[FIRRTL] Disallow reading from property ports (#8191)

According to the FIRRTL spec, the right hand side of a propassign must
have source flow.  This means that it is illegal to read from a property
output port, something that is allowed for regular HW ports.  This
change enforces this property during flow checking in the operation
verifiers.

lib/Dialect/FIRRTL/FIRRTLOps.cpp

@@ -3518,9 +3518,11 @@ static LogicalResult checkConnectFlow(Operation *connect) {
   // instance/memory input ports.
   auto srcFlow = foldFlow(src);
   if (!isValidSrc(srcFlow)) {
-    // A sink that is a port output or instance input used as a source is okay.
+    // A sink that is a port output or instance input used as a source is okay,
+    // as long as it is not a property.
     auto kind = getDeclarationKind(src);
-    if (kind != DeclKind::Port && kind != DeclKind::Instance) {
+    if (isa<PropertyType>(src.getType()) ||
+        (kind != DeclKind::Port && kind != DeclKind::Instance)) {
       auto srcRef = getFieldRefFromValue(src, /*lookThroughCasts=*/true);
       auto [srcName, rootKnown] = getFieldName(srcRef);
       auto diag = emitError(connect->getLoc());

test/Dialect/FIRRTL/errors.mlir

@@ -2199,6 +2199,29 @@ firrtl.circuit "Top" {
   }
 }
 
+// -----
+// Try to read from an output property port with sink flow.
+firrtl.circuit "Top" {
+  // expected-note @below {{the source was defined here}}
+  firrtl.module @Top(out %a : !firrtl.string, out %b : !firrtl.string) {
+    // expected-error @below {{connect has invalid flow: the source expression "b" has sink flow, expected source or duplex flow}}
+    firrtl.propassign %a, %b : !firrtl.string
+  }
+}
+
+// -----
+// Try to read from an input property instance port with sink flow.
+
+firrtl.circuit "Top" {
+  firrtl.module @Child(in %in : !firrtl.string) { }
+  firrtl.module @Top(out %out : !firrtl.string) {
+    // expected-note @below {{the source was defined here}}
+    %child_in = firrtl.instance child @Child(in in : !firrtl.string)
+    // expected-error @below {{connect has invalid flow: the source expression "child.in" has sink flow, expected source or duplex flow}}
+    firrtl.propassign %out, %child_in : !firrtl.string
+  }
+}
+
 // -----
 // Try to assign to the input port of an output object of a local object.
 // This fails because we can only assign directly to the ports of a local object