[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 #139019

Author: mizvekov

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
 ------------------------

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);
+}