[clang] fix nondeduced mismatch with nullptr template arguments (#124498)

In deduction, when comparing template arguments of value kind, we should
check if the value matches. Values of different types can still match.
For example, `short(0)` matches `int(0)`.

Values of nullptr kind always match each other, since there is only one
such possible value. Similarly to integrals, the type does not matter.

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -1037,6 +1037,7 @@ Bug Fixes to C++ Support
 - Fix template argument checking so that converted template arguments are
   converted again. This fixes some issues with partial ordering involving
   template template parameters with non-type template parameters.
+- Fix nondeduced mismatch with nullptr template arguments.
 - Clang now rejects declaring an alias template with the same name as its template parameter. (#GH123423)
 - Fixed the rejection of valid code when referencing an enumerator of an unscoped enum member with a prior declaration. (#GH124405)
 - Fixed immediate escalation of non-dependent expressions. (#GH123405)

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -2541,10 +2541,9 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
     return TemplateDeductionResult::NonDeducedMismatch;
 
   case TemplateArgument::NullPtr:
-    if (A.getKind() == TemplateArgument::NullPtr &&
-        S.Context.hasSameType(P.getNullPtrType(), A.getNullPtrType()))
+    // 'nullptr' has only one possible value, so it always matches.
+    if (A.getKind() == TemplateArgument::NullPtr)
       return TemplateDeductionResult::Success;
-
     Info.FirstArg = P;
     Info.SecondArg = A;
     return TemplateDeductionResult::NonDeducedMismatch;
@@ -2559,6 +2558,8 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
     return TemplateDeductionResult::NonDeducedMismatch;
 
   case TemplateArgument::StructuralValue:
+    // FIXME: structural equality will also compare types,
+    // but they should match iff they have the same value.
     if (A.getKind() == TemplateArgument::StructuralValue &&
         A.structurallyEquals(P))
       return TemplateDeductionResult::Success;

clang/test/SemaTemplate/cwg2398.cpp

@@ -697,15 +697,11 @@ namespace nttp_partial_order {
     template void f<B>(B<&A::m>);
   } // namespace t5
   namespace t6 {
-    // FIXME: This should pick the second overload.
     struct A {};
     using nullptr_t = decltype(nullptr);
     template<template<nullptr_t> class TT2> void f(TT2<nullptr>);
-    // new-note@-1 {{here}}
     template<template<A*>        class TT1> void f(TT1<nullptr>) {}
-    // new-note@-1 {{here}}
     template<A*> struct B {};
     template void f<B>(B<nullptr>);
-    // new-error@-1 {{ambiguous}}
   } // namespace t6
 } // namespace nttp_partial_order