[clang] [analyzer] Use AllocaRegion in MallocChecker (PR #72402)

[via cfe-commits]

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@@ -266,13 +266,18 @@ void CheckUseZeroAllocated1(void) {
 }
 
 char CheckUseZeroAllocated2(void) {
+  // FIXME: The return value of `alloca()` is modeled with `AllocaRegion`
+  // instead of `SymbolicRegion`, so the current implementation of
+  // `MallocChecker::checkUseZeroAllocated()` cannot handle it; and we get an
+  // unrelated, but suitable warning from core.uninitialized.UndefReturn.
   char *p = alloca(0);
-  return *p; // expected-warning {{Use of memory allocated with size zero}}
+  return *p; // expected-warning {{Undefined or garbage value returned to caller}}
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DonatNagyE wrote:

It seems that `alloca()` in general and `alloca(0)` in particular can mean many things, and I don't think that it's worth to create a specific error message because I cannot say anything concrete in it. This is a nonstandard function, and while we can model its "basic" behavior, I think that we shouldn't try to deal with its corner cases.

https://github.com/llvm/llvm-project/pull/72402