The checker

The editor checks the circuit at every change, as intrepyd would build it, so that what you simulate and verify is a circuit that makes sense.

• A block with a problem gets a badge: red for an error, yellow for a warning; its free input pins turn red.
• The status line of the editor counts the problems; a click there (or on the empty canvas, with nothing selected) lists them in the properties panel, and a click on one selects its block, opening the subsystem it is in. In VS Code they are in the Problems panel too.
• While there are errors, Simulate, Verify and MC/DC tests are off. Warnings do not stop them.

Errors are what would make the circuit wrong or impossible to build:

• an input not wired, or wired twice;
• a combinational loop, a loop of wires that no Unit delay breaks;
• a type mismatch along a wire: operands of different types, a number into a logic gate, a condition that is not a boolean;
• a constant that does not fit its type, an unknown operator, type or block;
• a block with no name, or two with the same name in one level;
• a wire that crosses the border of a subsystem.

Warnings point at what is probably a mistake:

• a result that goes nowhere;
• a circuit with no output and nothing to verify.

Every message, with what to do about it, is in Messages.