[clang][Interp] Check pointers for live-ness when returning them

[tbaeder]

Summary

We might be trying to return a pointer or reference to a local variable,
which doesn't work.

[cor3ntin]

LGTM. It may be worth submitting an issue for the current behavior which is wrong.
Do you want to add a quote to the standard? (https://eel.is/c++draft/expr.const#13.3)

clang/lib/AST/Interp/Interp.h

 //===----------------------------------------------------------------------===//
 // Returning values
 //===----------------------------------------------------------------------===//
 
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
   const T &Ret = S.Stk.pop<T>();
 
+  // Make sure returned pointers are live. We might be trying to return a
+  // pointer or reference to a local variable.
+  // Just return false, since a diagnostic has already been emitted in Sema.
+  if constexpr (std::is_same_v<T, Pointer>) {
+    // FIXME: We could be calling isLive() here, but the emitted diagnostics
+    // seem a little weird, at least if the returned expresion is of
+    // pointer type.
+    if (!Ret.isLive())
+      return false;
+  }
+
   assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
   if (!S.checkingPotentialConstantExpression() || S.Current->Caller)
     S.Current->popArgs();
 
   if (InterpFrame *Caller = S.Current->Caller) {
     PC = S.Current->getRetPC();
     delete S.Current;
     S.Current = Caller;

clang/test/AST/Interp/functions.cpp

 
 namespace CallWithArgs {
   /// This used to call problems during checkPotentialConstantExpression() runs.
   constexpr void g(int a) {}
   constexpr void f() {
     g(0);
   }
 }
+
+namespace ReturnLocalPtr {
+  constexpr int *p() {
+    int a = 12;
+    return &a; // ref-warning {{address of stack memory}} \
+               // expected-warning {{address of stack memory}}
+  }
+
+  /// GCC rejects the expression below, just like the new interpreter. The current interpreter
+  /// however accepts it and only warns about the function above returning an address to stack
+  /// memory. If we change the condition to 'p() != nullptr', it even succeeds.
+  static_assert(p() == nullptr, ""); // ref-error {{static assertion failed}} \
+                                     // expected-error {{not an integral constant expression}}
+
+  /// FIXME: The current interpreter emits diagnostics in the reference case below, but the
+  /// new one does not.
+  constexpr const int &p2() {
+    int a = 12; // ref-note {{declared here}}
+    return a; // ref-warning {{reference to stack memory associated with local variable}} \
+              // expected-warning {{reference to stack memory associated with local variable}}
+  }
+
+  static_assert(p2() == 12, ""); // ref-error {{not an integral constant expression}} \
+                                 // ref-note {{read of variable whose lifetime has ended}} \
+                                 // expected-error {{not an integral constant expression}}
+}