invalid_case_patterns

Use case expressions that are valid in Dart 3.0.

This rule is currently experimental and available as of Dart 3.0.0.

This rule has a quick fix available.

Details

Some case expressions that are valid in Dart 2.19 and below will become an error or have changed semantics when a library is upgraded to 3.0. This lint flags those expressions in order to ease migration to Dart 3.0.

Some valid switch cases in 2.19 will become compile errors in Dart 3.0:

• Set literals
• Parenthesized expressions
• Calls to identical().
• Unary operator expressions !, -, or ~ (except for - before an integer literal, which is a valid pattern and is fine)
• Binary operator expressions !=, ==, &, |, ^, ~/, >>, >>>, <<, +, -, *, /, %, <, <=, >, >=, ??.
• Conditional operator ?:
• .length calls on strings
• is and is! expressions

Examples of all of them:

switch (obj) {
  case {1}: // Set literal.
  case (1): // Parenthesized expression.
  case identical(1, 2): // `identical()` call.
  case -pi: // Unary operator.
  case 1 + 2: // Binary operator.
  case true ? 1 : 2: // Conditional operator.
  case 'hi'.length: // .length call.
  case i is int: // is expression.
}

Some valid switch cases in 2.19 are also syntactically valid patterns, but the pattern matching behavior may be different from the current constant equality behavior. They are:

List and map literals. A list or map literal can appear as a constant in a case:

switch (obj) {
  case [1, 2]: ...
  case {'k': 'v'}: ...
}

Currently, the case will only match if the incoming value has the same identity as the constant. So:

test(List<int> list) {
  switch (list) {
    case [1, 2]: print('Matched'); break;
    default: print('Did not match'); break;
  }
}

main() {
  test(const [1, 2]); // Prints "Matched".
  test([1, 2]); // Prints "Did not match".
}

With patterns, a list or map literal becomes a list or map pattern. The pattern destructures the incoming object and matches if the subpatterns all match. In other words, list and map pattern match using something more like deep equality.

With Dart 3.0, the above program prints “Matched” twice.

Constant constructor calls. Similar to collections, you can construct a constant instance of a class in a case:

class Point {
  final int x;
  final int y;
  const Point({this.x, this.y});
}

test(Point p) {
  switch (p) {
    case Point(x: 1, y: 2): print('Matched'); break;
    default: print('Did not match'); break;
  }
}

main() {
  test(const Point(1, 2)); // Prints "Matched".
  test(Point(1, 2)); // Prints "Did not match".
}

Again, like collections, the case currently only matches if the incoming value has the same identity. With patterns, the Point(...) syntax becomes an object pattern that destructures the incoming point, calls the x and y getters on it and then matches the results of those against the corresponding subpatterns.

In this example, it will print “Matched” twice.

Note that object patterns only support named fields. So any constant constructor in a case today that has positional arguments will become a compile-time error when parsed as a pattern. A constant constructor call with no arguments is a valid object pattern and only does a type test:

class Thing {
  const Thing();
}

test(Thing t) {
  switch (t) {
    case Thing(): print('Matched'); break;
    default: print('Did not match'); break;
  }
}

main() {
  test(const Thing()); // Prints "Matched".
  test(Thing()); // Prints "Did not match".
}

When interpreted as a pattern, this prints “Matched” twice.

Wildcards. Today, you can have a constant named _:

test(int n) {
  const _ = 3;
  switch (n) {
    case _: print('Matched'); break;
    default: print('Did not match'); break;
  }
}

main() {
  test(3); // Prints "Matched".
  test(5); // Prints "Did not match".
}

With patterns, the identifier _ is treated as a pattern that matches all values, so this prints “Matched” twice.

Logic operators. The logic operators && and || are valid constant expressions and also valid patterns. As a constant expression, they simply evaluate the expression to a boolean and match if the incoming value is equal to that boolean value. So:

test(bool b) {
  switch (b) {
    case true && false: print('Matched'); break;
    default: print('Did not match'); break;
  }
}

main() {
  test(false); // Prints "Matched".
  test(true); // Prints "Did not match".
}

With Dart 3.0, these become patterns. The above example prints “Did not match” twice because no boolean value can be both true and false.

Many of invalid cases can be mechanically changed to something that is valid both in Dart today and valid and means the same in Dart 3.0.

Parenthesized expressions: Provided the inner expression is one that’s not broken in Dart 3.0, just discard the parentheses.

List literals, map literals, set literals, and constant constructor calls: Put const before the literal or call. This turns it into a constant pattern which preserves the current behavior:

BAD:

case [1, 2]:
case {'k': 'v'}:
case {1, 2}:
case Point(1, 2):

GOOD:

case const [1, 2]:
case const {'k': 'v'}:
case const {1, 2}:
case const Point(1, 2):

• Wildcards: Rename the constant from _ to something else. Since the name is private, this can be done locally in the library without affecting other code.

• Everything else: For any other invalid expression, you have to hoist the expression out into a new named constant. For example, if you have code like this:

BAD:

switch (n) {
  case 1 + 2: ...
}

It can be fixed by changing it to:

GOOD:

const three = 1 + 2;

switch (n) {
  case three: ...
}

Usage

To enable the invalid_case_patterns rule, add invalid_case_patterns under linter > rules in your analysis_options.yaml file:

linter:
  rules:
    - invalid_case_patterns