[clang] create class-type injected NTTP with correct value kind (#101395)

A template parameter object is an lvalue, which was not being respected
for injected parameters.

Fixes #101394

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -173,6 +173,8 @@ Bug Fixes to C++ Support
 - Clang now correctly parses potentially declarative nested-name-specifiers in pointer-to-member declarators.
 - Fix a crash when checking the initialzier of an object that was initialized
   with a string literal. (#GH82167)
+- Fix a crash when matching template template parameters with templates which have
+  parameters of different class type. (#GH101394)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ASTContext.cpp

@@ -5588,11 +5588,19 @@ TemplateArgument ASTContext::getInjectedTemplateArg(NamedDecl *Param) {
     // of a real template argument.
     // FIXME: It would be more faithful to model this as something like an
     // lvalue-to-rvalue conversion applied to a const-qualified lvalue.
-    if (T->isRecordType())
+    ExprValueKind VK;
+    if (T->isRecordType()) {
+      // C++ [temp.param]p8: An id-expression naming a non-type
+      // template-parameter of class type T denotes a static storage duration
+      // object of type const T.
       T.addConst();
-    Expr *E = new (*this) DeclRefExpr(
-        *this, NTTP, /*RefersToEnclosingVariableOrCapture*/ false, T,
-        Expr::getValueKindForType(NTTP->getType()), NTTP->getLocation());
+      VK = VK_LValue;
+    } else {
+      VK = Expr::getValueKindForType(NTTP->getType());
+    }
+    Expr *E = new (*this)
+        DeclRefExpr(*this, NTTP, /*RefersToEnclosingVariableOrCapture=*/false,
+                    T, VK, NTTP->getLocation());
 
     if (NTTP->isParameterPack())
       E = new (*this)

clang/lib/AST/ExprClassification.cpp

@@ -485,12 +485,13 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
 /// ClassifyDecl - Return the classification of an expression referencing the
 /// given declaration.
 static Cl::Kinds ClassifyDecl(ASTContext &Ctx, const Decl *D) {
-  // C++ [expr.prim.general]p6: The result is an lvalue if the entity is a
-  //   function, variable, or data member and a prvalue otherwise.
+  // C++ [expr.prim.id.unqual]p3: The result is an lvalue if the entity is a
+  // function, variable, or data member, or a template parameter object and a
+  // prvalue otherwise.
   // In C, functions are not lvalues.
   // In addition, NonTypeTemplateParmDecl derives from VarDecl but isn't an
-  // lvalue unless it's a reference type (C++ [temp.param]p6), so we need to
-  // special-case this.
+  // lvalue unless it's a reference type or a class type (C++ [temp.param]p8),
+  // so we need to special-case this.
 
   if (const auto *M = dyn_cast<CXXMethodDecl>(D)) {
     if (M->isImplicitObjectMemberFunction())

clang/test/SemaTemplate/temp_arg_template_p0522.cpp

@@ -126,3 +126,21 @@ namespace GH62529 {
   template<class T4> B<A, T4> f();
   auto t = f<int>();
 } // namespace GH62529
+
+namespace GH101394 {
+  struct X {};
+  struct Y {
+    constexpr Y(const X &) {}
+  };
+
+  namespace t1 {
+    template<template<X> class> struct A {};
+    template<Y> struct B;
+    template struct A<B>;
+  } // namespace t1
+  namespace t2 {
+    template<template<Y> class> struct A {};
+    template<X> struct B;
+    template struct A<B>; // expected-error {{different template parameters}}
+  } // namespace t2
+} // namespace GH101394