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

[Corentin Jabot via cfe-commits]

================
@@ -7130,6 +7130,97 @@ 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");
+
+  // Fast path. Try to find the copy constructor which will be selected by
+  // overload resolution. Trivial copy constructor performs per-element copy.
+  if (auto *CXXRecord = ParamType->castAsCXXRecordDecl();
+      !CXXRecord->hasUserDeclaredConstructor()) {
+
+    if (CXXRecord->hasTrivialCopyConstructor() &&
+        CXXRecord->implicitCopyConstructorHasConstParam() &&
+        !CXXRecord->needsOverloadResolutionForCopyConstructor())
+      return false;
+
+  } else {
+    // If there is a copy constructor with const parameter and without explicit,
+    // it should be selected by overload resolution.
+    CXXConstructorDecl *CopyCtor = nullptr;
+
+    // If there is trailing requires clause, let overload resolution handle it.
+    bool FindCopyCtorWithTrailingRequiresClause = false;
----------------
cor3ntin wrote:

You can use `Ctor->isIneligibleOrNotSelected`.
I think Overload resolution is only needed in cases where the constructor selected for copy is itself a template

ie 

````cpp
struct S {
   constexpr S(auto&&);
};



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