Index: flang/lib/Semantics/resolve-names.cpp
===================================================================
--- flang/lib/Semantics/resolve-names.cpp
+++ flang/lib/Semantics/resolve-names.cpp
@@ -1089,6 +1089,9 @@
     bool sequence{false}; // is a sequence type
     const Symbol *type{nullptr}; // derived type being defined
     bool isStructure{false}; // is a DEC STRUCTURE
+    std::vector<std::tuple<SourceName, Scope *, DerivedTypeSpec *>>
+        delayedInstantiations; // We may have to delay instantiations of derived
+                               // types to avoid cycles.
   } derivedTypeInfo_;
   // In a ProcedureDeclarationStmt or ProcComponentDefStmt, this is
   // the interface name, if any.
@@ -5139,9 +5142,20 @@
       // Direct recursive use of a type in the definition of one of its
       // components: defer instantiation
     } else {
-      auto restorer{
-          GetFoldingContext().messages().SetLocation(currStmtSource().value())};
-      derived.Instantiate(currScope());
+      if (currScope().IsDerivedType() &&
+          GetAttrs().HasAny({Attr::POINTER, Attr::ALLOCATABLE})) {
+        // We risk ourselves to instantiate via a cycle of forward references
+        // an incomplete class. Delay what we wanted to instantiate and
+        // do that at the end of the derived-type-def.
+        // This works because initializers of these data components are
+        // not allowed (ALLOCATABLE) or they are very limited (POINTER).
+        derivedTypeInfo_.delayedInstantiations.emplace_back(
+            currStmtSource().value(), &currScope(), &derived);
+      } else {
+        auto restorer{GetFoldingContext().messages().SetLocation(
+            currStmtSource().value())};
+        derived.Instantiate(currScope());
+      }
     }
     SetDeclTypeSpec(type);
   }
@@ -5229,6 +5243,12 @@
   }
   Walk(std::get<std::optional<parser::TypeBoundProcedurePart>>(x.t));
   Walk(std::get<parser::Statement<parser::EndTypeStmt>>(x.t));
+  // Instantiate delayed types now that the type has been fully checked.
+  for (auto [source, scope, derivedTypeSpec] :
+      derivedTypeInfo_.delayedInstantiations) {
+    auto restorer{GetFoldingContext().messages().SetLocation(source)};
+    derivedTypeSpec->Instantiate(*scope);
+  }
   derivedTypeInfo_ = {};
   PopScope();
   return false;
Index: flang/test/Semantics/typeinfo05.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/typeinfo05.f90
@@ -0,0 +1,16 @@
+!RUN: bbc --dump-symbols %s | FileCheck %s
+!RUN: %flang_fc1 -fdebug-dump-symbols %s | FileCheck %s
+! Ensure that cycles via POINTERs do not instantiate incomplete derived
+! types that would lead to types whose sizeinbytes=0
+program main
+  implicit none
+  type t1
+    type(t2), pointer :: b
+  end type t1
+!CHECK: .dt.t1, SAVE, TARGET (CompilerCreated, ReadOnly): ObjectEntity type: TYPE(derivedtype) init:derivedtype(binding=NULL(),name=.n.t1,sizeinbytes=40_8,uninstantiated=NULL(),kindparameter=NULL(),lenparameterkind=NULL(),component=.c.t1,procptr=NULL(),special=NULL(),specialbitset=0_4,hasparent=0_1,noinitializationneeded=0_1,nodestructionneeded=1_1,nofinalizationneeded=1_1)
+  type :: t2
+    type(t1), pointer :: a
+  end type t2
+! CHECK: .dt.t2, SAVE, TARGET (CompilerCreated, ReadOnly): ObjectEntity type: TYPE(derivedtype) init:derivedtype(binding=NULL(),name=.n.t2,sizeinbytes=40_8,uninstantiated=NULL(),kindparameter=NULL(),lenparameterkind=NULL(),component=.c.t2,procptr=NULL(),special=NULL(),specialbitset=0_4,hasparent=0_1,noinitializationneeded=0_1,nodestructionneeded=1_1,nofinalizationneeded=1_1)
+end program main
+
Index: flang/test/Semantics/typeinfo06.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/typeinfo06.f90
@@ -0,0 +1,16 @@
+!RUN: bbc --dump-symbols %s | FileCheck %s
+!RUN: %flang_fc1 -fdebug-dump-symbols %s | FileCheck %s
+! Ensure that cycles via ALLOCATABLEs do not instantiate incomplete derived
+! types that would lead to types whose sizeinbytes=0
+program main
+  implicit none
+  type t1
+    type(t2), allocatable :: b
+  end type t1
+!CHECK: .dt.t1, SAVE, TARGET (CompilerCreated, ReadOnly): ObjectEntity type: TYPE(derivedtype) init:derivedtype(binding=NULL(),name=.n.t1,sizeinbytes=40_8,uninstantiated=NULL(),kindparameter=NULL(),lenparameterkind=NULL(),component=.c.t1,procptr=NULL(),special=NULL(),specialbitset=0_4,hasparent=0_1,noinitializationneeded=0_1,nodestructionneeded=0_1,nofinalizationneeded=1_1)
+  type :: t2
+    type(t1), allocatable :: a
+  end type t2
+! CHECK: .dt.t2, SAVE, TARGET (CompilerCreated, ReadOnly): ObjectEntity type: TYPE(derivedtype) init:derivedtype(binding=NULL(),name=.n.t2,sizeinbytes=40_8,uninstantiated=NULL(),kindparameter=NULL(),lenparameterkind=NULL(),component=.c.t2,procptr=NULL(),special=NULL(),specialbitset=0_4,hasparent=0_1,noinitializationneeded=0_1,nodestructionneeded=0_1,nofinalizationneeded=1_1)
+end program main
+