[clang] [LifetimeSafety] Propagate origins through BaseToDerived casts (PR #231896)

[via cfe-commits]

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@@ -0,0 +1,71 @@
+// RUN: %clang_cc1 -fsyntax-only -Wlifetime-safety -Wno-dangling -verify %s
+
+struct Base {};
+struct Derived : Base {};
+Derived *DowncastReturnsStackAddress() {
+  Derived d;
+  Base *b = &d;
+  // expected-warning at -1 {{stack memory associated with local variable 'd' is returned}}
+  return static_cast<Derived *>(b); // expected-note {{returned here}}
+}
+
+// A downcast that does not outlive the pointee is fine.
+void use(Derived *);
+void DowncastWithinScopeIsOk() {
+  Derived d;
+  Base *b = &d;
+  use(static_cast<Derived *>(b)); // ok
+}
+
+struct RefBase {};
+struct RefDerived : RefBase {};
+RefDerived *DowncastReferenceReturnsStackAddress() {
+  RefDerived d;
+  RefBase &b = d;
+  // expected-warning at -1 {{stack memory associated with local variable 'd' is returned}}
+  RefDerived &r = static_cast<RefDerived &>(b);
+  return &r;      // expected-note {{returned here}}
+}
+
+void use(Derived &);
+void DowncastReferenceWithinScopeIsOk() {
+  Derived d;
+  Base &b = d;
+  use(static_cast<Derived &>(b)); // ok
+}
+
+// ****************************************************************************
+// Different origin shapes for BaseToDerived and DerivedToBase casts.
+// ****************************************************************************
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NeKon69 wrote:

I don't think we usually write such comments? Especially with stars and everything. This probably doesn't matter that much though.

https://github.com/llvm/llvm-project/pull/231896