Fix CRTP partial specialization instantiation crash.

[erichkeane]

Summary

Fixes #60778.

When instantiating the body of a class template specialization that was
instantiated from a partial specialization, we were incorrectly
collecting template arguments from the primary template, which resulted
in the template arguments list being inaccurate. In the example from
the issue, we were trying to substitute the boolean 'false' into the
type on Nested, which caused an assertion.

[alexander-shaposhnikov]

LG

clang/docs/ReleaseNotes.rst

- Fix lambdas and other anonymous function names not respecting ``-fdebug-prefix-map``
   (`#62192 <https://github.com/llvm/llvm-project/issues/62192>`_)
 - Fix crash when attempting to pass a non-pointer type as first argument of
   ``__builtin_assume_aligned``.
   (`#62305 <https://github.com/llvm/llvm-project/issues/62305>`_)
 - A default argument for a non-type template parameter is evaluated and checked
   at the point where it is required. This fixes:
   (`#62224 <https://github.com/llvm/llvm-project/issues/62224>`_) and
   (`#62596 <https://github.com/llvm/llvm-project/issues/62596>`_)
+- Fix an assertion when instantiating the body of a Class Template Specialization
+  when it had been instantiated from a partial template specialization with different
+  template arguments on the containing class. This fixes:
+  (`#60778 <https://github.com/llvm/llvm-project/issues/60778>`_).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a bug where attribute annotations on type specifiers (enums, classes,
   structs, unions, and scoped enums) were not properly ignored, resulting in

clang/lib/Sema/SemaTemplateInstantiate.cpp

if (!SkipForSpecialization)
           ClassTemplSpec->getTemplateInstantiationArgs().asArray(),
           /*Final=*/false);
 
     // If this class template specialization was instantiated from a
     // specialized member that is a class template, we're done.
     assert(ClassTemplSpec->getSpecializedTemplate() && "No class template?");
     if (ClassTemplSpec->getSpecializedTemplate()->isMemberSpecialization())
       return Response::Done();
+
+    // If this was instantiated from a partial template specialization, we need
+    // to get the next level of declaration context from the partial
+    // specialization, as the ClassTemplateSpecializationDecl's
+    // DeclContext/LexicalDeclContext will be for the primary template.
+    if (auto *InstFromPartialTempl = ClassTemplSpec->getSpecializedTemplateOrPartial()
+                      .dyn_cast<ClassTemplatePartialSpecializationDecl *>())
+      return Response::ChangeDecl(InstFromPartialTempl->getLexicalDeclContext());
   }
   return Response::UseNextDecl(ClassTemplSpec);
 }
 
 Response HandleFunction(const FunctionDecl *Function,
                         MultiLevelTemplateArgumentList &Result,
                         const FunctionDecl *Pattern, bool RelativeToPrimary,
                         bool ForConstraintInstantiation) {

clang/test/SemaTemplate/partial-spec-instantiate.cpp

namespace IgnorePartialSubstitution {
 
   // This used to pick the primary template, because we got confused and
   // thought that template parameter 0 was the current partially-substituted
   // pack (from `helper`) during the deduction for the partial specialization.
   void f() { helper<S>(); }
 
   _Static_assert(S::value, "");
 }
+
+namespace GH60778 {
+  template <bool B = false> class ClassTemplate {
+  public:
+      template <typename T, typename = void> class Nested {};
+  };
+
+  template <typename DerivedType> class Base {};
+
+  template <>
+  template <typename T>
+  class ClassTemplate<>::Nested<T> : public Base<ClassTemplate<>::Nested<T> > {};
+
+  void use() {
+    // This should instantiate the body of Nested with the template arguments
+    // from the Partial Specialization. This would previously get confused and
+    // get the template arguments from the primary template instead.
+    ClassTemplate<>::Nested<int> instantiation;
+  }
+}