diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -84,6 +84,10 @@
   scope" -- i.e., not scoped by `BLOCK` constructs.
   As most (but not all) compilers implement `BLOCK` scoping of construct
   names, so does f18, with a portability warning.
+* 15.6.4 paragraph 2 prohibits an implicitly typed statement function
+  from sharing the same name as a symbol in its scope's host, if it
+  has one.
+  We accept this usage with a portability warning.
 
 ## Extensions, deletions, and legacy features supported by default
 
diff --git a/flang/lib/Semantics/check-declarations.cpp b/flang/lib/Semantics/check-declarations.cpp
--- a/flang/lib/Semantics/check-declarations.cpp
+++ b/flang/lib/Semantics/check-declarations.cpp
@@ -1044,6 +1044,15 @@
     if (auto msg{evaluate::CheckStatementFunction(
             symbol, *stmtFunction, context_.foldingContext())}) {
       SayWithDeclaration(symbol, std::move(*msg));
+    } else if (details.result().flags().test(Symbol::Flag::Implicit)) {
+      // 15.6.4 p2 weird requirement
+      if (const Symbol *
+          host{symbol.owner().parent().FindSymbol(symbol.name())}) {
+        evaluate::AttachDeclaration(
+            messages_.Say(symbol.name(),
+                "An implicitly typed statement function should not appear when the same symbol is available in its host scope"_port_en_US),
+            *host);
+      }
     }
   }
   if (IsElementalProcedure(symbol)) {
diff --git a/flang/test/Semantics/stmt-func01.f90 b/flang/test/Semantics/stmt-func01.f90
--- a/flang/test/Semantics/stmt-func01.f90
+++ b/flang/test/Semantics/stmt-func01.f90
@@ -34,6 +34,7 @@
   integer :: sf9
   !ERROR: Defining expression of statement function 'sf9' cannot be converted to its result type INTEGER(4)
   sf9(n) = "bad"
+  sf10 = 1.
  contains
   real function explicit(x,y)
     integer, intent(in) :: x
@@ -44,4 +45,8 @@
     real :: arr(2)
     arr = [1., 2.]
   end function
+  subroutine foo
+    !PORTABILITY: An implicitly typed statement function should not appear when the same symbol is available in its host scope
+    sf10(x) = 2.*x
+  end subroutine
 end