3a309ba4eb
This PR improves the error message encountered in the case of a type class instance resolution failure, and adds an error explanation that discusses the common new-user case of binary operation overloading and points to the `trace.Meta.synthInstance` option for advanced debugging. ## Example ```lean4 def f (x : String) := x + x ``` Before: ``` failed to synthesize HAdd String String ?m.5 Hint: Additional diagnostic information may be available using the `set_option diagnostics true` command. ``` After: ``` failed to synthesize instance of type class HAdd String String ?m.5 Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command. Error code: lean.failedToSynthesizeTypeclassInstance [View explanation](https://lean-lang.org/doc/reference/latest/find/?domain=Manual.errorExplanation&name=lean.failedToSynthesizeTypeclassInstance) ``` The error message is changed in three important ways: * Explains *what* failed to synthesize, using the "type class" terminology that's more likely to be recognized than the "instance" terminology * Points to the `trace.Meta.synthInstance` option which is otherwise nearly undiscoverable but is quite powerful (see also leanprover/reference-manual#663 which is adding commentary on this option) * Gives an error explanation link (which won't actually work until the next release after this is merged) which prioritizes the common-case explanation of using the wrong binary operation
21 lines
575 B
Text
21 lines
575 B
Text
example : True := by
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if 1 + 1 = 2 then _ else ?_
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case pos => trivial
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fail_if_success case neg => contradiction
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· contradiction
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example (p : Prop) : True := by
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if p then ?foo else trivial
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case foo => trivial
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/-! Should not accidentally leak `open Classical` into branches. -/
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/--
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error: failed to synthesize instance of type class
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Decidable p
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Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
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-/
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#guard_msgs in
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example (p : Prop) : True := by
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if p then exact decide p else trivial
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