[Clang][Sema] ASTContext::getUnconstrainedType propagates dependence (#92425)

When the argument passed to `ASTContext::getUnconstrainedType` is an
unconstrained `AutoType`, will return the argument unchanged. However,
when called with a constrained `AutoType`, an unconstrained,
non-dependent `AutoType` will be returned even if the argument was
dependent. Consider the following:
```
template<typename T>
concept C = sizeof(T) == sizeof(int);

template<auto N>
struct A;

template<C auto N>
struct A<N>; // error: class template partial specialization is not more specialized than the primary template
```
When comparing the template parameters for equivalence,
`ASTContext::getUnconstrainedType` is used to remove the constraints per
[temp.over.link] p6 sentence 2. For the template
parameter `N` of the class template, it returns a dependent `AutoType`.
For the template parameter `N` of the class template partial
specialization, it returns a non-dependent `AutoType`. We subsequently
compare the adjusted types and find they are not equivalent, thus we
consider the partial specialization to not be more specialized than the
primary template per [temp.func.order] p6.2.2.

This patch changes `ASTContext::getUnconstrainedType` such that the
dependence of a constrained `AutoType` will propagate to the returned
unconstrained `AutoType`. This causes the above example to be correctly
accepted, fixing #77377.

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -723,6 +723,7 @@ Bug Fixes to C++ Support
 - Clang now ignores template parameters only used within the exception specification of candidate function
   templates during partial ordering when deducing template arguments from a function declaration or when
   taking the address of a function template.
+- Fix a bug with checking constrained non-type template parameters for equivalence. Fixes (#GH77377).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ASTContext.cpp

@@ -5910,7 +5910,8 @@ QualType ASTContext::getUnconstrainedType(QualType T) const {
   if (auto *AT = CanonT->getAs<AutoType>()) {
     if (!AT->isConstrained())
       return T;
-    return getQualifiedType(getAutoType(QualType(), AT->getKeyword(), false,
+    return getQualifiedType(getAutoType(QualType(), AT->getKeyword(),
+                                        AT->isDependentType(),
                                         AT->containsUnexpandedParameterPack()),
                             T.getQualifiers());
   }

clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p2.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// expected-no-diagnostics
+
+namespace GH77377 {
+  template<typename T>
+  concept C = sizeof(T) == sizeof(int);
+
+  template<auto N>
+  struct A;
+
+  template<C auto N>
+  struct A<N>;
+}