Why do signals from "under-specified data types" diagnostic not use the data type specified under "default for underspecified data type" in Simulink R2026a?

I have enabled the "Underspecified data types" diagnostic action in Simulink R2026a to give a warning in order to identify underspecified signals in my model. I can successfully see them in the following warning message: 

The data types for some signals in this model are under-specified. Simulink is using a heuristic to select these data types. The heuristic leads to reasonable choices in most cases. Usage of the heuristic can be avoided by setting signal data types using Signal Specification blocks or signal objects. The list of signals with under-specified data types are:  

  
Caused by: 

Signal with under-specified data types: 'Input Port 3' of '<model_name>'. 
Signal with under-specified data types: 'Input Port 1' of '<model_name>/<subsystem_name>'. 
Signal with under-specified data types: 'Input Port 2' of '<model_name>/<subsystem_name>'.  

I would then expect these under-specified signals to see the data type that I defined under the "Default for underspecified data type" configuration parameter. However, the under-specified signals do not always use that data type. 

Why is this happening and is this expected? 

Answer 1

[MathWorks Support Team]

This is expected as the "Default for underspecified data type" configuration parameter has a stricter definition for an under-specified data type compared to the "Underspecified data types" diagnostic. Thus, the two have different trigger conditions. 

As of Simulink R2026a, the "Underspecified data types" diagnostic triggers when Simulink cannot infer the signal data type. In such scenarios, Simulink may set the signal data type through some propagation rules before using the data type specified for "Default for underspecified data type". For more information about propagation of signal properties, see the Simulink Propagation documentation page.