[clang-tools-extra] [clangd] Fix crash on completion with out-of-range position (PR #196112)

[Krzysztof Jędruczyk via cfe-commits]

https://github.com/kjedruczyk created https://github.com/llvm/llvm-project/pull/196112

shouldRunCompletion() checked the Expected<> from positionToOffset() via operator!() but never consumed the error with takeError(). This caused an assertion failure when a TriggerCharacter completion request had a position beyond the document bounds.

LLM was used to generate the unit test.

Fixes: #196072 

>From 6ce84e65d9ba636e10aa72ac96b057808fc3f4e5 Mon Sep 17 00:00:00 2001
From: Krzysztof Jedruczyk <kjedruczyk at bloomberg.net>
Date: Wed, 6 May 2026 17:04:24 +0100
Subject: [PATCH] [clangd] Fix crash on completion with out-of-range position

shouldRunCompletion() checked the Expected<> from positionToOffset() via
operator!() but never consumed the error with takeError(). This caused an
assertion failure when a TriggerCharacter completion request had a position
beyond the document bounds.
---
 clang-tools-extra/clangd/ClangdLSPServer.cpp  |  3 ++-
 .../clangd/unittests/ClangdLSPServerTests.cpp | 20 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
index ebd42abd2dd61..811c3b33d27ca 100644
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -1801,7 +1801,8 @@ bool ClangdLSPServer::shouldRunCompletion(
   if (!Offset) {
     vlog("could not convert position '{0}' to offset for file '{1}'",
          Params.position, Params.textDocument.uri.file());
-    return true;
+    llvm::consumeError(Offset.takeError());
+    return true; // completion code will log the error for invalid position.
   }
   return allowImplicitCompletion(*Code, *Offset);
 }
diff --git a/clang-tools-extra/clangd/unittests/ClangdLSPServerTests.cpp b/clang-tools-extra/clangd/unittests/ClangdLSPServerTests.cpp
index 95bf5e54fc792..0aca1433614dc 100644
--- a/clang-tools-extra/clangd/unittests/ClangdLSPServerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ClangdLSPServerTests.cpp
@@ -492,6 +492,26 @@ TEST_F(LSPTest, DiagModuleTest) {
   EXPECT_THAT(Client.diagnostics("foo.cpp"),
               llvm::ValueIs(testing::ElementsAre(diagMessage(DiagMsg))));
 }
+
+// Regression test for https://github.com/llvm/llvm-project/issues/196072.
+TEST_F(LSPTest, CompletionOutOfRangePosition) {
+  auto &Client = start();
+  Client.didOpen("foo.cpp", "int x;");
+  auto &Reply = Client.call(
+      "textDocument/completion",
+      llvm::json::Object{
+          {"textDocument", Client.documentID("foo.cpp")},
+          {"position", llvm::json::Object{{"line", 97}, {"character", 0}}},
+          {"context",
+           llvm::json::Object{
+               {"triggerKind", 2},
+               {"triggerCharacter", ">"},
+           }},
+      });
+  auto Result = Reply.take();
+  ASSERT_TRUE(!!Result) << "Expected a response, not a server crash";
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang