This bug is a regression introduced with 0.36 (was not present in 0.34 releases, nor earlier versions I tried).

When creating a fast-vapi file for parallel compilation using --fast-vapi, the compiler will error-out when a virtual/abstract method throws an error.

$ cat exception.vala
public class Test {
    public virtual void test () throws Error {
        throw new Error (-1, 0, "Sample error");
    }
}
$ valac --version
Vala 0.34.6
$ valac --fast-vapi exception.vapi exception.vala && echo No Error
No Error
$ dev/vala/compiler/valac --version
Vala 0.36.0
$ dev/vala/compiler/valac --fast-vapi exception.vapi exception.vala
exception.vala:2.5-2.28: error: overriding method `Test.test' is incompatible with base method `Test.test': incompatible error type `Error'.
    public virtual void test () throws Error {
    ^^^^^^^^^^^^^^^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)