Fix E0730: Type Parameter Defaults Only Allowed in Function Items

Tue, 21 Apr 2026 17:32:04 +0800

1. Symptoms

When compiling Rust code with default type parameters in invalid contexts, you will encounter the E0730 error. The compiler produces a clear message indicating the restriction.

error[E0730]: default type parameters are only allowed in function items
 --> src/main.rs:5:1
 |
5 | impl<T = i32> Trait for Foo<T> {}
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

The error specifically states that default type parameters are only allowed in function items. This means you cannot use the = DefaultValue syntax for type parameters in impl blocks, type aliases, or other non-function contexts.

Compiler Errors on ErrorVault — Developer Error Code Dictionary

Fix E0730: Type Parameter Defaults Only Allowed in Function Items

1. Symptoms