[clang] Improve deduction of reference typed NTTP (llvm#110393)

This improves the existing workaround for a core issue introduced in
CWG1770.

When performing template argument deduction for an NTTP which the
parameter side is a reference, instead of dropping the references for
both sides, just make the argument be same reference typed as the
parameter, in case the argument is not already a reference type.

Fixes llvm#73460

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -465,6 +465,7 @@ Bug Fixes to C++ Support
 - Fixed an assertion failure in debug mode, and potential crashes in release mode, when
   diagnosing a failed cast caused indirectly by a failed implicit conversion to the type of the constructor parameter.
 - Fixed an assertion failure by adjusting integral to boolean vector conversions (#GH108326)
+- Fixed an issue deducing non-type template arguments of reference type. (#GH73460)
 - Mangle friend function templates with a constraint that depends on a template parameter from an enclosing template as members of the enclosing class. (#GH110247)
 - Fixed an issue in constraint evaluation, where type constraints on the lambda expression
   containing outer unexpanded parameters were not correctly expanded. (#GH101754)

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -442,18 +442,18 @@ DeduceNonTypeTemplateArgument(Sema &S, TemplateParameterList *TemplateParams,
 
   // FIXME: It's not clear how deduction of a parameter of reference
   // type from an argument (of non-reference type) should be performed.
-  // For now, we just remove reference types from both sides and let
-  // the final check for matching types sort out the mess.
-  ValueType = ValueType.getNonReferenceType();
-  if (ParamType->isReferenceType())
-    ParamType = ParamType.getNonReferenceType();
-  else
-    // Top-level cv-qualifiers are irrelevant for a non-reference type.
-    ValueType = ValueType.getUnqualifiedType();
+  // For now, we just make the argument have same reference type as the
+  // parameter.
+  if (ParamType->isReferenceType() && !ValueType->isReferenceType()) {
+    if (ParamType->isRValueReferenceType())
+      ValueType = S.Context.getRValueReferenceType(ValueType);
+    else
+      ValueType = S.Context.getLValueReferenceType(ValueType);
+  }
 
   return DeduceTemplateArgumentsByTypeMatch(
       S, TemplateParams, ParamType, ValueType, Info, Deduced,
-      TDF_SkipNonDependent,
+      TDF_SkipNonDependent | TDF_IgnoreQualifiers,
       PartialOrdering ? PartialOrderingKind::NonCall
                       : PartialOrderingKind::None,
       /*ArrayBound=*/NewDeduced.wasDeducedFromArrayBound(), HasDeducedAnyParam);

clang/test/SemaTemplate/temp_arg_nontype_cxx1z.cpp

@@ -613,3 +613,11 @@ struct {
 template<typename T>
 using a = s<f(T::x)>;
 }
+
+namespace GH73460 {
+  template <class T, T, T> struct A;
+  template <class T, T n> struct A<T, n, n> {};
+
+  int j;
+  template struct A<int&, j, j>;
+} // namespace GH73460