Uhm, could you please explain what do you mean by 'serialized'?

It needs to be preserved when writing an ICE into a PCH / module file. See the ASTWriter / ASTReader.

Aha. I did add handling there but it raises questions:

1. This will silently break with different AST serialization versions. I'm not sure how to handle it, since VERSION_MINOR isn't even read back.
2. Does this need a test? How to write one? Like ./test/PCH/include-timestamp.cpp, using llvm-bcanalyzer?

Don't worry about breaking the serialization format. We do not maintain AST file format compatibility in general (neither between major releases nor between any two arbitrary SVN revisions). [We should probably maintain file format compatibility between patch releases, but I don't think that works right now because we check the full version number including the patch level when reading a file.]

Please do add a test: what you need to do is create a PCH containing an implicit cast expression and then import that AST file and do anything to check that the value is preserved (such as inspecting the output of -ast-dump). Maybe you could add this to the existing test/PCH/cxx_exprs.cpp test, which already does most of what you want, but doesn't have any FileCheck tests on the -ast-dump output yet.

Uhm, so i have tried to add a test.
With git master, the -ast-dump from the first runline of test/PCH/cxx_exprs.cpp contains:

|-TypedefDecl 0x55a348020b88 </build/clang/test/PCH/cxx_exprs.h:5:1, col:44> col:44 referenced static_cast_result 'typeof (static_cast<void *>(0))':'void *'
| `-TypeOfExprType 0x55a348020b50 'typeof (static_cast<void *>(0))' sugar
|   |-ParenExpr 0x55a348020b28 <col:19, col:42> 'void *'
|   | `-CXXStaticCastExpr 0x55a348020af8 <col:20, col:41> 'void *' static_cast<void *> <NoOp>
|   |   `-ImplicitCastExpr 0x55a348020ae0 <col:40> 'void *' <NullToPointer>
|   |     `-IntegerLiteral 0x55a348020aa8 <col:40> 'int' 0
|   `-PointerType 0x55a347fe6390 'void *'
|     `-BuiltinType 0x55a347fe5b90 'void'

But the last runline (which uses PCH) does not have that.
Unfortunately i know next to nothing about PCH / AST serialization, so i'm not sure how to workaround that yet.
It does contain `-<undeserialized declarations>, so maybe i have to somehow force full deserialization?