[clang] [clang][analyzer] Improved PointerSubChecker (PR #93676)

[Donát Nagy via cfe-commits]

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@@ -0,0 +1,74 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=alpha.core.PointerSub -verify %s
+
+void f1(void) {
+  int x, y, z[10];
+  int d = &y - &x; // expected-warning{{Subtraction of two pointers that do not point into the same array is undefined behavior}}
+  d = z - &y; // expected-warning{{Subtraction of two pointers that do not point into the same array is undefined behavior}}
+  d = &x - &x; // expected-warning{{Subtraction of two pointers that do not point into the same array is undefined behavior}}
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NagyDonat wrote:

This corner case is explicitly allowed by the standard: non-array variables act as if they were single-element arrays and it's valid to do (trivial) pointer arithmetic on them.

Even calculating the past-the-end pointer of a non-array object (e.g. `&x + 1`) is a valid (but unusual) operation; however the [description on cppreference](https://en.cppreference.com/w/cpp/language/operator_arithmetic) suggests that it's UB to take the difference `(&x+1)-&x`. You should probably look this up in the standard itself if you want to be accurate.



https://github.com/llvm/llvm-project/pull/93676