[clang] fix nondeduced mismatch with nullptr template arguments

[mizvekov]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/124498.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+4-3)
• (modified) clang/test/SemaTemplate/cwg2398.cpp (-4)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 27574924a14a92..0556879251f89d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -996,6 +996,7 @@ 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.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 2b96692727a7c8..1e1fce10e7c017 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/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;
diff --git a/clang/test/SemaTemplate/cwg2398.cpp b/clang/test/SemaTemplate/cwg2398.cpp
index 137b94ba2641de..dccb17c48d3256 100644
--- a/clang/test/SemaTemplate/cwg2398.cpp
+++ b/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

[zyn0217]

Thanks

[cor3ntin]

LGTM

[cor3ntin]

Suggested change

	// but they should match iff they have the same value.
	// FIXME: Arguments of structural types should match when their values are the same,
	// even when their types are different.