TE127 — Parameter pattern struct does not match its annotation

Why this fires¶

A named struct pattern in parameter position (fn f(StructName { ... }: T)) names a different struct than the parameter's own type annotation.

struct Point { x: int }
struct Circle { r: int }

fn f(Point { x }: Circle) { print(x) }
//   ^^^^^^^^^^^^^^^^^^^ error[TE127]: parameter pattern names struct 'Point'
//                        but the annotation says 'Circle'

The same check applies to effect handler-arm patterns, where the operation's declared parameter type plays the role of the annotation:

struct Point { x: int, y: int }
struct Circle { r: int }
effect Gfx { fn draw(p: Point) }

fn f() {
    handle work() with {
        Gfx::draw(Circle { r }) => r,
        //        ^^^^^^^^^^^^ error[TE127]: ... names struct 'Circle' but 'Gfx::draw'
        //                       declares parameter type 'Point'
    }
}

Fix it¶

Make the pattern name the same struct as the annotation (or the operation's declared parameter type):

fn f(Point { x }: Point) { print(x) }   // ok

If the annotation itself is wrong, fix the annotation instead.

Parameter pattern struct does not match its annotation

Why this fires¶

Fix it¶

See also¶

See also