[Clang][Concepts] Fix the constraint equivalence checking involving parameter packs (#102131)

We established an instantiation scope in order for constraint
equivalence checking to properly map the uninstantiated parameters.

That mechanism mapped even packs to themselves. Consequently, parameter
packs e.g. appearing in a function call, were not expanded. So they
would end up becoming `SubstTemplateTypeParmPackType`s that circularly
depend on the canonical declaration of the function template, which is
not yet determined, hence the spurious error.

No release note as I plan to backport it to 19.

Fixes #101735

---------

Co-authored-by: cor3ntin <[email protected]>

Author: zyn0217

clang/lib/Sema/SemaConcept.cpp

@@ -977,8 +977,30 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   // equivalence.
   LocalInstantiationScope ScopeForParameters(S);
   if (auto *FD = DeclInfo.getDecl()->getAsFunction())
-    for (auto *PVD : FD->parameters())
-      ScopeForParameters.InstantiatedLocal(PVD, PVD);
+    for (auto *PVD : FD->parameters()) {
+      if (!PVD->isParameterPack()) {
+        ScopeForParameters.InstantiatedLocal(PVD, PVD);
+        continue;
+      }
+      // This is hacky: we're mapping the parameter pack to a size-of-1 argument
+      // to avoid building SubstTemplateTypeParmPackTypes for
+      // PackExpansionTypes. The SubstTemplateTypeParmPackType node would
+      // otherwise reference the AssociatedDecl of the template arguments, which
+      // is, in this case, the template declaration.
+      //
+      // However, as we are in the process of comparing potential
+      // re-declarations, the canonical declaration is the declaration itself at
+      // this point. So if we didn't expand these packs, we would end up with an
+      // incorrect profile difference because we will be profiling the
+      // canonical types!
+      //
+      // FIXME: Improve the "no-transform" machinery in FindInstantiatedDecl so
+      // that we can eliminate the Scope in the cases where the declarations are
+      // not necessarily instantiated. It would also benefit the noexcept
+      // specifier comparison.
+      ScopeForParameters.MakeInstantiatedLocalArgPack(PVD);
+      ScopeForParameters.InstantiatedLocalPackArg(PVD, PVD);
+    }
 
   std::optional<Sema::CXXThisScopeRAII> ThisScope;
 

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

@@ -599,3 +599,26 @@ template <class DerT>
 unsigned long DerivedCollection<DerTs...>::index() {}
 
 } // namespace GH72557
+
+namespace GH101735 {
+
+template <class, class>
+concept True = true;
+
+template <typename T>
+class A {
+  template <typename... Ts>
+  void method(Ts&... ts)
+    requires requires (T t) {
+      { t.method(static_cast<Ts &&>(ts)...) } -> True<void>;
+    };
+};
+
+template <typename T>
+template <typename... Ts>
+void A<T>::method(Ts&... ts)
+  requires requires (T t) {
+    { t.method(static_cast<Ts &&>(ts)...) } -> True<void>;
+  } {}
+
+}