[clang] [Sema][AArch64] Emit error for mismatched VLs on streaming mode transitions (PR #159131)

[Sander de Smalen via cfe-commits]

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@@ -3980,6 +3980,14 @@ def warn_sme_locally_streaming_has_vl_args_returns : Warning<
   "%select{returning|passing}0 a VL-dependent argument %select{from|to}0 a locally streaming function is undefined"
   " behaviour when the streaming and non-streaming vector lengths are different at runtime">,
   InGroup<AArch64SMEAttributes>, DefaultIgnore;
+def warn_sme_streaming_compatible_vl_mismatch : Warning<
+  "%select{returning|passing}0 a VL-dependent argument %select{from|to}0 a %select{non-streaming|streaming}1"
+  " function is undefined behaviour when the streaming-compatible caller is in %select{streaming|non-streaming}1"
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sdesmalen-arm wrote:

nit:
```suggestion
  " function is undefined behaviour when the streaming-compatible caller is %select{|not}1 in streaming mode"
```

https://github.com/llvm/llvm-project/pull/159131