[clang][Interp] Fix modifying const objects in functions calls in ctors

The current frame might not be a constructor for the object we're
initializing, but a parent frame might.

Author: tbaederr

clang/lib/AST/Interp/Interp.cpp

@@ -405,10 +405,16 @@ bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
 
   // The This pointer is writable in constructors and destructors,
   // even if isConst() returns true.
-  if (const Function *Func = S.Current->getFunction();
-      Func && (Func->isConstructor() || Func->isDestructor()) &&
-      Ptr.block() == S.Current->getThis().block()) {
-    return true;
+  // TODO(perf): We could be hitting this code path quite a lot in complex
+  // constructors. Is there a better way to do this?
+  if (S.Current->getFunction()) {
+    for (const InterpFrame *Frame = S.Current; Frame; Frame = Frame->Caller) {
+      if (const Function *Func = Frame->getFunction();
+          Func && (Func->isConstructor() || Func->isDestructor()) &&
+          Ptr.block() == Frame->getThis().block()) {
+        return true;
+      }
+    }
   }
 
   if (!Ptr.isBlockPointer())

clang/test/AST/Interp/records.cpp

@@ -1512,3 +1512,28 @@ namespace OnePastEndAndBack {
   constexpr const Base *d = c - 1;
   static_assert(d == &a, "");
 }
+
+namespace BitSet {
+  class Bitset {
+    unsigned Bit = 0;
+
+  public:
+    constexpr Bitset() {
+      int Init[2] = {1,2};
+      for (auto I : Init)
+        set(I);
+    }
+    constexpr void set(unsigned I) {
+      this->Bit++;
+      this->Bit = 1u << 1;
+    }
+  };
+
+  struct ArchInfo {
+    Bitset DefaultExts;
+  };
+
+  constexpr ArchInfo ARMV8A = {
+    Bitset()
+  };
+}