[clang] [SYCL] Error on C inputs when compiling with -fsycl (PR #200318)

[Srividya Sundaram via cfe-commits]

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@@ -30,3 +30,7 @@
 // RUN: %clangxx -### -fsycl -sycl-std=2017 --no-offloadlib -- %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO-OFFLOADLIB
 // RUN: %clang_cl -### -fsycl -sycl-std=2017 --no-offloadlib -- %s 2>&1 | FileCheck %s --check-prefix=CHECK-NO-OFFLOADLIB
 // CHECK-NO-OFFLOADLIB-NOT: warning: unknown argument ignored in clang-cl: '--no-offloadlib'
+
+// Verify that -fsycl with a C input is an error (enforced in the frontend).
+// RUN: not %clang -fsycl -x c %s 2>&1 | FileCheck %s --check-prefix=CHECK-C-INPUT
+// CHECK-C-INPUT: error: invalid argument 'C' not allowed with '-fsycl'
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srividya-sundaram wrote:

I'd like to keep this test since it validates the full driver → frontend path, confirming that invoking `clang -fsycl -x c` also rejects C inputs, not just direct cc1 invocations. It's a lightweight sanity check for the end-to-end behavior

https://github.com/llvm/llvm-project/pull/200318