[flang] Warn about violations of an obscure requirement (15.6.4 p2)

The Fortran 2018 standard, perhaps as an attempt to prevent ambiguity
in  older compilers, requires that a statement function appear in an
explicit type declaration statement if its name is also accessible
from a host scope.  F18 processes the specification parts of inner
procedures first, so we don't need this requirement to prevent
ambiguity, and can only really check it retrospectively after name
resolution.  Emit a portability warning when appropriate.

Differential Revision: https://reviews.llvm.org/D145100

flang/docs/Extensions.md

@@ -84,6 +84,10 @@ end
   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
 

flang/lib/Semantics/check-declarations.cpp

@@ -1044,6 +1044,15 @@ void CheckHelper::CheckSubprogram(
     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)) {

flang/test/Semantics/stmt-func01.f90

@@ -34,6 +34,7 @@ program main
   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 @@ program main
     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