Clangd sometimes fails to deduce non-dependent type inside template about clangd HOT 3 OPEN

If we look at the AST of waldo, we'll see the call expression find_if is modeled as an UnresolvedLookupExpr, and thus the type of such is dependent.

I think there's improvement room regarding the clang side, such as we can eagerly perform the lookup of call expressions with non-dependent arguments, and thereby, the result type would be non-dependent at the time clangd consumes the AST. But I could also envision some earlier diagnostics brought over by such drastic changes might break some existing codebases.

We can definitely invent some heuristics at clangd side - I presume that's basically a duplicate of a somewhat CXXNameLookup.

from clangd.

Change the second argument to find_if() to not be a lambda

Fun enough that the lambda expression plays the trick here - probably we are somehow depending on the lambda's dependency?

from clangd.

While I haven't checked the details, my guess is that, since lambdas are modelled as local classes, and the local class is declared inside the function's DeclContext, it therefore becomes a different type in every instantiation of the function template.

If that's the case, there might be an opportunity to optimize the frontend itself here, to avoid instantiating find_if separately for each instantiation of waldo.

from clangd.