[clang][bytecode] Fix two small builtin_constant_p cases (#137587)

Only accept string literals if we're pointing to the first index and do
accept complex literals.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -321,8 +321,11 @@ bool CheckBCPResult(InterpState &S, const Pointer &Ptr) {
   if (Ptr.isTypeidPointer())
     return true;
 
+  if (Ptr.getType()->isAnyComplexType())
+    return true;
+
   if (const Expr *Base = Ptr.getDeclDesc()->asExpr())
-    return isa<StringLiteral>(Base);
+    return isa<StringLiteral>(Base) && Ptr.getIndex() == 0;
   return false;
 }
 

clang/test/AST/ByteCode/builtin-constant-p.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++20 -fexperimental-new-constant-interpreter -verify=expected,both %s
-// RUN: %clang_cc1 -std=c++20 -verify=ref,both %s
+// RUN: %clang_cc1 -std=c++20 -verify=expected,both %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++20 -verify=ref,both      %s
 
 using intptr_t = __INTPTR_TYPE__;
 
@@ -129,3 +129,14 @@ void g() {
   const float f = __builtin_is_constant_evaluated();
   static_assert(fold(f == 0.0f));
 }
+
+void test17(void) {
+#define ASSERT(...) { enum { folded = (__VA_ARGS__) }; int arr[folded ? 1 : -1]; }
+#define T(...) ASSERT(__builtin_constant_p(__VA_ARGS__))
+#define F(...) ASSERT(!__builtin_constant_p(__VA_ARGS__))
+
+  T(3i + 5);
+  T("string literal");
+  F("string literal" + 1); // both-warning {{adding}} \
+                           // both-note {{use array indexing}}
+}