[Driver][SYCL] Adjust default setting implementation

Modify setting implementation to be more inline with existing flow.  This
also allows for MSVC /std option settings to override.

Signed-off-by: Michael D Toguchi <[email protected]>

Author: mdtoguchi

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5078,13 +5078,6 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                                  options::OPT_fno_trigraphs))
       if (A != Std)
         A->render(Args, CmdArgs);
-  } else if (IsSYCL && types::isCXX(InputType)) {
-    // For DPC++, we default to -std=c++17 for all compilations.  Use of -std
-    // on the command line will override.
-    CmdArgs.push_back("-std=c++17");
-
-    Args.AddLastArg(CmdArgs, options::OPT_ftrigraphs,
-                    options::OPT_fno_trigraphs);
   } else {
     // Honor -std-default.
     //
@@ -5097,6 +5090,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                                 /*Joined=*/true);
     else if (IsWindowsMSVC)
       ImplyVCPPCXXVer = true;
+    else if (IsSYCL)
+      // For DPC++, we default to -std=c++17 for all compilations.  Use of -std
+      // on the command line will override.
+      CmdArgs.push_back("-std=c++17");
 
     Args.AddLastArg(CmdArgs, options::OPT_ftrigraphs,
                     options::OPT_fno_trigraphs);
@@ -5665,7 +5662,11 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
     if (LanguageStandard.empty()) {
       if (IsMSVC2015Compatible)
-        LanguageStandard = "-std=c++14";
+        if (IsSYCL)
+          // For DPC++, C++17 is the default.
+          LanguageStandard = "-std=c++17";
+        else
+          LanguageStandard = "-std=c++14";
       else
         LanguageStandard = "-std=c++11";
     }

clang/test/Driver/sycl-offload.c

@@ -821,5 +821,11 @@
 // CHECK-STD: clang{{.*}} "-fsyntax-only" {{.*}} "-std=c++17"
 // CHECK-STD: clang{{.*}} "-emit-obj" {{.*}} "-std=c++17"
 
+// -std=c++17 override check
+// RUN: %clangxx -### -c -fsycl -std=c++14 -xc++ %s 2>&1 | FileCheck -check-prefix=CHECK-STD-OVR %s
+// RUN: %clang_cl -### -c -fsycl /std:c++14 -TP %s 2>&1 | FileCheck -check-prefix=CHECK-STD-OVR %s
+// CHECK-STD-OVR: clang{{.*}} "-std=c++14"
+// CHECK-STD-OVR-NOT: clang{{.*}} "-std=c++17"
+
 // TODO: SYCL specific fail - analyze and enable
 // XFAIL: windows-msvc