[clang] e8f198d - Fix pack deduction to only deduce the arity of packs that are actually

[Hans Wennborg via cfe-commits]

Cherry-picked in e241c8fe6d2e6d83e9fb32bd34da8ffcdc0dd83d. Thanks!

On Thu, Jan 16, 2020 at 1:22 AM Richard Smith <richard at metafoo.co.uk> wrote:
>
> Hi Hans,
>
> Please consider this bugfix for the Clang 10 release branch.
>
> On Wed, 15 Jan 2020 at 16:21, Richard Smith via cfe-commits <cfe-commits at lists.llvm.org> wrote:
>>
>>
>> Author: Richard Smith
>> Date: 2020-01-15T16:21:08-08:00
>> New Revision: e8f198dd9e9dabed8d50276465906e7c8827cada
>>
>> URL: https://github.com/llvm/llvm-project/commit/e8f198dd9e9dabed8d50276465906e7c8827cada
>> DIFF: https://github.com/llvm/llvm-project/commit/e8f198dd9e9dabed8d50276465906e7c8827cada.diff
>>
>> LOG: Fix pack deduction to only deduce the arity of packs that are actually
>> expanded by the deduced pack.
>>
>> We recently started also deducing the arity of separately-expanded packs
>> that are merely mentioned within the pack in question, which is
>> incorrect.
>>
>> Added:
>>
>>
>> Modified:
>>     clang/lib/Sema/SemaTemplateDeduction.cpp
>>     clang/test/SemaTemplate/deduction.cpp
>>
>> Removed:
>>
>>
>>
>> ################################################################################
>> diff  --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
>> index 1b9f1b2144d1..048a50a741e4 100644
>> --- a/clang/lib/Sema/SemaTemplateDeduction.cpp
>> +++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
>> @@ -724,38 +724,48 @@ class PackDeductionScope {
>>      // Compute the set of template parameter indices that correspond to
>>      // parameter packs expanded by the pack expansion.
>>      llvm::SmallBitVector SawIndices(TemplateParams->size());
>> +    llvm::SmallVector<TemplateArgument, 4> ExtraDeductions;
>>
>>      auto AddPack = [&](unsigned Index) {
>>        if (SawIndices[Index])
>>          return;
>>        SawIndices[Index] = true;
>>        addPack(Index);
>> +
>> +      // Deducing a parameter pack that is a pack expansion also constrains the
>> +      // packs appearing in that parameter to have the same deduced arity. Also,
>> +      // in C++17 onwards, deducing a non-type template parameter deduces its
>> +      // type, so we need to collect the pending deduced values for those packs.
>> +      if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(
>> +              TemplateParams->getParam(Index))) {
>> +        if (auto *Expansion = dyn_cast<PackExpansionType>(NTTP->getType()))
>> +          ExtraDeductions.push_back(Expansion->getPattern());
>> +      }
>> +      // FIXME: Also collect the unexpanded packs in any type and template
>> +      // parameter packs that are pack expansions.
>>      };
>>
>> -    // First look for unexpanded packs in the pattern.
>> -    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
>> -    S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
>> -    for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
>> -      unsigned Depth, Index;
>> -      std::tie(Depth, Index) = getDepthAndIndex(Unexpanded[I]);
>> -      if (Depth == Info.getDeducedDepth())
>> -        AddPack(Index);
>> -    }
>> +    auto Collect = [&](TemplateArgument Pattern) {
>> +      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
>> +      S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
>> +      for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
>> +        unsigned Depth, Index;
>> +        std::tie(Depth, Index) = getDepthAndIndex(Unexpanded[I]);
>> +        if (Depth == Info.getDeducedDepth())
>> +          AddPack(Index);
>> +      }
>> +    };
>> +
>> +    // Look for unexpanded packs in the pattern.
>> +    Collect(Pattern);
>>      assert(!Packs.empty() && "Pack expansion without unexpanded packs?");
>>
>>      unsigned NumNamedPacks = Packs.size();
>>
>> -    // We can also have deduced template parameters that do not actually
>> -    // appear in the pattern, but can be deduced by it (the type of a non-type
>> -    // template parameter pack, in particular). These won't have prevented us
>> -    // from partially expanding the pack.
>> -    llvm::SmallBitVector Used(TemplateParams->size());
>> -    MarkUsedTemplateParameters(S.Context, Pattern, /*OnlyDeduced*/true,
>> -                               Info.getDeducedDepth(), Used);
>> -    for (int Index = Used.find_first(); Index != -1;
>> -         Index = Used.find_next(Index))
>> -      if (TemplateParams->getParam(Index)->isParameterPack())
>> -        AddPack(Index);
>> +    // Also look for unexpanded packs that are indirectly deduced by deducing
>> +    // the sizes of the packs in this pattern.
>> +    while (!ExtraDeductions.empty())
>> +      Collect(ExtraDeductions.pop_back_val());
>>
>>      return NumNamedPacks;
>>    }
>>
>> diff  --git a/clang/test/SemaTemplate/deduction.cpp b/clang/test/SemaTemplate/deduction.cpp
>> index 1f1c30a8b4ab..7268912dd6c5 100644
>> --- a/clang/test/SemaTemplate/deduction.cpp
>> +++ b/clang/test/SemaTemplate/deduction.cpp
>> @@ -546,3 +546,21 @@ namespace designators {
>>
>>    static_assert(f({.a = 1, .b = 2}) == 3, ""); // expected-error {{no matching function}}
>>  }
>> +
>> +namespace nested_packs {
>> +  template<typename ...T, typename ...U> void f(T (*...f)(U...)); // expected-note {{deduced packs of
>> diff erent lengths for parameter 'U' (<> vs. <int>)}}
>> +  void g() { f(g); f(g, g); f(g, g, g); }
>> +  void h(int) { f(h); f(h, h); f(h, h, h); }
>> +  void i() { f(g, h); } // expected-error {{no matching function}}
>> +
>> +#if __cplusplus >= 201703L
>> +  template<auto ...A> struct Q {};
>> +  template<typename ...T, T ...A, T ...B> void q(Q<A...>, Q<B...>); // #q
>> +  void qt(Q<> q0, Q<1, 2> qii, Q<1, 2, 3> qiii) {
>> +    q(q0, q0);
>> +    q(qii, qii);
>> +    q(qii, qiii); // expected-error {{no match}} expected-note@#q {{deduced packs of
>> diff erent lengths for parameter 'T' (<int, int> vs. <int, int, int>)}}
>> +    q(q0, qiii); // expected-error {{no match}} expected-note@#q {{deduced packs of
>> diff erent lengths for parameter 'T' (<> vs. <int, int, int>)}}
>> +  }
>> +#endif
>> +}
>>
>>
>>
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