[clang] [clang][AST] Fix source range of class template implicit instantiations. (PR #156011)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Why is this the correct behavior, rather than the other way around?

The patch seems to leave no way to query the source range of this declaration, rather than the definition.

On the other hand, querying the source range of the definition is currently possible, if you are explicit about it.

https://github.com/llvm/llvm-project/pull/156011