[clang] [Clang][Sema] Reject template arguments not equivalent to their copies (part of P2308R1) (PR #193754)

[Yanzuo Liu via cfe-commits]

================
@@ -7130,6 +7130,71 @@ static bool CheckTemplateArgumentPointerToMember(
   return true;
 }
 
+// P2308R1  C++26 [temp.arg.nontype]p4:
+//   ... If, for the initialization from any candidate initializer,
+//     - ...
+//     - the initialization would cause P to not be
+//       template-argument-equivalent ([temp.type]) to v,
+//   the program is ill-formed.
+static bool CheckTemplateArgumentCopyEquivalence(Sema &S, NamedDecl *Param,
+                                                 QualType ParamType,
+                                                 const APValue &Value,
+                                                 SourceLocation ArgLoc) {
+  assert(ParamType->isRecordType() && "no need to check copy equivalence");
+
+  if (ParamType->castAsCXXRecordDecl()->isTriviallyCopyTemplateParam())
+    return false;
+
+  SourceLocation ParamLoc = Param->getLocation();
+
+  // Instead of creating a variable (C++26 [temp.arg.nontype]p3),
+  // create a template parameter object to represent the candidate initializer.
+  // They are equivalent during creating a copy.
+  auto *CandidateInitializer =
+      S.BuildDeclRefExpr(S.Context.getTemplateParamObjectDecl(ParamType, Value),
+                         ParamType.withConst(), VK_LValue, ArgLoc);
+  InitializationKind Kind = InitializationKind::CreateForInit(
+      ArgLoc, /*DirectInit=*/false, CandidateInitializer);
+  Expr *Inits[1] = {CandidateInitializer};
+  InitializedEntity Entity =
+      InitializedEntity::InitializeTemplateParameter(ParamType, Param);
+  InitializationSequence InitSeq(S, Entity, Kind, Inits);
+  ExprResult Result = InitSeq.Perform(S, Entity, Kind, Inits);
+  if (Result.isInvalid())
+    return S.Diag(ParamLoc, diag::note_template_arg_requires_copy);
+
+  if (cast<CXXConstructExpr>(Result.get())->getConstructor()->isTrivial()) {
+    ParamType->castAsCXXRecordDecl()->setTriviallyCopyTemplateParam();
+    return false;
+  }
----------------
zwuis wrote:

AFAIK, to deal with `explicit`, we need to perform name lookup, and traverse all constructors to find the copy constructor.

Please let me know whether you are happy with it or not.

https://github.com/llvm/llvm-project/pull/193754