[Sema] Diagnose cross-file operator redeclarations

Previously we permitted operator redeclarations
across files as the user could shadow imported
operator decls on a per-file basis, and we would
prefer an imported operator decl over one in
another file.

However with the new operator lookup
implementation, operators in the current module
always shadow imported decls. As such, we should
start diagnosing cross-file redeclarations when
the new lookup logic is enabled.

Resolves rdar://48731166

Author: hamishknight

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -436,18 +436,23 @@ static void checkOperatorOrPrecedenceGroupRedeclaration(
     if (other == decl || other->isInvalid())
       continue;
 
-    // Emit a redeclaration error if the two declarations occur in the same
-    // source file. We currently allow redeclarations across source files to
-    // allow the user to shadow operator decls from imports, as we currently
-    // favor those decls over ones from other files.
-    // FIXME: Once we prefer operator decls from the same module, start
-    // diagnosing redeclarations across files.
+    bool shouldDiagnose = false;
     if (currentFile == other->getDeclContext()->getParentSourceFile()) {
-      // Make sure we get the diagnostic ordering to be sensible.
+      // For a same-file redeclaration, make sure we get the diagnostic ordering
+      // to be sensible.
       if (decl->getLoc().isValid() && other->getLoc().isValid() &&
           ctx.SourceMgr.isBeforeInBuffer(decl->getLoc(), other->getLoc())) {
         std::swap(decl, other);
       }
+      shouldDiagnose = true;
+    } else {
+      // If the declarations are in different files, only diagnose if we've
+      // enabled the new operator lookup behaviour where decls in the current
+      // module are now favored over imports.
+      shouldDiagnose = ctx.LangOpts.EnableNewOperatorLookup;
+    }
+
+    if (shouldDiagnose) {
       ctx.Diags.diagnose(decl, diagID);
       ctx.Diags.diagnose(other, noteID);
       decl->setInvalid();