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problem with type comparison (`<:`) transitiveness when `.type` was used as upper-type-boundaries (`>:`) and then `|`-ed together #5878

ibaklan opened this Issue Feb 8, 2019 · 0 comments


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ibaklan commented Feb 8, 2019

Problem was disclosed when I was making experiments with possible different implementations of HasThisType (As I can know from error messages direct |-ing of .type-types is prohibited, but taking into account possibility of definition of upper-bounded (>:) abstract types, one still can compose some quasi-|-ing of that .type-types which ends up with problems/inconsistencies - in particular <:-relationship transitiveness happen to be missing - automatic interference of constraints by transitiveness of <: does't work here)

Initially problem was found in following form (complete demonstrating snippet here)

    trait HasThisType {
      type ThisB >: this.type

      type ThisC1 >: ThisB
      type ThisC2 >: ThisB
      type ThisD >: ThisC1 | ThisC2
      type ThisD1 = ThisC1 | ThisC2

      def testThisTypeInner(): Unit = {
        val thisB: ThisB = this
        val thisC: ThisC1 = this
        // fails with "[E007] Type Mismatch Error"
        //val thisD: ThisD = this
        val thisDProven: ThisD = this : ThisC1

        // test with "pure" `|` of abstract types (without `>:`) gives the same result

        // fails with "[E007] Type Mismatch Error"
        //val thisD1: ThisD1 = this
        val thisD1Proven: ThisD1 = this : ThisC1

Here one could observe that this.type <: this.ThisB <: this.ThisD1 so taking into account transitiveness of <: one may expect that this.type <: this.ThisD1 - but "it doesn't" (compiler could not infer that relationship).
So that assignment val thisD1: ThisD1 = this treated as invalid, while "down-casting" this to ThisB for example, "resolves the problem", and val thisD1: ThisD1 = this : ThisB become compile-able again.

After looking closer to that problem it is clearly seen that transitiveness is broken only on the step
this.type <: ThisB, other relationship - like
ThisB <: ThisC1 <: ThisD1 <: ThisD works as expected (compiler easily infer that ThisB <: ThisD for example)

Other form of this problem (which one may immediately compose from previous one) could be formulated in terms of 2-wo separate val-s (like val foo1 and val foo2) and 2-wo upper bounded abstract types (like type Foo1ThisB >: foo1.type and type Foo2ThisB >: foo2.type)
Then demonstration snippet could look like following (complete snippet here)

    trait HasDoubleFooLike {
      val foo1: Any
      val foo2: Any

      type Foo1ThisB >: foo1.type
      type Foo2ThisB >: foo2.type

      type FooXThisB >: Foo1ThisB | Foo2ThisB
      type FooXThisB1 = Foo1ThisB | Foo2ThisB

      def testFooThisTypeInner(): Unit = {
        val fooLike1: Foo1ThisB = foo1
        val fooLike2: Foo2ThisB = foo2
        // fails with "[E007] Type Mismatch Error"
        //val fooXLike1: FooXThisB = foo1
        val fooXLike1Proven: FooXThisB = foo1: Foo1ThisB
        // fails with "[E007] Type Mismatch Error"
        //val fooXLike2: FooXThisB = foo2
        val fooXLike2Proven: FooXThisB = foo2: Foo2ThisB

        // test with "pure" `|` (without `>:`) gives the same result

        // fails with "[E007] Type Mismatch Error"
        //val fooX1Like1: FooXThisB1 = foo1
        val fooX1Like1Proven: FooXThisB1 = foo1: Foo1ThisB
        // fails with "[E007] Type Mismatch Error"
        //val fooX1Like2: FooXThisB1 = foo2
        val fooX1Like2Proven: FooXThisB1 = foo2: Foo2ThisB

Here again transitiveness is broken on step foo1.type <: Foo1ThisB and foo2.type <: Foo2ThisB so that
foo1.type <: this.Foo1ThisB <: this.FooXThisB and foo2.type <: this.Foo2ThisB <: this.FooXThisB - are OK.
But that could not be reduced to foo1.type <: this.FooXThisB and foo2.type <: this.FooXThisB - compiler fails to infer that, disregarding that it should directly follow from transitiveness of <: relationship.

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