[concepts] Extract function template pack arguments from the current …

…instantiation if possible (#80594)

Before the constraint substitution, we employ
`getTemplateInstantiationArgs`, which in turn attempts to inspect
`TemplateArgument`s from the function template. For parameter packs from
their parent contexts, we used to extract the arguments from the
specialization type, in which could result in non-canonical argument
types e.g. `PackExpansionType`.

This may break the contract that, during a tree transformation, in
`TreeTransform::TryExpandParameterPacks`, the corresponding
`TemplateArgument`s for an `UnexpandedParameterPack` are expected to be
of `Pack` kinds if we're expanding template parameters.

Fixes #72557.

clang/docs/ReleaseNotes.rst

@@ -197,6 +197,9 @@ Bug Fixes to C++ Support
   Fixes (`#67976 <https://github.com/llvm/llvm-project/issues/67976>`_)
 - Fix crash and diagnostic with const qualified member operator new.
   Fixes (`#79748 <https://github.com/llvm/llvm-project/issues/79748>`_)
+- Fixed a crash where substituting into a requires-expression that involves parameter packs
+  during the equivalence determination of two constraint expressions.
+  (`#72557 <https://github.com/llvm/llvm-project/issues/72557>`_)
 - Fix a crash when specializing an out-of-line member function with a default
   parameter where we did an incorrect specialization of the initialization of
   the default parameter.

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -241,10 +241,38 @@ Response HandleFunctionTemplateDecl(const FunctionTemplateDecl *FTD,
 
     while (const Type *Ty = NNS ? NNS->getAsType() : nullptr) {
       if (NNS->isInstantiationDependent()) {
-        if (const auto *TSTy = Ty->getAs<TemplateSpecializationType>())
+        if (const auto *TSTy = Ty->getAs<TemplateSpecializationType>()) {
+          ArrayRef<TemplateArgument> Arguments = TSTy->template_arguments();
+          // Prefer template arguments from the injected-class-type if possible.
+          // For example,
+          // ```cpp
+          // template <class... Pack> struct S {
+          //   template <class T> void foo();
+          // };
+          // template <class... Pack> template <class T>
+          //           ^^^^^^^^^^^^^ InjectedTemplateArgs
+          //           They're of kind TemplateArgument::Pack, not of
+          //           TemplateArgument::Type.
+          // void S<Pack...>::foo() {}
+          //        ^^^^^^^
+          //        TSTy->template_arguments() (which are of PackExpansionType)
+          // ```
+          // This meets the contract in
+          // TreeTransform::TryExpandParameterPacks that the template arguments
+          // for unexpanded parameters should be of a Pack kind.
+          if (TSTy->isCurrentInstantiation()) {
+            auto *RD = TSTy->getCanonicalTypeInternal()->getAsCXXRecordDecl();
+            if (ClassTemplateDecl *CTD = RD->getDescribedClassTemplate())
+              Arguments = CTD->getInjectedTemplateArgs();
+            else if (auto *Specialization =
+                         dyn_cast<ClassTemplateSpecializationDecl>(RD))
+              Arguments =
+                  Specialization->getTemplateInstantiationArgs().asArray();
+          }
           Result.addOuterTemplateArguments(
-              const_cast<FunctionTemplateDecl *>(FTD), TSTy->template_arguments(),
+              const_cast<FunctionTemplateDecl *>(FTD), Arguments,
               /*Final=*/false);
+        }
       }
 
       NNS = NNS->getPrefix();

clang/test/SemaTemplate/concepts-out-of-line-def.cpp

@@ -581,3 +581,21 @@ void S<T>::test(T target, U... value)
   }
 {}
 } // namespace GH74447
+
+namespace GH72557 {
+
+template <typename...>
+concept IsAnyOf = true;
+
+template <class... DerTs> struct DerivedCollection {
+  template <class DerT>
+    requires IsAnyOf<DerTs...>
+  unsigned long index();
+};
+
+template <class... DerTs>
+template <class DerT>
+  requires IsAnyOf<DerTs...>
+unsigned long DerivedCollection<DerTs...>::index() {}
+
+} // namespace GH72557