[clang] fix missing initialization of original number of expansions

[mizvekov]

Summary

When expanding undeclared function parameters, we should initialize
the original number of expansions, if known, before trying to expand
them, otherwise a length mismatch with an outer pack might not be
diagnosed.

Fixes PR56094.

Signed-off-by: Matheus Izvekov <[e-mail]>

[aaron.ballman]

This looks reasonable to me, but I leave it to others to do the final sign-off. Please be sure to add a release note for the fix though.

clang/docs/ReleaseNotes.rst

- Fix a crash when evaluating a multi-dimensional array's array filler
   expression is element-dependent. This fixes
   `Issue 50601 <https://github.com/llvm/llvm-project/issues/56016>`_.
 - Fixed a crash-on-valid with consteval evaluation of a list-initialized
   constructor for a temporary object. This fixes
   `Issue 55871 <https://github.com/llvm/llvm-project/issues/55871>`_.
 - Fix `#57008 <https://github.com/llvm/llvm-project/issues/57008>`_ - Builtin
   C++ language extension type traits instantiated by a template with unexpected
   number of arguments cause an assertion fault.
+- Fix multi-level pack expansion of undeclared function parameters.
+  This fixes `Issue 56094 <https://github.com/llvm/llvm-project/issues/56094>`_.
+
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now correctly diagnose as ill-formed a constant expression where an
   enum without a fixed underlying type is set to a value outside the range of
   the enumeration's values. Due to the extended period of time this bug was
   present in major C++ implementations (including Clang), this error has the
   ability to be downgraded into a warning (via: -Wno-error=enum-constexpr-conversion)

clang/lib/Sema/TreeTransform.h

#endif
     QualType OldType = ParamTypes[i];
     bool IsPackExpansion = false;
     Optional<unsigned> NumExpansions;
     QualType NewType;
     if (const PackExpansionType *Expansion
                                        = dyn_cast<PackExpansionType>(OldType)) {
       // We have a function parameter pack that may need to be expanded.
       QualType Pattern = Expansion->getPattern();
+      NumExpansions = Expansion->getNumExpansions();
       SmallVector<UnexpandedParameterPack, 2> Unexpanded;
       getSema().collectUnexpandedParameterPacks(Pattern, Unexpanded);
 
       // Determine whether we should expand the parameter packs.
       bool ShouldExpand = false;
       bool RetainExpansion = false;
       if (getDerived().TryExpandParameterPacks(Loc, SourceRange(),
                                                Unexpanded,

clang/test/CXX/temp/temp.decls/temp.variadic/p5.cpp

 int fn() {
   FooAlias<> a;
   bar(a);
 
   FooAlias<int> b;
   bar(b);
 }
 }
+
+namespace pr56094 {
+template <typename... T> struct D {
+  template <typename... U> using B = int(int (*...p)(T, U));
+  // expected-error@-1 {{pack expansion contains parameter pack 'U' that has a different length (1 vs. 2) from outer parameter packs}}
+  template <typename U1, typename U2> D(B<U1, U2> *);
+  // expected-note@-1 {{in instantiation of template type alias 'B' requested here}}
+};
+using t1 = D<float>::B<int>;
+// expected-note@-1 {{in instantiation of template class 'pr56094::D<float>' requested here}}
+
+template <bool...> struct F {};
+template <class...> struct G {};
+template <bool... I> struct E {
+  template <bool... U> using B = G<F<I, U>...>;
+  // expected-error@-1 {{pack expansion contains parameter pack 'U' that has a different length (1 vs. 2) from outer parameter packs}}
+  template <bool U1, bool U2> E(B<U1, U2> *);
+  // expected-note@-1 {{in instantiation of template type alias 'B' requested here}}
+};
+using t2 = E<true>::B<false>;
+// expected-note@-1 {{in instantiation of template class 'pr56094::E<true>' requested here}}
+} // namespace pr56094