[clang] Represent array refs as TemplateArgument::Declaration

This returns (probably temporarily) array-referring NTTP behavior to which was prior to #78041 to avoid regressions.

Author: bolshakov-a

clang/lib/Sema/SemaTemplate.cpp

@@ -7417,9 +7417,9 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
  if (ArgResult.isInvalid())
  return ExprError();
 
- // Prior to C++20, enforce restrictions on possible template argument
- // values.
- if (!getLangOpts().CPlusPlus20 && Value.isLValue()) {
+ if (Value.isLValue()) {
+  APValue::LValueBase Base = Value.getLValueBase();
+  auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
  // For a non-type template-parameter of pointer or reference type,
  // the value of the constant expression shall not refer to
  assert(ParamType->isPointerType() || ParamType->isReferenceType() ||
@@ -7428,33 +7428,37 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
  // -- a string literal
  // -- the result of a typeid expression, or
  // -- a predefined __func__ variable
- APValue::LValueBase Base = Value.getLValueBase();
- auto *VD = const_cast<ValueDecl *>(Base.dyn_cast<const ValueDecl *>());
  if (Base &&
  (!VD ||
  isa<LifetimeExtendedTemporaryDecl, UnnamedGlobalConstantDecl>(VD))) {
  Diag(Arg->getBeginLoc(), diag::err_template_arg_not_decl_ref)
  << Arg->getSourceRange();
  return ExprError();
  }
- // -- a subobject [until C++20]
- if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 &&
- VD && VD->getType()->isArrayType() &&
+
+ if (Value.hasLValuePath() && Value.getLValuePath().size() == 1 && VD &&
+ VD->getType()->isArrayType() &&
  Value.getLValuePath()[0].getAsArrayIndex() == 0 &&
  !Value.isLValueOnePastTheEnd() && ParamType->isPointerType()) {
- // Per defect report (no number yet):
- // ... other than a pointer to the first element of a complete array
- // object.
- } else if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
- Value.isLValueOnePastTheEnd()) {
- Diag(StartLoc, diag::err_non_type_template_arg_subobject)
- << Value.getAsString(Context, ParamType);
- return ExprError();
+ SugaredConverted = TemplateArgument(VD, ParamType);
+ CanonicalConverted = TemplateArgument(
+ cast<ValueDecl>(VD->getCanonicalDecl()), CanonParamType);
+ return ArgResult.get();
+ }
+
+ // -- a subobject [until C++20]
+ if (!getLangOpts().CPlusPlus20) {
+ if (!Value.hasLValuePath() || Value.getLValuePath().size() ||
+ Value.isLValueOnePastTheEnd()) {
+ Diag(StartLoc, diag::err_non_type_template_arg_subobject)
+ << Value.getAsString(Context, ParamType);
+ return ExprError();
+ }
+ assert((VD || !ParamType->isReferenceType()) &&
+ "null reference should not be a constant expression");
+ assert((!VD || !ParamType->isNullPtrType()) &&
+ "non-null value of type nullptr_t?");
  }
- assert((VD || !ParamType->isReferenceType()) &&
- "null reference should not be a constant expression");
- assert((!VD || !ParamType->isNullPtrType()) &&
- "non-null value of type nullptr_t?");
  }
 
  if (Value.isAddrLabelDiff())