[Clang][Sema] Don't consider top-level cv-qualifiers in template partial orderings (#81449)

This fixes a regression since
340eac0,
from which we compared function parameter types with cv-qualifiers taken
into account. However, as per [dcl.fct]/p5:

> After producing the list of parameter types, any top-level
cv-qualifiers modifying
> a parameter type are deleted when forming the function type.

Thus, I think we should use `hasSameUnqualifiedType` for type
comparison.

This fixes #75404.

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -228,6 +228,8 @@ Bug Fixes to C++ Support
   or non-constant more accurately. Previously, only a subset of the initializer
   elements were considered, misclassifying some initializers as constant. Fixes
   some of (`#80510 <https://github.com/llvm/llvm-project/issues/80510>`).
+- Clang now ignores top-level cv-qualifiers on function parameters in template partial orderings.
+  (`#75404 <https://github.com/llvm/llvm-project/issues/75404>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5642,9 +5642,12 @@ FunctionTemplateDecl *Sema::getMoreSpecializedTemplate(
                                       Sema::TPL_TemplateParamsEquivalent))
     return nullptr;
 
+  // [dcl.fct]p5:
+  // Any top-level cv-qualifiers modifying a parameter type are deleted when
+  // forming the function type.
   for (unsigned i = 0; i < NumParams1; ++i)
-    if (!Context.hasSameType(FD1->getParamDecl(i)->getType(),
-                             FD2->getParamDecl(i)->getType()))
+    if (!Context.hasSameUnqualifiedType(FD1->getParamDecl(i)->getType(),
+                                        FD2->getParamDecl(i)->getType()))
       return nullptr;
 
   // C++20 [temp.func.order]p6.3:

clang/test/CXX/over/over.match/over.match.best/p1-2a.cpp

@@ -97,13 +97,16 @@ namespace non_template
   static_assert(is_same_v<decltype(baz<int>()), int>); // expected-error {{call to 'baz' is ambiguous}}
   static_assert(is_same_v<decltype(bar<int>()), void>); // expected-error {{call to 'bar' is ambiguous}}
 
+  // Top-level cv-qualifiers are ignored in template partial ordering per [dcl.fct]/p5.
+  //   After producing the list of parameter types, any top-level cv-qualifiers modifying
+  //   a parameter type are deleted when forming the function type.
   template<typename T>
-  constexpr int goo(int a) requires AtLeast2<int> && true { // expected-note {{candidate function}}
+  constexpr int goo(T a) requires AtLeast2<T> && true {
     return 1;
   }
 
   template<typename T>
-  constexpr int goo(const int b) requires AtLeast2<int> { // expected-note {{candidate function}}
+  constexpr int goo(const T b) requires AtLeast2<T> {
     return 2;
   }
 
@@ -122,7 +125,6 @@ namespace non_template
     return 2;
   }
 
-  // By temp.func.order-6.2.2, this is ambiguous because parameter a and b have different types.
-  static_assert(goo<int>(1) == 1); // expected-error {{call to 'goo' is ambiguous}}
+  static_assert(goo<int>(1) == 1);
   static_assert(doo<int>(2) == 1);
 }