efriedma added inline comments.
================
Comment at: clang/lib/Sema/SemaExpr.cpp:10724
+ // Subtracting from a null pointer should produce a warning.
+ if (LHS.get()->IgnoreParenCasts()->isNullPointerConstant(
+ Context, Expr::NPC_ValueDependentIsNotNull))
----------------
IgnoreParenCasts() seems a little weird here; e.g. this code thinks
`(char*)(char)256` isn't null? Not sure how much it practically matters,
though, given this is just a warning.
================
Comment at: clang/test/Sema/pointer-addition.c:34
+ f = (char*)(f - (char*)0); // expected-warning {{performing pointer
arithmetic on a null pointer has undefined behavior}}
+ f = (char*)((char*)0 - (char*)0); // expected-warning {{performing pointer
arithmetic on a null pointer has undefined behavior}} expected-warning
{{performing pointer arithmetic on a null pointer has undefined behavior}}
}
----------------
This is what I was afraid would happen.
C++ has a specific carveout in the standard that "null-null" isn't undefined. C
doesn't, but I'm not sure warning is practically useful there.
In any case, printing the same warning twice isn't useful.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D98798/new/
https://reviews.llvm.org/D98798
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