[clang] Serialization: support hashing null template arguments (#141890)

clang/docs/ReleaseNotes.rst

@@ -821,6 +821,7 @@ Miscellaneous Clang Crashes Fixed
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 - Fixed crash when ``-print-stats`` is enabled in compiling IR files. (#GH131608)
+- Fix code completion crash involving PCH serialzied templates. (#GH139019)
 
 OpenACC Specific Changes
 ------------------------

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;

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);
+}