[clang] [Clang] Correctly construct template arguments for template template parameters (PR #76811)

[Erich Keane via cfe-commits]

erichkeane wrote:

So these sort of crashes are simply that the generated Multi-level Template Argument List doesn't match what is in the AST.  This function (getInstantiationArgs) tries to 'recreate' them, so if it is 'wrong' than we get a crash.  I suspect we're just missing some sort of awkward case in this situation.  I'd suggest looking to see if you can figure out where the correct arguments are.

One thing that Concepts did was make it so that this function is used in many more cases, and at many more times. I wouldn't be surprised if it is now being used in situations where the decl-context hasn't been appropriately updated (that is, is either not set at all, or is in the uninstantiated version).  In those cases, we have to special case them/figure out what they are supposed to be and use those isntead.

https://github.com/llvm/llvm-project/pull/76811