[Clang][Sema] fix a bug on template partial specialization (#89862)

attempt to fix
#68885 (comment)
Deduction of NTTP whose type is `decltype(auto)` would create an
implicit cast expression to dependent type and makes the type of primary
template definition (`InjectedClassNameSpecialization`) and its partial
specialization different. Prevent emitting cast expression to make clang
knows their types are identical by removing `CTAK == CTAK_Deduced` when
the type is `decltype(auto)`.

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

Author: jcsxky

clang/docs/ReleaseNotes.rst

@@ -611,6 +611,7 @@ Bug Fixes to C++ Support
   immediate function context.
 - Fix CTAD for ``std::initializer_list``. This allows ``std::initializer_list{1, 2, 3}`` to be deduced as
   ``std::initializer_list<int>`` as intended.
+- Fix a bug on template partial specialization whose template parameter is `decltype(auto)`.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -7706,7 +7706,7 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     // FIXME: The language rules don't say what happens in this case.
     // FIXME: We get an opaque dependent type out of decltype(auto) if the
     // expression is merely instantiation-dependent; is this enough?
-    if (CTAK == CTAK_Deduced && Arg->isTypeDependent()) {
+    if (Arg->isTypeDependent()) {
       auto *AT = dyn_cast<AutoType>(DeducedT);
       if (AT && AT->isDecltypeAuto()) {
         SugaredConverted = TemplateArgument(Arg);

clang/test/SemaCXX/identical-type-primary-partial-specialization.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+
+template <decltype(auto) a>
+struct S { // expected-note {{previous definition is here}}
+    static constexpr int i = 42;
+};
+
+template <decltype(auto) a>
+struct S<a> { // expected-error {{class template partial specialization does not specialize any template argument; to define the primary template, remove the template argument list}} \
+              // expected-error {{redefinition of 'S'}}
+    static constexpr int i = 0;
+};