[c++20] P1907R1: Support for generalized non-type template arguments …

…of scalar type. (#78041)

Previously committed as 9e08e51, and
reverted because a dependency commit was reverted, then committed again
as 4b57400 and reverted again because
"dependency commit" 5a391d3 was
reverted. But it doesn't seem that 5a391d3 was a real dependency
for this.

This commit incorporates 4b57400 and
18e093f by Richard Smith (@zygoloid),
with some minor fixes, most notably:

- `UncommonValue` renamed to `StructuralValue`

- `VK_PRValue` instead of `VK_RValue` as default kind in lvalue and
member pointer handling branch in
`BuildExpressionFromNonTypeTemplateArgumentValue`;

- handling of `StructuralValue` in `IsTypeDeclaredInsideVisitor`;

- filling in `SugaredConverted` along with `CanonicalConverted`
parameter in `Sema::CheckTemplateArgument`;

- minor cleanup in
`TemplateInstantiator::transformNonTypeTemplateParmRef`;

- `TemplateArgument` constructors refactored;

- `ODRHash` calculation for `UncommonValue`;

- USR generation for `UncommonValue`;

- more correct MS compatibility mangling algorithm (tested on MSVC ver.
19.35; toolset ver. 143);

- IR emitting fixed on using a subobject as a template argument when the
corresponding template parameter is used in an lvalue context;

- `noundef` attribute and opaque pointers in `template-arguments` test;

- analysis for C++17 mode is turned off for templates in
`warn-bool-conversion` test; in C++17 and C++20 mode, array reference
used as a template argument of pointer type produces template argument
of UncommonValue type, and
`BuildExpressionFromNonTypeTemplateArgumentValue` makes
`OpaqueValueExpr` for it, and `DiagnoseAlwaysNonNullPointer` cannot see
through it; despite of "These cases should not warn" comment, I'm not
sure about correct behavior; I'd expect a suggestion to replace `if` by
`if constexpr`;

- `temp.arg.nontype/p1.cpp` and `dr18xx.cpp` tests fixed.

clang-tools-extra/clangd/DumpAST.cpp

@@ -143,6 +143,7 @@ class DumpVisitor : public RecursiveASTVisitor<DumpVisitor> {
       TEMPLATE_ARGUMENT_KIND(Declaration);
       TEMPLATE_ARGUMENT_KIND(Template);
       TEMPLATE_ARGUMENT_KIND(TemplateExpansion);
+      TEMPLATE_ARGUMENT_KIND(StructuralValue);
 #undef TEMPLATE_ARGUMENT_KIND
     }
     llvm_unreachable("Unhandled ArgKind enum");

clang-tools-extra/clangd/FindTarget.cpp

@@ -1039,6 +1039,7 @@ class ExplicitReferenceCollector
     case TemplateArgument::Pack:
     case TemplateArgument::Type:
     case TemplateArgument::Expression:
+    case TemplateArgument::StructuralValue:
       break; // Handled by VisitType and VisitExpression.
     };
     return RecursiveASTVisitor::TraverseTemplateArgumentLoc(A);

clang/docs/ReleaseNotes.rst

@@ -183,6 +183,9 @@ C++ Language Changes
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
+- Implemented `P1907R1 <https://wg21.link/P1907R1>` which extends allowed non-type template argument
+  kinds with e.g. floating point values and pointers and references to subobjects.
+  This feature is still experimental. Accordingly, `__cpp_nontype_template_args` was not updated.
 
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/ODRHash.h

@@ -25,6 +25,7 @@
 
 namespace clang {
 
+class APValue;
 class Decl;
 class IdentifierInfo;
 class NestedNameSpecifier;
@@ -101,6 +102,8 @@ class ODRHash {
   // Save booleans until the end to lower the size of data to process.
   void AddBoolean(bool value);
 
+  void AddStructuralValue(const APValue &);
+
   static bool isSubDeclToBeProcessed(const Decl *D, const DeclContext *Parent);
 
 private:

clang/include/clang/AST/PropertiesBase.td

@@ -808,6 +808,20 @@ let Class = PropertyTypeCase<TemplateArgument, "Integral"> in {
     return TemplateArgument(ctx, value, type, isDefaulted);
   }]>;
 }
+let Class = PropertyTypeCase<TemplateArgument, "StructuralValue"> in {
+  def : Property<"value", APValue> {
+    let Read = [{ node.getAsStructuralValue() }];
+  }
+  def : Property<"type", QualType> {
+    let Read = [{ node.getStructuralValueType() }];
+  }
+  def : Property<"isDefaulted", Bool> {
+    let Read = [{ node.getIsDefaulted() }];
+  }
+  def : Creator<[{
+    return TemplateArgument(ctx, type, value, isDefaulted);
+  }]>;
+}
 let Class = PropertyTypeCase<TemplateArgument, "Template"> in {
   def : Property<"name", TemplateName> {
     let Read = [{ node.getAsTemplateOrTemplatePattern() }];

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -850,6 +850,7 @@ bool RecursiveASTVisitor<Derived>::TraverseTemplateArgument(
   case TemplateArgument::Declaration:
   case TemplateArgument::Integral:
   case TemplateArgument::NullPtr:
+  case TemplateArgument::StructuralValue:
     return true;
 
   case TemplateArgument::Type:
@@ -882,6 +883,7 @@ bool RecursiveASTVisitor<Derived>::TraverseTemplateArgumentLoc(
   case TemplateArgument::Declaration:
   case TemplateArgument::Integral:
   case TemplateArgument::NullPtr:
+  case TemplateArgument::StructuralValue:
     return true;
 
   case TemplateArgument::Type: {

clang/include/clang/AST/TemplateArgumentVisitor.h

@@ -37,6 +37,7 @@ class Base {
       DISPATCH(Declaration);
       DISPATCH(NullPtr);
       DISPATCH(Integral);
+      DISPATCH(StructuralValue);
       DISPATCH(Template);
       DISPATCH(TemplateExpansion);
       DISPATCH(Expression);
@@ -59,6 +60,7 @@ class Base {
   VISIT_METHOD(Declaration);
   VISIT_METHOD(NullPtr);
   VISIT_METHOD(Integral);
+  VISIT_METHOD(StructuralValue);
   VISIT_METHOD(Template);
   VISIT_METHOD(TemplateExpansion);
   VISIT_METHOD(Expression);

clang/include/clang/AST/TemplateBase.h

@@ -50,6 +50,7 @@ template <> struct PointerLikeTypeTraits<clang::Expr *> {
 
 namespace clang {
 
+class APValue;
 class ASTContext;
 class Expr;
 struct PrintingPolicy;
@@ -80,6 +81,13 @@ class TemplateArgument {
     /// that was provided for an integral non-type template parameter.
     Integral,
 
+    /// The template argument is a non-type template argument that can't be
+    /// represented by the special-case Declaration, NullPtr, or Integral
+    /// forms. These values are only ever produced by constant evaluation,
+    /// so cannot be dependent.
+    /// TODO: merge Declaration, NullPtr and Integral into this?
+    StructuralValue,
+
     /// The template argument is a template name that was provided for a
     /// template template parameter.
     Template,
@@ -130,6 +138,14 @@ class TemplateArgument {
     };
     void *Type;
   };
+  struct V {
+    LLVM_PREFERRED_TYPE(ArgKind)
+    unsigned Kind : 31;
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned IsDefaulted : 1;
+    APValue *Value;
+    void *Type;
+  };
   struct A {
     LLVM_PREFERRED_TYPE(ArgKind)
     unsigned Kind : 31;
@@ -156,37 +172,42 @@ class TemplateArgument {
   union {
     struct DA DeclArg;
     struct I Integer;
+    struct V Value;
     struct A Args;
     struct TA TemplateArg;
     struct TV TypeOrValue;
   };
 
+  void initFromType(QualType T, bool IsNullPtr, bool IsDefaulted);
+  void initFromDeclaration(ValueDecl *D, QualType QT, bool IsDefaulted);
+  void initFromIntegral(const ASTContext &Ctx, const llvm::APSInt &Value,
+                        QualType Type, bool IsDefaulted);
+  void initFromStructural(const ASTContext &Ctx, QualType Type,
+                          const APValue &V, bool IsDefaulted);
+
 public:
   /// Construct an empty, invalid template argument.
   constexpr TemplateArgument() : TypeOrValue({Null, 0, /* IsDefaulted */ 0}) {}
 
   /// Construct a template type argument.
   TemplateArgument(QualType T, bool isNullPtr = false,
                    bool IsDefaulted = false) {
-    TypeOrValue.Kind = isNullPtr ? NullPtr : Type;
-    TypeOrValue.IsDefaulted = IsDefaulted;
-    TypeOrValue.V = reinterpret_cast<uintptr_t>(T.getAsOpaquePtr());
+    initFromType(T, isNullPtr, IsDefaulted);
   }
 
-  /// Construct a template argument that refers to a
-  /// declaration, which is either an external declaration or a
-  /// template declaration.
+  /// Construct a template argument that refers to a (non-dependent)
+  /// declaration.
   TemplateArgument(ValueDecl *D, QualType QT, bool IsDefaulted = false) {
-    assert(D && "Expected decl");
-    DeclArg.Kind = Declaration;
-    DeclArg.IsDefaulted = IsDefaulted;
-    DeclArg.QT = QT.getAsOpaquePtr();
-    DeclArg.D = D;
+    initFromDeclaration(D, QT, IsDefaulted);
   }
 
   /// Construct an integral constant template argument. The memory to
   /// store the value is allocated with Ctx.
-  TemplateArgument(ASTContext &Ctx, const llvm::APSInt &Value, QualType Type,
+  TemplateArgument(const ASTContext &Ctx, const llvm::APSInt &Value,
+                   QualType Type, bool IsDefaulted = false);
+
+  /// Construct a template argument from an arbitrary constant value.
+  TemplateArgument(const ASTContext &Ctx, QualType Type, const APValue &Value,
                    bool IsDefaulted = false);
 
   /// Construct an integral constant template argument with the same
@@ -297,7 +318,7 @@ class TemplateArgument {
   /// Retrieve the type for a type template argument.
   QualType getAsType() const {
     assert(getKind() == Type && "Unexpected kind");
-    return QualType::getFromOpaquePtr(reinterpret_cast<void*>(TypeOrValue.V));
+    return QualType::getFromOpaquePtr(reinterpret_cast<void *>(TypeOrValue.V));
   }
 
   /// Retrieve the declaration for a declaration non-type
@@ -315,7 +336,7 @@ class TemplateArgument {
   /// Retrieve the type for null non-type template argument.
   QualType getNullPtrType() const {
     assert(getKind() == NullPtr && "Unexpected kind");
-    return QualType::getFromOpaquePtr(reinterpret_cast<void*>(TypeOrValue.V));
+    return QualType::getFromOpaquePtr(reinterpret_cast<void *>(TypeOrValue.V));
   }
 
   /// Retrieve the template name for a template name argument.
@@ -371,6 +392,14 @@ class TemplateArgument {
   /// default template parameter.
   bool getIsDefaulted() const { return (bool)TypeOrValue.IsDefaulted; }
 
+  /// Get the value of a StructuralValue.
+  const APValue &getAsStructuralValue() const { return *Value.Value; }
+
+  /// Get the type of a StructuralValue.
+  QualType getStructuralValueType() const {
+    return QualType::getFromOpaquePtr(Value.Type);
+  }
+
   /// If this is a non-type template argument, get its type. Otherwise,
   /// returns a null QualType.
   QualType getNonTypeTemplateArgumentType() const;
@@ -516,6 +545,7 @@ class TemplateArgumentLoc {
     assert(Argument.getKind() == TemplateArgument::NullPtr ||
            Argument.getKind() == TemplateArgument::Integral ||
            Argument.getKind() == TemplateArgument::Declaration ||
+           Argument.getKind() == TemplateArgument::StructuralValue ||
            Argument.getKind() == TemplateArgument::Expression);
   }
 
@@ -541,13 +571,9 @@ class TemplateArgumentLoc {
   /// - Fetches the full source range of the argument.
   SourceRange getSourceRange() const LLVM_READONLY;
 
-  const TemplateArgument &getArgument() const {
-    return Argument;
-  }
+  const TemplateArgument &getArgument() const { return Argument; }
 
-  TemplateArgumentLocInfo getLocInfo() const {
-    return LocInfo;
-  }
+  TemplateArgumentLocInfo getLocInfo() const { return LocInfo; }
 
   TypeSourceInfo *getTypeSourceInfo() const {
     if (Argument.getKind() != TemplateArgument::Type)
@@ -575,6 +601,11 @@ class TemplateArgumentLoc {
     return LocInfo.getAsExpr();
   }
 
+  Expr *getSourceStructuralValueExpression() const {
+    assert(Argument.getKind() == TemplateArgument::StructuralValue);
+    return LocInfo.getAsExpr();
+  }
+
   NestedNameSpecifierLoc getTemplateQualifierLoc() const {
     if (Argument.getKind() != TemplateArgument::Template &&
         Argument.getKind() != TemplateArgument::TemplateExpansion)
@@ -606,8 +637,7 @@ class TemplateArgumentListInfo {
 public:
   TemplateArgumentListInfo() = default;
 
-  TemplateArgumentListInfo(SourceLocation LAngleLoc,
-                           SourceLocation RAngleLoc)
+  TemplateArgumentListInfo(SourceLocation LAngleLoc, SourceLocation RAngleLoc)
       : LAngleLoc(LAngleLoc), RAngleLoc(RAngleLoc) {}
 
   // This can leak if used in an AST node, use ASTTemplateArgumentListInfo
@@ -626,21 +656,15 @@ class TemplateArgumentListInfo {
     return Arguments.data();
   }
 
-  llvm::ArrayRef<TemplateArgumentLoc> arguments() const {
-    return Arguments;
-  }
+  llvm::ArrayRef<TemplateArgumentLoc> arguments() const { return Arguments; }
 
   const TemplateArgumentLoc &operator[](unsigned I) const {
     return Arguments[I];
   }
 
-  TemplateArgumentLoc &operator[](unsigned I) {
-    return Arguments[I];
-  }
+  TemplateArgumentLoc &operator[](unsigned I) { return Arguments[I]; }
 
-  void addArgument(const TemplateArgumentLoc &Loc) {
-    Arguments.push_back(Loc);
-  }
+  void addArgument(const TemplateArgumentLoc &Loc) { Arguments.push_back(Loc); }
 };
 
 /// Represents an explicit template argument list in C++, e.g.,

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5154,8 +5154,6 @@ def err_non_type_template_arg_subobject : Error<
   "non-type template argument refers to subobject '%0'">;
 def err_non_type_template_arg_addr_label_diff : Error<
   "template argument / label address difference / what did you expect?">;
-def err_non_type_template_arg_unsupported : Error<
-  "non-type template argument of type %0 is not yet supported">;
 def err_template_arg_not_convertible : Error<
   "non-type template argument of type %0 cannot be converted to a value "
   "of type %1">;
@@ -5207,9 +5205,6 @@ def err_template_arg_not_object_or_func : Error<
   "non-type template argument does not refer to an object or function">;
 def err_template_arg_not_pointer_to_member_form : Error<
   "non-type template argument is not a pointer to member constant">;
-def err_template_arg_member_ptr_base_derived_not_supported : Error<
-  "non-type template argument of pointer-to-member type %1 that refers "
-  "to member %q0 of a different class is not supported yet">;
 def err_template_arg_invalid : Error<
   "non-type template argument '%0' is invalid">;
 def ext_template_arg_extra_parens : ExtWarn<

clang/include/clang/Sema/Sema.h

@@ -8595,8 +8595,8 @@ class Sema final {
                                           QualType ParamType,
                                           SourceLocation Loc);
   ExprResult
-  BuildExpressionFromIntegralTemplateArgument(const TemplateArgument &Arg,
-                                              SourceLocation Loc);
+  BuildExpressionFromNonTypeTemplateArgument(const TemplateArgument &Arg,
+                                             SourceLocation Loc);
 
   /// Enumeration describing how template parameter lists are compared
   /// for equality.

clang/lib/AST/ASTContext.cpp

@@ -6754,6 +6754,11 @@ ASTContext::getCanonicalTemplateArgument(const TemplateArgument &Arg) const {
     case TemplateArgument::Integral:
       return TemplateArgument(Arg, getCanonicalType(Arg.getIntegralType()));
 
+    case TemplateArgument::StructuralValue:
+      return TemplateArgument(*this,
+                              getCanonicalType(Arg.getStructuralValueType()),
+                              Arg.getAsStructuralValue());
+
     case TemplateArgument::Type:
       return TemplateArgument(getCanonicalType(Arg.getAsType()),
                               /*isNullPtr*/ false, Arg.getIsDefaulted());

clang/lib/AST/ASTImporter.cpp

@@ -823,6 +823,17 @@ ASTNodeImporter::import(const TemplateArgument &From) {
                             From.getIsDefaulted());
   }
 
+  case TemplateArgument::StructuralValue: {
+    ExpectedType ToTypeOrErr = import(From.getStructuralValueType());
+    if (!ToTypeOrErr)
+      return ToTypeOrErr.takeError();
+    Expected<APValue> ToValueOrErr = import(From.getAsStructuralValue());
+    if (!ToValueOrErr)
+      return ToValueOrErr.takeError();
+    return TemplateArgument(Importer.getToContext(), *ToTypeOrErr,
+                            *ToValueOrErr);
+  }
+
   case TemplateArgument::Template: {
     Expected<TemplateName> ToTemplateOrErr = import(From.getAsTemplate());
     if (!ToTemplateOrErr)
@@ -3572,6 +3583,8 @@ class IsTypeDeclaredInsideVisitor
     case TemplateArgument::NullPtr:
       // FIXME: The type is not allowed to be in the function?
       return CheckType(Arg.getNullPtrType());
+    case TemplateArgument::StructuralValue:
+      return CheckType(Arg.getStructuralValueType());
     case TemplateArgument::Pack:
       for (const auto &PackArg : Arg.getPackAsArray())
         if (checkTemplateArgument(PackArg))

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -685,6 +685,9 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
     return IsStructurallyEquivalent(Context, Arg1.getAsExpr(),
                                     Arg2.getAsExpr());
 
+  case TemplateArgument::StructuralValue:
+    return Arg1.structurallyEquals(Arg2);
+
   case TemplateArgument::Pack:
     return IsStructurallyEquivalent(Context, Arg1.pack_elements(),
                                     Arg2.pack_elements());

clang/lib/AST/Decl.cpp

@@ -343,6 +343,10 @@ LinkageComputer::getLVForTemplateArgumentList(ArrayRef<TemplateArgument> Args,
       LV.merge(getTypeLinkageAndVisibility(Arg.getNullPtrType()));
       continue;
 
+    case TemplateArgument::StructuralValue:
+      LV.merge(getLVForValue(Arg.getAsStructuralValue(), computation));
+      continue;
+
     case TemplateArgument::Template:
     case TemplateArgument::TemplateExpansion:
       if (TemplateDecl *Template =