[clang] [PAC] Re-sign a pointer to a noexcept member function when it is converted to a pointer to a member function without noexcept (PR #109056)

[John McCall via cfe-commits]

================
@@ -2419,8 +2419,13 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return Visit(const_cast<Expr*>(E));
 
   case CK_NoOp: {
-    return CE->changesVolatileQualification() ? EmitLoadOfLValue(CE)
-                                              : Visit(const_cast<Expr *>(E));
+    if (CE->changesVolatileQualification())
+      return EmitLoadOfLValue(CE);
+    auto V = Visit(const_cast<Expr *>(E));
+    if (CGF.CGM.getCodeGenOpts().PointerAuth.CXXMemberFunctionPointers &&
+        CE->getType()->isMemberFunctionPointerType())
+      V = CGF.CGM.getCXXABI().EmitMemberPointerConversion(CGF, CE, V);
+    return V;
----------------
rjmccall wrote:

Okay, but that's coincidental, right?  The language doesn't semantically guarantee that that conversion is a no-op, it just happens to be under normal conditions, even with function pointer type diversity enabled.

What cast kind do we use if you cast a function pointer to a completely different type?

https://github.com/llvm/llvm-project/pull/109056