KEEP for custom equals in inline classes

proposals/custom-equals-in-value-classes.md

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+# Custom equals in value classes
+
+* **Type**: Design proposal
+* **Authors**: Vladislav Grechko
+* **Status**: Prototype implemented
+* **Issue**: [KT-24874](https://youtrack.jetbrains.com/issue/KT-24874/)
+
+## Summary
+
+Allow overriding `equals` from `Any` and declaring so-called *typed equals* in value classes.
+
+## Motivation and use cases
+
+### Collections
+
+Suppose we want to implement wrapper for an object that implements `List` interface. We want this wrapper to be
+light-weight and avoid allocation of wrapper object. This might be achieved by declaring following inline class:
+
+```Kotlin
+@JvmInline
+value class SingletonList<T>(val element: T) : List<T> {
+    ...
+}
+```
+
+Since different implementations of `List` must be equal if they contain same elements, we have to
+customize `equals(other: Any?)` in `SingletonList`.
+
+> Note that passing instance of `SingletonList` as argument to non-inline function that takes `List`  will lead to
+> boxing and allocation a new object. For inline functions, boxing can be avoided when IR inliner is implemented.
+> Related issue:
+> [KT-40391](https://youtrack.jetbrains.com/issue/KT-40391/Inline-function-boxes-inline-class-type-argument)
+>
+
+### Units of measurement
+
+Inline classes are well suited to represent units of measurement. It might be useful to establish custom equivalence
+relation for them.
+
+```Kotlin
+@JvmInline
+value class Degrees(val value: Double)
+
+
+fun foo() {
+    // customize equality relation to make the set contain a single value
+    val uniqueAngles = setOf(Degrees(90.0), Degrees(-270.0))
+}
+```
+
+```Kotlin
+@JvmInline
+value class Minutes(val value: Int)
+
+@JvmInline
+value class Seconds(val value: Int)
+
+
+fun foo() {
+    // customize equality relation to make the set contain a single value
+    val uniqueTimeIntervals = setOf(Minutes(1), Seconds(60))
+}
+```
+
+## Proposal
+
+### Typed equals
+
+Simply overriding `equals(other: Any?)` in value class would require boxing of the right-hand side operand of
+every `'=='` comparison:
+
+```Kotlin
+@JvmInline
+value class Degrees(val value: Double) {
+    override fun equals(other: Any?): Boolean {
+        ...
+    }
+}
+
+fun foo() {
+    // have to box to pass to equals(other: Any?)
+    println(Degrees(0) == Degrees(45))
+}
+```
+
+That is why in [KEEP for inline classes](https://github.com/Kotlin/KEEP/blob/master/proposals/inline-classes.md) a new
+concept of **typed equals** was proposed:
+
+```Kotlin
+@JvmInline
+value class Degrees(val value: Double) {
+    @TypedEquals
+    fun equals(other: Degrees) = (value - other.value) % 360.0 == 0.0
+}
+```
+
+Type of `other` will be unboxed during compilation and passing argument will not require boxing.
+
+More precise, we define typed equals as a function such that:
+
+* Has name `"equals"`
+* Declared as a member function of value class
+* Has a single parameter which type is a star-projection of enclosing class
+    * We will elaborate on this restriction in the next section
+* Returns `Boolean`
+* Annotated with `@TypedEquals` annotation
+
+Typed equals, as well as equals from `Any`, must define an equivalence relation, i.e. be symmetric, reflexive,
+transitive and consistent.
+
+We forbid typed equals to have type parameters, as well as default values for value parameter.
+
+> From now, we will be calling equals from `Any` *untyped* equals, in contrast, to *typed* one.
+
+### Interaction of typed and untyped equals
+
+We need to establish consistency of typed and untyped equals, i.e. the following property:
+> For any `x`, `y` such that `x`, `y` refer instances of an inline class, it must be true
+> that `x.equals(y) == (x as Any).equals(y)`
+
+We propose the following:
+
+* If only typed equals is declared:
+    * Generate consistent untyped one. Implementation of untyped equals will be trying to cast the argument to
+      corresponding value class type and passing to typed equals if succeeded.
+* If only untyped equals is declared:
+    * Generate consistent typed one. Implementation of typed equals will be boxing argument and passing it to untyped
+      equals. Implement special diagnostics to warn programmers about negative performance impact of boxing in
+      auto-generated code
+* If both equals methods are declared:
+    * Consider programmer responsible for their consistency
+
+Note, that in the first case on JVM level we will be casting argument to the raw type of value class, thus we will know
+nothing about its type arguments. We emphasize this fact by requiring the parameter of typed equals to be
+star-projection of the value class.
+
+### Custom `hashCode`
+
+It is obvious that the ability of overriding `equals` makes us allow overriding of `hashCode`.
+
+## Other questions
+
+### Inline classes over inline classes
+
+Suppose we have an inline class `B` over inline class `A` and `A` declares typed equals. In this case,
+default-generated equals in `B` will be comparing it underlying values using custom equals of `A`.
+
+### Interaction with Valhalla
+
+In its current prototype version `20-valhalla+20-75` Valhalla supports customization of equals and hashCode in values
+classes. Therefore, we will be able to compile Kotlin value classes with custom equals to Valhalla values classes.
+
+```Java
+value class ValueClass {
+    int x;
+    public ValueClass(int x) {
+        this.x = x;
+    }
+    public boolean equals(ValueClass other) {
+        return (x - other.x) % 2 == 0;
+    }
+    @Override
+    public boolean equals(Object other) {
+        if (other instanceof ValueClass) {
+            return equals((ValueClass)other);
+        }
+        return false;
+    }
+    @Override
+    public int hashCode() {
+        return 42;
+    }
+}
+class Main {
+    public static void main(String[] args) {
+        ValueClass zeroVal = new ValueClass(0);
+        ValueClass oneVal = new ValueClass(2);
+
+        Object zeroObj = zeroVal;
+
+        System.out.println(zeroVal.equals(oneVal));		// true
+        System.out.println(zeroObj.equals(oneVal));		// true
+        System.out.println(zeroObj.hashCode());			// 42
+    }
+}
+```
+
+## Possible future improvements
+
+### Typed equals in non-value classes
+
+Introducing typed equals in non-value classes could help to get rid of boilerplate code:
+
+```Kotlin
+class A {
+    fun equals(other: A) {
+        //do check
+        ...
+    }
+}
+```
+
+instead of:
+
+```Kotlin
+class A {
+    override fun equals(other: Any?) {
+        if (other is A) {
+            //do check
+            ...
+        }
+        return false
+    }
+}
+```
+
+### Resolving references to typed equals in IDE
+
+Resolving `'=='` operation to typed equals in IDE could let user navigate from invocation to declaration.