[flang] Include missing internal interfaces in .mod files

Interfaces which are internal to a procedure need to be included in
module files if (and only if) they are referenced in the interface of
the procedure. That is, they are needed if they are the interfaces of
dummy or return value procedures.

Fixes #53420

Differential Revision: https://reviews.llvm.org/D121738

flang/lib/Semantics/mod-file.cpp

@@ -1057,6 +1057,30 @@ void SubprogramSymbolCollector::Collect() {
       if (needed) {
         need_.push_back(symbol);
       }
+    } else if (symbol.has<SubprogramDetails>()) {
+      // An internal subprogram is needed if it is used as interface
+      // for a dummy or return value procedure.
+      bool needed{false};
+      const auto hasInterface{[&symbol](const Symbol *s) -> bool {
+        // Is 's' a procedure with interface 'symbol'?
+        if (s) {
+          if (const auto *sDetails{s->detailsIf<ProcEntityDetails>()}) {
+            const ProcInterface &sInterface{sDetails->interface()};
+            if (sInterface.symbol() == &symbol) {
+              return true;
+            }
+          }
+        }
+        return false;
+      }};
+      for (const Symbol *dummyArg : details.dummyArgs()) {
+        needed = needed || hasInterface(dummyArg);
+      }
+      needed =
+          needed || (details.isFunction() && hasInterface(&details.result()));
+      if (needed && needSet_.insert(symbol).second) {
+        need_.push_back(symbol);
+      }
     }
   }
 }

flang/test/Semantics/modfile46.f90

@@ -0,0 +1,55 @@
+! RUN: %python %S/test_modfile.py %s %flang_fc1
+! Ensure that interfaces, which are internal to procedures and are used to
+! define the interface of dummy or return value procedures, are included in
+! .mod files.
+module m
+  implicit none
+contains
+  function f(x)
+    real, intent(in) :: x
+    abstract interface
+       subroutine used_int(x, p)
+         implicit none
+         real, intent(out) :: x
+         interface
+            subroutine inner_int(x)
+              implicit none
+              real, intent(out) :: x
+            end subroutine inner_int
+         end interface
+         procedure(inner_int) :: p
+       end subroutine used_int
+
+       pure logical function unused_int(i)
+         implicit none
+         integer, intent(in) :: i
+       end function unused_int
+    end interface
+    procedure(used_int), pointer :: f
+
+    f => null()
+  contains
+    subroutine internal()
+    end subroutine internal
+  end function f
+end module m
+
+!Expect: m.mod
+!module m
+!contains
+!function f(x)
+!real(4),intent(in)::x
+!procedure(used_int),pointer::f
+!abstract interface
+!subroutine used_int(x,p)
+!real(4),intent(out)::x
+!procedure(inner_int)::p
+!interface
+!subroutine inner_int(x)
+!real(4),intent(out)::x
+!end
+!end interface
+!end
+!end interface
+!end
+!end