improve error message when accessing an optional map property

Feature Spec

Assuming we have an optional Map, and we want to access one of its properties,
optional_map.get("property") (or even optional_map?.get("property")) will throw the following error: "property access unsupported on type Map?"
this error message might not be clear enough to guide the user on how to fix the error.

(At the end I solved it this way:

let m = optional_map ?? {property: "x"};
m.get("property");

)

Use Cases

accessing a property on an optional map

Implementation Notes

We will also support optional_map?.property in the future

Component

Language Design

Community Note

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No file, test, or entry point is named. Start by locating the compiler diagnostic for property access on an optional Map, then define a clearer correction-oriented message for the shown access patterns and verify that the optional-map examples produce it.