Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -11071,8 +11071,6 @@
   "specialization %0">;
 def err_coroutine_promise_incompatible_return_functions : Error<
   "the coroutine promise type %0 declares both 'return_value' and 'return_void'">;
-def err_coroutine_promise_requires_return_function : Error<
-  "the coroutine promise type %0 must declare either 'return_value' or 'return_void'">;
 def note_coroutine_promise_implicit_await_transform_required_here : Note<
   "call to 'await_transform' implicitly required by 'co_await' here">;
 def note_coroutine_promise_suspend_implicitly_required : Note<
Index: clang/lib/Sema/SemaCoroutine.cpp
===================================================================
--- clang/lib/Sema/SemaCoroutine.cpp
+++ clang/lib/Sema/SemaCoroutine.cpp
@@ -1414,14 +1414,25 @@
         << LRValue.getLookupName();
     return false;
   } else if (!HasRVoid && !HasRValue) {
-    // FIXME: The PDTS currently specifies this case as UB, not ill-formed.
-    // However we still diagnose this as an error since until the PDTS is fixed.
-    S.Diag(FD.getLocation(),
-           diag::err_coroutine_promise_requires_return_function)
-        << PromiseRecordDecl;
-    S.Diag(PromiseRecordDecl->getLocation(), diag::note_defined_here)
-        << PromiseRecordDecl;
-    return false;
+    // [stmt.return.coroutine]p3:
+    // If p.return_­void() is a valid expression, flowing off the end of a
+    // coroutine's function-body is equivalent to a co_­return with no operand;
+    // otherwise flowing off the end of a coroutine's function-body results in
+    // undefined behavior.
+    //
+    // We need to set 'Fallthrough' otherwise the other analysis part might
+    // think the coroutine has defined a return_value method. So it might emit
+    // false positive warning. e.g.,
+    //
+    //    promise_without_return_func foo() {
+    //        co_await something();
+    //    }
+    //
+    // Then AnalysisBasedWarning would emit a warning about `foo()` lacks a
+    // co_return statements.
+    Fallthrough = S.ActOnNullStmt(PromiseRecordDecl->getLocation());
+    if (Fallthrough.isInvalid())
+      return false;
   } else if (HasRVoid) {
     // If the unqualified-id return_void is found, flowing off the end of a
     // coroutine is equivalent to a co_return with no operand. Otherwise,
Index: clang/test/SemaCXX/coroutines-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutines-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutines-exp-namespace.cpp
@@ -978,19 +978,6 @@
   co_return; //expected-note {{function is a coroutine due to use of 'co_return' here}}
 }
 
-struct bad_promise_no_return_func { // expected-note {{'bad_promise_no_return_func' defined here}}
-  coro<bad_promise_no_return_func> get_return_object();
-  suspend_always initial_suspend();
-  suspend_always final_suspend() noexcept;
-  void unhandled_exception();
-};
-// FIXME: The PDTS currently specifies this as UB, technically forbidding a
-// diagnostic.
-coro<bad_promise_no_return_func> no_return_value_or_return_void() {
-  // expected-error@-1 {{'bad_promise_no_return_func' must declare either 'return_value' or 'return_void'}}
-  co_await a;
-}
-
 struct bad_await_suspend_return {
   bool await_ready();
   // expected-error@+1 {{return type of 'await_suspend' is required to be 'void' or 'bool' (have 'char')}}
Index: clang/test/SemaCXX/coroutines.cpp
===================================================================
--- clang/test/SemaCXX/coroutines.cpp
+++ clang/test/SemaCXX/coroutines.cpp
@@ -970,16 +970,18 @@
   co_return; //expected-note {{function is a coroutine due to use of 'co_return' here}}
 }
 
-struct bad_promise_no_return_func { // expected-note {{'bad_promise_no_return_func' defined here}}
-  coro<bad_promise_no_return_func> get_return_object();
+struct promise_no_return_func {
+  coro<promise_no_return_func> get_return_object();
   suspend_always initial_suspend();
   suspend_always final_suspend() noexcept;
   void unhandled_exception();
 };
-// FIXME: The PDTS currently specifies this as UB, technically forbidding a
-// diagnostic.
-coro<bad_promise_no_return_func> no_return_value_or_return_void() {
-  // expected-error@-1 {{'bad_promise_no_return_func' must declare either 'return_value' or 'return_void'}}
+// [dcl.fct.def.coroutine]p6:
+// If searches for the names return_­void and return_­value in the scope of the promise type each find any declarations, the program is ill-formed.
+// [Note 1: If return_­void is found, flowing off the end of a coroutine is equivalent to a co_­return with no operand. Otherwise, flowing off the end of a coroutine results in undefined behavior ([stmt.return.coroutine]). — end note]
+//
+// So it isn't ill-formed if the promise doesn't define return_value and return_void. It is just a UB.
+coro<promise_no_return_func> no_return_value_or_return_void() {
   co_await a;
 }