[compiler-rt] [TySan] Fix struct access with different bases (PR #120412)

[Florian Hahn via llvm-commits]

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@@ -131,6 +131,14 @@ static bool isAliasingLegalUp(tysan_type_descriptor *TDA,
           break;
       }
 
+      // This offset can't be negative. Therefore we must be accessing something
+      // partially inside the last type
+      // We shouldn't check this if we are on the first member, Idx will
+      // underflow The first member can be offset in rare cases such as
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fhahn wrote:

I think with that we are missing the case where we are accessing the last field (I think the only case covered by the test), so we should probably stick with the current check. 

It would be good to add a test cases where we need to adjust Idx for other indices.

(also comments still need adjusting/reflowing

https://github.com/llvm/llvm-project/pull/120412