Eval string one past end reland (#137091)

Relands #137078 after updating clang/test/AST/ByteCode/cxx20.cpp to
account for diagnostic outputs that differ between Linux and macOS.

Author: hnrklssn

clang/lib/AST/ExprConstant.cpp

@@ -2232,10 +2232,15 @@ 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 +119,14 @@ 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}} \
+                                       // both-note {{comparison against pointer '&"test1"[6]' that points past the end of a complete object has unspecified value}}
+
 namespace UninitializedFields {
   class A {
   public:

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

@@ -2203,6 +2203,8 @@ 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 &&