[clang][Interp] Protect ArrayDecay ops against dummy pointers

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1891,6 +1891,9 @@ inline bool ArrayElemPop(InterpState &S, CodePtr OpPC, uint32_t Index) {
 inline bool ArrayDecay(InterpState &S, CodePtr OpPC) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
 
+  if (Ptr.isDummy())
+    return false;
+
   if (!Ptr.isUnknownSizeArray()) {
     S.Stk.push<Pointer>(Ptr.atIndex(0));
     return true;

clang/test/AST/Interp/arrays.cpp

@@ -598,3 +598,23 @@ namespace NonConstReads {
   const int y = 0;
   int yy[y];
 }
+
+namespace SelfComparison {
+  struct S {
+    int field;
+    static int static_field;
+    int array[4];
+  };
+
+  struct T {
+    int field;
+    static int static_field;
+    int array[4];
+    S s;
+  };
+
+  int struct_test(S s1, S s2, S *s3, T t) {
+    return s3->array[t.field] == s3->array[t.field];  // expected-warning {{self-comparison always evaluates to true}} \
+                                                      // ref-warning {{self-comparison always evaluates to true}}
+  };
+}

clang/test/SemaCXX/self-comparison.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++2a
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++2a -fexperimental-new-constant-interpreter
 
 int foo(int x) {
   return x == x; // expected-warning {{self-comparison always evaluates to true}}