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

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,7 @@
+// Verify that a .c file compiled with -fsycl is an error.
+// RUN: not %clang -### -fsycl %s 2>&1 | FileCheck -check-prefix ERR %s
+// ERR: error: invalid argument '{{.*}}sycl-c-warn.c' not allowed with '-fsycl'
+
+// Verify that explicitly forcing -x c with -fsycl is also an error.
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AaronBallman wrote:

I guess I'm not seeing why the driver would provide a more user-friendly error message; I think the messages would be the same either way? But if it turns out that my suggestion has a poor user experience, I don't insist it only be done from the frontend -- I am mostly trying to avoid duplicating the logic for checking for a C compilation unit. Everything eventually funnels through the frontend, so it seems most reasonable to catch it there instead of from the driver.

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