Index: flang/docs/Extensions.md
===================================================================
--- flang/docs/Extensions.md
+++ flang/docs/Extensions.md
@@ -490,3 +490,10 @@
   application codes that expect exterior symbols whose names match
   components to be visible in a derived-type definition's default initialization
   expressions, and so f18 follows that precedent.
+
+* 19.3.1p1 "Within its scope, a local identifier of an entity of class (1)
+  or class (4) shall not be the same as a global identifier used in that scope..."
+  is read so as to allow the name of a module, submodule, main program,
+  or `BLOCK DATA` subprogram to also be the name of an local entity in its
+  scope, with a portability warning, since that global name is not actually
+  capable of being "used" in its scope.
Index: flang/lib/Semantics/check-declarations.cpp
===================================================================
--- flang/lib/Semantics/check-declarations.cpp
+++ flang/lib/Semantics/check-declarations.cpp
@@ -1805,6 +1805,31 @@
     if (scope.kind() == Scope::Kind::BlockData) {
       CheckBlockData(scope);
     }
+    if (auto name{scope.GetName()}) {
+      auto iter{scope.find(*name)};
+      if (iter != scope.end()) {
+        const char *kind{nullptr};
+        switch (scope.kind()) {
+        case Scope::Kind::Module:
+          kind = scope.symbol()->get<ModuleDetails>().isSubmodule()
+              ? "submodule"
+              : "module";
+          break;
+        case Scope::Kind::MainProgram:
+          kind = "main program";
+          break;
+        case Scope::Kind::BlockData:
+          kind = "BLOCK DATA subprogram";
+          break;
+        default:;
+        }
+        if (kind) {
+          messages_.Say(iter->second->name(),
+              "Name '%s' declared in a %s should not have the same name as the %s"_port_en_US,
+              *name, kind, kind);
+        }
+      }
+    }
     CheckGenericOps(scope);
   }
 }
Index: flang/test/Semantics/OpenMP/omp-declare-target03.f90
===================================================================
--- flang/test/Semantics/OpenMP/omp-declare-target03.f90
+++ flang/test/Semantics/OpenMP/omp-declare-target03.f90
@@ -12,6 +12,7 @@
   !ERROR: The module name or main program name cannot be in a DECLARE TARGET directive
   !$omp declare target (mod1)
 
+  !PORTABILITY: Name 'main' declared in a main program should not have the same name as the main program
   !ERROR: The module name or main program name cannot be in a DECLARE TARGET directive
   !$omp declare target (main)
 end
Index: flang/test/Semantics/OpenMP/omp-threadprivate03.f90
===================================================================
--- flang/test/Semantics/OpenMP/omp-threadprivate03.f90
+++ flang/test/Semantics/OpenMP/omp-threadprivate03.f90
@@ -13,6 +13,7 @@
   !ERROR: The module name or main program name cannot be in a THREADPRIVATE directive
   !$omp threadprivate(mod1)
 
+  !PORTABILITY: Name 'main' declared in a main program should not have the same name as the main program
   !ERROR: The module name or main program name cannot be in a THREADPRIVATE directive
   !$omp threadprivate(main)
 
Index: flang/test/Semantics/bind-c02.f90
===================================================================
--- flang/test/Semantics/bind-c02.f90
+++ flang/test/Semantics/bind-c02.f90
@@ -18,6 +18,7 @@
   !ERROR: Only variable and named common block can be in BIND statement
   bind(c) :: sub
 
+  !PORTABILITY: Name 'm' declared in a module should not have the same name as the module
   bind(c) :: m ! no error for implicit type variable
 
   type my_type
Index: flang/test/Semantics/resolve05.f90
===================================================================
--- flang/test/Semantics/resolve05.f90
+++ flang/test/Semantics/resolve05.f90
@@ -1,12 +1,20 @@
 ! RUN: %python %S/test_errors.py %s %flang_fc1
 program p
-  integer :: p ! this is ok
+  !PORTABILITY: Name 'p' declared in a main program should not have the same name as the main program
+  integer :: p
 end
 module m
-  integer :: m ! this is ok
+  !PORTABILITY: Name 'm' declared in a module should not have the same name as the module
+  integer :: m
 end
 submodule(m) sm
-  integer :: sm ! this is ok
+  !PORTABILITY: Name 'sm' declared in a submodule should not have the same name as the submodule
+  integer :: sm
+end
+block data bd
+  !PORTABILITY: Name 'bd' declared in a BLOCK DATA subprogram should not have the same name as the BLOCK DATA subprogram
+  type bd
+  end type
 end
 module m2
   type :: t
Index: flang/test/Semantics/resolve20.f90
===================================================================
--- flang/test/Semantics/resolve20.f90
+++ flang/test/Semantics/resolve20.f90
@@ -37,7 +37,8 @@
   type :: bad3
   end type
 
-  type :: m ! the name of a module can be used as a local identifier
+  !PORTABILITY: Name 'm' declared in a module should not have the same name as the module
+  type :: m
   end type m
 
   !ERROR: EXTERNAL attribute was already specified on 'a'