[llvm-branch-commits] [clang] Backport: [clang] Serialization: support hashing null template arguments (PR #141957)

[Matheus Izvekov via llvm-branch-commits]

https://github.com/mizvekov created https://github.com/llvm/llvm-project/pull/141957

When performing overload resolution during code completion, clang will allow incomplete substitutions in more places than would be allowed for valid code, because for completion to work well, it needs clang to keep going so it can explore the space of possibilities.

Notably, we accept instantiating declarations will null template arguments, and this works fine, except that when lazily loading serialzied templated declarations, the template argument hasher assumes null arguments can't be used.

This patch makes the hasher happily accept that.

Fixes https://github.com/llvm/llvm-project/issues/139019

>From 7759bb57c24390797ee34fa58a5e1234f5aa9369 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Thu, 29 May 2025 00:28:21 -0300
Subject: [PATCH] [clang] Serialization: support hashing null template
 arguments

When performing overload resolution during code completion,
clang will allow incomplete substitutions in more places
than would be allowed for valid code, because for completion to work well,
it needs clang to keep going so it can explore the space of possibilities.

Notably, we accept instantiating declarations will null template arguments,
and this works fine, except that when lazily loading serialzied templated
declarations, the template argument hasher assumes null arguments can't
be used.

This patch makes the hasher happily accept that.

Fixes https://github.com/llvm/llvm-project/issues/139019
---
 clang/docs/ReleaseNotes.rst                   |  1 +
 .../Serialization/TemplateArgumentHasher.cpp  |  4 ++-
 clang/test/CodeCompletion/GH139019.cpp        | 26 +++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/CodeCompletion/GH139019.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 7b84210fddab3..262bf4e3d4f5b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1118,6 +1118,7 @@ Miscellaneous Clang Crashes Fixed
 
 - Fixed a crash when an unscoped enumeration declared by an opaque-enum-declaration within a class template
   with a dependent underlying type is subject to integral promotion. (#GH117960)
+- Fix code completion crash involving PCH serialzied templates. (#GH139019)
 
 OpenACC Specific Changes
 ------------------------
diff --git a/clang/lib/Serialization/TemplateArgumentHasher.cpp b/clang/lib/Serialization/TemplateArgumentHasher.cpp
index 598f098f526d0..5fd6941256fe2 100644
--- a/clang/lib/Serialization/TemplateArgumentHasher.cpp
+++ b/clang/lib/Serialization/TemplateArgumentHasher.cpp
@@ -65,7 +65,9 @@ void TemplateArgumentHasher::AddTemplateArgument(TemplateArgument TA) {
 
   switch (Kind) {
   case TemplateArgument::Null:
-    llvm_unreachable("Expected valid TemplateArgument");
+    // These can occur in incomplete substitutions performed with code
+    // completion (see PartialOverloading).
+    break;
   case TemplateArgument::Type:
     AddQualType(TA.getAsType());
     break;
diff --git a/clang/test/CodeCompletion/GH139019.cpp b/clang/test/CodeCompletion/GH139019.cpp
new file mode 100644
index 0000000000000..fed35b38362a1
--- /dev/null
+++ b/clang/test/CodeCompletion/GH139019.cpp
@@ -0,0 +1,26 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/test.hpp -emit-pch -o %t/1.pch
+// RUN: %clang_cc1 -std=c++20 %t/test.cpp -include-pch %t/1.pch -code-completion-at=%t/test.cpp:7:17
+
+//--- test.hpp
+#pragma once
+class provider_t
+{
+  public:
+    template<class T>
+    void emit(T *data)
+    {}
+};
+
+//--- test.cpp
+#include "test.hpp"
+
+void test()
+{
+    provider_t *focus;
+    void *data;
+    focus->emit(&data);
+}