[Clang][Sema] Use StructuralValues to model dependent NTTP arguments (#93556)

This patch takes Richard's approach of no longer modeling dependent NTTP
arguments with TemplateParamObjectDecls. Clang used to do so, which left
behind a problem in that we might mess up dependent and non-dependent
arguments that boil down to the same canonical type because there's a
default argument on the NTTP.

The problem of "canonical expression" is still present because this
patch doesn't touch the profiling part. Namely, #92292 seems different.

Fixes #84052

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -810,6 +810,7 @@ Bug Fixes to C++ Support
 - Clang now diagnoses unexpanded parameter packs in attributes. (Fixes #GH93269).
 - Clang now allows ``@$``` in raw string literals. Fixes (#GH93130).
 - Fix an assertion failure when checking invalid ``this`` usage in the wrong context. (Fixes #GH91536).
+- Clang no longer models dependent NTTP arguments as ``TemplateParamObjectDecl`` s. Fixes (#GH84052).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/TemplateBase.cpp

@@ -221,8 +221,13 @@ static const ValueDecl *getAsSimpleValueDeclRef(const ASTContext &Ctx,
 
   // We model class non-type template parameters as their template parameter
   // object declaration.
-  if (V.isStruct() || V.isUnion())
+  if (V.isStruct() || V.isUnion()) {
+    // Dependent types are not supposed to be described as
+    // TemplateParamObjectDecls.
+    if (T->isDependentType() || T->isInstantiationDependentType())
+      return nullptr;
     return Ctx.getTemplateParamObjectDecl(T, V);
+  }
 
   // Pointers and references with an empty path use the special 'Declaration'
   // representation.

clang/test/SemaTemplate/temp_arg_nontype_cxx2c.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -std=c++20 -Wconversion -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++2c -Wconversion -verify %s
 
 struct Test {
     int a = 0;
@@ -102,3 +102,24 @@ void bar() {
 }
 
 }
+
+namespace GH84052 {
+
+template <class... T>
+concept C = sizeof(T...[1]) == 1; // #C
+
+struct A {};
+
+template <class T, C<T> auto = A{}> struct Set {}; // #Set
+
+template <class T> void foo() {
+  Set<T> unrelated;
+}
+
+Set<bool> sb;
+Set<float> sf;
+// expected-error@-1 {{constraints not satisfied for class template 'Set'}}
+// expected-note@#Set {{because 'C<decltype(GH84052::A{}), float>' evaluated to false}}
+// expected-note@#C {{evaluated to false}}
+
+} // namespace GH84052