44 lines
1.5 KiB
Text
44 lines
1.5 KiB
Text
/-
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Here is an artificial example where temporary metavariables created by typeclass resolution
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would otherwise leak into nested typeclass resolution unless we catch it.
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-/
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class Foo (α : Type) : Type := (x : Unit)
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class Bar (α : Type) : Type := (x : Unit)
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class HasParam (α : Type) [Bar α] : Type := (x : Unit)
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instance FooToBar (α : Type) [Foo α] : Bar α := {x:=()}
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-- Note: there is an implicit `FooToBar` inside the second argument of the return type.
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instance HasParamInst (α : Type) [h : Foo α] : HasParam α := {x:=()}
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class Top : Type := (x : Unit)
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-- Note: `AllFoo` is a weird, universal instance.
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instance AllFoo (α : Type) : Foo α := {x:=()}
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/-
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When the subgoal `[@HasParam α (@FooToBar α (AllFoo α))]` is triggered, `α` is not known.
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This happens for two reasons:
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1. The return type `Top` does not include `α`.
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2. The universal `AllFoo α` instance obviates the need to take a building-block class as argument.
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Thus we would be tempted to call nested typeclass resolution on `Foo <tmp-mvar>`.
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-/
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instance Bad (α : Type) [HasParam α] : Top := Top.mk ()
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def foo [Top] : Unit := ()
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set_option pp.all true
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set_option trace.class_instances true
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set_option trace.type_context.complete_instance true
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#check @foo _
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/-
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[class_instances] class-instance resolution trace
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[class_instances] (0) ?x_0 : Top := @Bad ?x_1 ?x_2
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[class_instances] (1) ?x_2 : @HasParam ?x_1 (@FooToBar ?x_1 (AllFoo ?x_1)) := @HasParamInst ?x_3 ?x_4
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[type_context.complete_instance] would have synthed: Foo ?x_3
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-/
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