[flang] Compile the output of -fdebug-unparse-with-modules

The output of a compilation with the -fdebug-unparse-with-modules
option comprises its normal unparsed output along with the regenerated
contents of any modules that were required from module files.
This is handy for producing stand-alone test cases.

The modules' contents are generated by the same code that writes module
files, so they can contain some USE associations to private entities
in other modules that are necessary to complete local declarations,
usually initializers.  Such USE associations to private entities are
not flagged as fatal errors when modules are read from module files,
but they currently are caught when the output produced by this option
is being read back in to the compiler.

Handle this case by softening the error to a warning when one module
uses a private entity from another with an alias containing the
non-conforming '$' character.  (I could have omitted the message
altogether, but there are other valid warnings that will occur due
to undefined function result variables; further, I didn't want to
provide a general hole around the protection of private names.)

flang/docs/ModFiles.md

@@ -98,7 +98,14 @@ Entities that have been included in a module by means of USE association
 are represented in the module file with `USE` statements.
 Name aliases are sometimes necessary when an entity from another
 module is needed for a declaration and conflicts with another
-entity of the same name.
+entity of the same name, or is `PRIVATE`.
+These aliases have currency symbols (`$`) in them.
+When a module
+is parsed from a module file, no error is emitted for associating
+such an alias with a `PRIVATE` name.
+A module parsed from another source file that is not a module file
+(notably, the output of the `-fdebug-unparse-with-modules` option)
+will emit only warnings.
 
 ## Reading and writing module files
 

flang/lib/Semantics/resolve-names.cpp

@@ -3325,9 +3325,19 @@ ModuleVisitor::SymbolRename ModuleVisitor::AddUse(
     // Privacy is not enforced in module files so that generic interfaces
     // can be resolved to private specific procedures in specification
     // expressions.
-    Say(useName, "'%s' is PRIVATE in '%s'"_err_en_US, MakeOpName(useName),
-        useModuleScope_->GetName().value());
-    return {};
+    // Local names that contain currency symbols ('$') are created by the
+    // module file writer when a private name in another module is needed to
+    // process a local declaration.  These can show up in the output of
+    // -fdebug-unparse-with-modules, too, so go easy on them.
+    if (currScope().IsModule() &&
+        localName.ToString().find("$") != std::string::npos) {
+      Say(useName, "'%s' is PRIVATE in '%s'"_warn_en_US, MakeOpName(useName),
+          useModuleScope_->GetName().value());
+    } else {
+      Say(useName, "'%s' is PRIVATE in '%s'"_err_en_US, MakeOpName(useName),
+          useModuleScope_->GetName().value());
+      return {};
+    }
   }
   auto &localSymbol{MakeSymbol(localName)};
   DoAddUse(useName, localName, localSymbol, *useSymbol);

flang/test/Semantics/resolve11.f90

@@ -86,3 +86,17 @@ subroutine s5
   use m5, only: foo, fun
   print *, fun() ! ok
 end
+
+module m6
+  !WARNING: 'foo' is PRIVATE in 'm5'
+  use m5, only: name$with$dollar => foo
+  !ERROR: 'foo' is PRIVATE in 'm5'
+  use m5, only: normal_name => foo
+end
+
+subroutine s6
+  !The special dispensation for USE association of private names to local
+  !aliases with '$' in them only applies to modules.
+  !ERROR: 'foo' is PRIVATE in 'm5'
+  use m5, only: name$with$dollar => foo
+end