[SYCL] Make sycl::exception nothrow copy constructible

sycl/include/sycl/exception.hpp

@@ -97,7 +97,9 @@ class __SYCL_EXPORT exception : public std::exception {
   cl_int get_cl_code() const;
 
 private:
-  std::string MMsg;
+  // Exceptions must be noexcept copy constructible, so cannot use std::string
+  // directly.
+  std::shared_ptr<std::string> MMsg;
   pi_int32 MPIErr;
   std::shared_ptr<context> MContext;
 
@@ -108,8 +110,9 @@ class __SYCL_EXPORT exception : public std::exception {
       : exception(std::string(Msg), PIErr, Context) {}
   exception(const std::string &Msg, const pi_int32 PIErr,
             std::shared_ptr<context> Context = nullptr)
-      : MMsg(Msg + " " + detail::codeToString(PIErr)), MPIErr(PIErr),
-        MContext(Context) {}
+      : MMsg(std::make_shared<std::string>(Msg + " " +
+                                           detail::codeToString(PIErr))),
+        MPIErr(PIErr), MContext(Context) {}
 
   // base constructors used by SYCL 1.2.1 exception subclasses
   exception(std::error_code ec, const char *Msg, const pi_int32 PIErr,
@@ -122,7 +125,8 @@ class __SYCL_EXPORT exception : public std::exception {
     MPIErr = PIErr;
   }
 
-  exception(const std::string &Msg) : MMsg(Msg), MContext(nullptr) {}
+  exception(const std::string &Msg)
+      : MMsg(std::make_shared<std::string>(Msg)), MContext(nullptr) {}
 
   // base constructor for all SYCL 2020 constructors
   // exception(context *ctxPtr, std::error_code ec, const std::string

sycl/source/exception.cpp

@@ -65,12 +65,12 @@ exception::exception(context Ctx, int EV, const std::error_category &ECat)
 // protected base constructor for all SYCL 2020 constructors
 exception::exception(std::error_code EC, std::shared_ptr<context> SharedPtrCtx,
                      const std::string &WhatArg)
-    : MMsg(WhatArg + ReservedForErrorcode), MPIErr(PI_ERROR_INVALID_VALUE),
-      MContext(SharedPtrCtx) {
+    : MMsg(std::make_shared<std::string>(WhatArg + ReservedForErrorcode)),
+      MPIErr(PI_ERROR_INVALID_VALUE), MContext(SharedPtrCtx) {
   // For compatibility with previous implementation, we are "hiding" the
   // std::error_code in the MMsg string, behind the null string terminator
-  const int StringTermPoint = MMsg.length() - strlen(ReservedForErrorcode);
-  char *ReservedPtr = &MMsg[StringTermPoint];
+  const int StringTermPoint = MMsg->length() - strlen(ReservedForErrorcode);
+  char *ReservedPtr = &(*MMsg)[StringTermPoint];
   ReservedPtr[0] = '\0';
   ReservedPtr++;
   // insert error code
@@ -79,9 +79,9 @@ exception::exception(std::error_code EC, std::shared_ptr<context> SharedPtrCtx,
 }
 
 const std::error_code &exception::code() const noexcept {
-  const char *WhatStr = MMsg.c_str();
+  const char *WhatStr = MMsg->c_str();
   // advance to inner string-terminator
-  int StringTermPoint = MMsg.length() - strlen(ReservedForErrorcode);
+  int StringTermPoint = MMsg->length() - strlen(ReservedForErrorcode);
   if (StringTermPoint >= 0) {
     const char *ReservedPtr = &WhatStr[StringTermPoint];
     // check for string terminator, which denotes a SYCL 2020 exception
@@ -100,7 +100,7 @@ const std::error_category &exception::category() const noexcept {
   return code().category();
 }
 
-const char *exception::what() const noexcept { return MMsg.c_str(); }
+const char *exception::what() const noexcept { return MMsg->c_str(); }
 
 bool exception::has_context() const { return (MContext != nullptr); }
 

sycl/test/regression/exception_nothrow_copy_constructible.cpp

@@ -0,0 +1,11 @@
+// RUN: %clangxx -fsycl -fsyntax-only -Xclang -verify %s
+// expected-no-diagnostics
+
+#include <sycl/sycl.hpp>
+#include <type_traits>
+
+int main() {
+  static_assert(std::is_nothrow_copy_constructible_v<sycl::exception>,
+                "sycl::exception is not nothrow copy constructible.");
+  return 0;
+}