[ConstEval] Fix crash when comparing strings past the end

When `ArePotentiallyOverlappingStringLiterals`, added in
llvm#109208, compares string
literals it drops the front of the string with the greatest offset from
its base pointer. The number of characters dropped is equal to the
difference between the two strings' offsets from their base pointers.
This would trigger an assert when the resulting offset is past the end
of the object. Not only are one-past-the-end pointers legal constructs,
the compiler should not crash even when faced with illegal constructs.

rdar://149865910

Author: hnrklssn

clang/lib/AST/ExprConstant.cpp

@@ -2234,10 +2234,15 @@ static bool ArePotentiallyOverlappingStringLiterals(const EvalInfo &Info,
   // within RHS. We don't need to look at the characters of one string that
   // would appear before the start of the other string if they were merged.
   CharUnits Offset = RHS.Offset - LHS.Offset;
-  if (Offset.isNegative())
+  if (Offset.isNegative()) {
+    if (LHSString.Bytes.size() < (size_t)-Offset.getQuantity())
+      return false;
     LHSString.Bytes = LHSString.Bytes.drop_front(-Offset.getQuantity());
-  else
+  } else {
+    if (RHSString.Bytes.size() < (size_t)Offset.getQuantity())
+      return false;
     RHSString.Bytes = RHSString.Bytes.drop_front(Offset.getQuantity());
+  }
 
   bool LHSIsLonger = LHSString.Bytes.size() > RHSString.Bytes.size();
   StringRef Longer = LHSIsLonger ? LHSString.Bytes : RHSString.Bytes;

clang/test/AST/ByteCode/cxx20.cpp

@@ -119,6 +119,15 @@ constexpr auto b3 = name1() == name1(); // ref-error {{must be initialized by a
 constexpr auto b4 = name1() == name2();
 static_assert(!b4);
 
+constexpr auto bar(const char *p) { return p + __builtin_strlen(p); }
+constexpr auto b5 = bar(p1) == p1;
+static_assert(!b5);
+constexpr auto b6 = bar(p1) == ""; // ref-error {{must be initialized by a constant expression}} \
+                                   // ref-note {{comparison of addresses of potentially overlapping literals}}
+constexpr auto b7 = bar(p1) + 1 == ""; // both-error {{must be initialized by a constant expression}} \
+                                       // ref-note {{comparison against pointer '&"test1"[6]' that points past the end of a complete object has unspecified value}} \
+                                       // expected-note {{comparison against pointer '&"test1"[6] + 1' that points past the end of a complete object has unspecified value}}
+
 namespace UninitializedFields {
   class A {
   public:

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -2199,6 +2199,8 @@ namespace BuiltinStrlen {
   static_assert(__builtin_strlen("foo") == 3, "");
   static_assert(__builtin_strlen("foo\0quux") == 3, "");
   static_assert(__builtin_strlen("foo\0quux" + 4) == 4, "");
+  static_assert(__builtin_strlen("foo") + 1 + "foo" == "foo", ""); // expected-error {{static assertion expression is not an integral constant expression}}
+  // expected-note@-1 {{comparison against pointer '&"foo"[4]' that points past the end of a complete object has unspecified value}}
 
   constexpr bool check(const char *p) {
     return __builtin_strlen(p) == 3 &&