[clang][Sema] Skip access check on arrays of zero-length element

[danix800]

Summary

Bound check on array of zero-sized element isn't meaningful.

Fixes https://github.com/llvm/llvm-project/issues/64564

[jacquesguan]

LGTM

clang/docs/ReleaseNotes.rst

 ^^^^^^^^^^^^^^^^^^^^^^^
 
 Miscellaneous Clang Crashes Fixed
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a crash when parsing top-level ObjC blocks that aren't properly
   terminated. Clang should now also recover better when an @end is missing
   between blocks.
   `Issue 64065 <https://github.com/llvm/llvm-project/issues/64065>`_
+- Fixed a crash when check array access on zero-length element.
+  `Issue 64564 <https://github.com/llvm/llvm-project/issues/64564>`_
 
 Target Specific Changes
 -----------------------
 
 AMDGPU Support
 ^^^^^^^^^^^^^^
 
 X86 Support

clang/lib/Sema/SemaChecking.cpp

if (index.isUnsigned() || !index.isNegative()) {
       unsigned AddrBits = ASTC.getTargetInfo().getPointerWidth(
           EffectiveType->getCanonicalTypeInternal().getAddressSpace());
       if (index.getBitWidth() < AddrBits)
         index = index.zext(AddrBits);
       std::optional<CharUnits> ElemCharUnits =
           ASTC.getTypeSizeInCharsIfKnown(EffectiveType);
       // PR50741 - If EffectiveType has unknown size (e.g., if it's a void
       // pointer) bounds-checking isn't meaningful.
-      if (!ElemCharUnits)
+      if (!ElemCharUnits || ElemCharUnits->isZero())
         return;
       llvm::APInt ElemBytes(index.getBitWidth(), ElemCharUnits->getQuantity());
       // If index has more active bits than address space, we already know
       // we have a bounds violation to warn about.  Otherwise, compute
       // address of (index + 1)th element, and warn about bounds violation
       // only if that address exceeds address space.
       if (index.getActiveBits() <= AddrBits) {
         bool Overflow;

clang/test/Sema/array-bounds-zero-length-elem-gh64564.c

+// RUN: %clang_cc1 -triple i686-apple-darwin -verify %s
+
+int a[][0]; // expected-warning {{tentative array definition assumed to have one element}}
+void gh64564_1(void) {
+  int b = a[0x100000000][0];
+}
+
+typedef struct {} S;
+S s[]; // expected-warning {{tentative array definition assumed to have one element}}
+void gh64564_2(void) {
+  S t = s[0x100000000];
+}