[clang] [clang][Sema] Handle function parameter packs in `PackDeductionScope::addPacks` (PR #215235)

[Yanzuo Liu via cfe-commits]

================
@@ -935,6 +935,11 @@ class PackDeductionScope {
       S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
       for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
         unsigned Depth, Index;
+
+        // Function parameter packs cannot be deduced.
+        if (isa_and_nonnull<ParmVarDecl>(
+                dyn_cast<NamedDecl *>(Unexpanded[I].first)))
+          continue;
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zwuis wrote:

> ... This is confusing, for example
> 
> ```cpp
> void f(auto... param) {
>   auto g = [](decltype(param)... pp) {};
>   g(param...);
> }
> ```
> 
> in such context, `pp` are indeed deduced from (though substituted) `param...`.

There is no deduction triggered by `g(param...)`. `pp` is expanded at the same time when `decltype(param)...` is expanded. We can observe exact parameter types of the lambda if we remove `g(param...)` and instantiate `f`.

https://github.com/llvm/llvm-project/pull/215235