`ASTReader::FinishedDeserializing()` calls
`adjustDeductedFunctionResultType(...)` [0], which in turn calls
`FunctionDecl::getMostRecentDecl()`[1]. In modules builds,
`getMostRecentDecl()` may reach out to the `ASTReader` and start
deserializing again. Starting deserialization starts `ReadTimer`;
however, `FinishedDeserializing()` doesn't call `stopTimer()` until
after it's call to `adjustDeductedFunctionResultType(...)` [2]. As a
result, we hit an assert checking that we don't start an already started
timer [3]. To fix this, we simply don't start the timer if it's already
running.
Unfortunately I don't have a test case for this yet as modules builds
are notoriously difficult to reduce.
[0]:
4d2288d318/clang/lib/Serialization/ASTReader.cpp (L11053)
[1]:
4d2288d318/clang/lib/AST/ASTContext.cpp (L3804)
[2]:
4d2288d318/clang/lib/Serialization/ASTReader.cpp (L11065-L11066)
[3]:
4d2288d318/llvm/lib/Support/Timer.cpp (L150)
904b4f5a27