3fc99eef10
This PR adds two validation checks to `addInstance` that provide early feedback for common mistakes in instance declarations: 1. **Non-class instance check**: errors when an instance target type is not a type class. This catches the common mistake of writing `instance` for a plain structure. Previously handled by the `nonClassInstance` linter in Batteries (`Batteries.Tactic.Lint.TypeClass`), this is now checked directly at declaration time. 2. **Impossible argument check**: errors when an instance has arguments that cannot be inferred by instance synthesis. Specifically, it flags arguments that are not instance-implicit and do not appear in any subsequent instance-implicit argument or in the return type. Previously such instances would be silently accepted but could never be synthesised. Supersedes #13237 and #13333.
28 lines
390 B
Text
28 lines
390 B
Text
/-!
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# `instance` creates a theorem if the class is a `Prop`
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https://github.com/leanprover/lean4/issues/5672
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-/
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class A : Prop
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instance a : A where
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/--
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info: theorem a : A :=
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{ }
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-/
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#guard_msgs in #print a
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/-!
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Uses `def` variable inclusion rules
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-/
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section
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variable (x : Nat)
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@[reducible]
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def b : A := by
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cases x <;> exact {}
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/-- info: b (x : Nat) : A -/
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#guard_msgs in #check b
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end
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