From f7b573f80a33e7d6aba2d7a35a1a9f1a14a7ffca Mon Sep 17 00:00:00 2001
From: Sergei Slipchenko <faergeek@gmail.com>
Date: Mon, 26 Jan 2026 17:26:50 +0400
Subject: [PATCH] fix(diagnostics): assert adjusted diagnostic position #37210

Problem:
During diagnostic position adjustment we may go out of bounds
when trying to get line's length. But it's not clear what kind of
input triggers that.

Solution:
Assert and print relevant input values.
---
 runtime/lua/vim/diagnostic.lua | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index 763f057df2..c1f5ff3832 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -1390,7 +1390,25 @@ function M.set(namespace, bufnr, diagnostics, opts)
         -- avoid ending an extmark before start of the line
         if end_col == 0 then
           end_row = end_row - 1
-          end_col = #lines[end_row + 1]
+
+          local end_line = lines[end_row + 1]
+
+          if not end_line then
+            error(
+              'Failed to adjust diagnostic position to the end of a previous line. #lines in a buffer: '
+                .. #lines
+                .. ', lnum: '
+                .. diagnostic.lnum
+                .. ', col: '
+                .. diagnostic.col
+                .. ', end_lnum: '
+                .. diagnostic.end_lnum
+                .. ', end_col: '
+                .. diagnostic.end_col
+            )
+          end
+
+          end_col = #end_line
         end
       end