[Feature] Default template arguments

What feature would you like to see?

Overrideable default values for template arguments like in C++.

struct Array<T = u32, auto Size = 10> {
  T data[Size];
};

How will this feature be useful to you and others?

I have a large number of "optional" structs in my patterns

struct Optional<T, U> {
    U isSome;
    if (isSome != 0) {
        T data;
    }
};

99% of the time U is a u8, but sometimes it is a u32. It would be nice to only have to specify U in the rare case rather than every time.

struct Optional<T, U = u8> {
    U isSome;
    if (isSome != 0) {
        T data;
    }
};

Optional<SomeOtherStruct> common_optional @ 0x00;
Optional<SomeOtherStruct, u32> uncommon_optional @ 0x10;

Request Type

• I can provide a PoC for this feature or am willing to work on it myself and submit a PR

Additional context?

No response

The issue defines the requested default-template syntax through the Array and Optional examples, but names no source files, tests, or parser entry points. Start by locating the template parsing and type-resolution code, then identify existing tests for template arguments. Done means omitted arguments use their declared defaults while explicitly supplied arguments still override them.