[clang][Interp] Delay compiling functions that don't have a body yet

Sometimes, isDefined() returns true, even though the function doesn't
have a body yet, but will have one later. This is for example the case
when referring to a class member function via a member pointer before
the member function has been fully parsed. Reject them at first and
compile them later.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeEmitter.cpp

@@ -147,7 +147,8 @@ Function *ByteCodeEmitter::compileFunc(const FunctionDecl *FuncDecl) {
   assert(Func);
   // For not-yet-defined functions, we only create a Function instance and
   // compile their body later.
-  if (!FuncDecl->isDefined()) {
+  if (!FuncDecl->isDefined() ||
+      (FuncDecl->willHaveBody() && !FuncDecl->hasBody())) {
     Func->setDefined(false);
     return Func;
   }

clang/test/AST/Interp/memberpointers.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -std=c++14 -fexperimental-new-constant-interpreter -verify=expected,both %s
+// RUN: %clang_cc1 -std=c++23 -fexperimental-new-constant-interpreter -verify=expected,both %s
 // RUN: %clang_cc1 -std=c++14 -verify=ref,both %s
+// RUN: %clang_cc1 -std=c++23 -verify=ref,both %s
 
 namespace MemberPointers {
   struct A {