LineEdit.edit_clear(buf)
+    LineEdit.edit_transpose_chars(buf)
+    @test content(buf) == ""
+    @test position(buf) == 0
 end
 
 @testset "edit_word_transpose" begin