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Robert J. Simmons 429734c02f
feat: suggest deriving an instance when the instance might be derivable (#11346) 
This PR modifies the error message for type synthesis failure for the
case where the type class in question is potentially derivable using a
`deriving` command. Also changes the error explanation for type class
instance synthesis failure with an illustration of this pattern.

## Example

```lean4
inductive MyColor where
  | chartreuse | sienna | thistle
def forceColor (oc : Option MyColor) :=
  oc.get!
```

Before this PR, this gives the potentially confusing impression that
Lean may have decided that `MyColor` is _not_ inhabited — people used to
Rust may be especially inclined towards this confusion.
```
failed to synthesize instance of type class
  Inhabited MyColor

Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
```

After this PR, a targeted hint suggests precisely the command that will
fix the issue:
```
error: failed to synthesize instance of type class
  Inhabited MyColor

Hint: Adding the command `deriving instance Inhabited for MyColor` may allow Lean to derive the missing instance.
```
2025-12-01 14:28:15 +00:00
..
Compiler feat: dedicated fix operator for well-founded recursion on Nat (#7965) 2025-12-01 12:51:55 +00:00
Data feat: reduce server memory consumption (#11162) 2025-12-01 10:53:23 +00:00
DocString chore: minor String API improvements (#11439) 2025-12-01 11:44:14 +00:00
Elab feat: suggest deriving an instance when the instance might be derivable (#11346) 2025-12-01 14:28:15 +00:00
ErrorExplanations feat: suggest deriving an instance when the instance might be derivable (#11346) 2025-12-01 14:28:15 +00:00
Language chore: revert "refactor: port shell option processing to Lean" (#11378) 2025-11-26 09:28:48 +00:00
LibrarySuggestions feat: set_library_suggestions makes auxiliary def, rather than storing Syntax 2025-11-29 01:08:47 +11:00
Linter feat: document that backward options may disappear (#11304) 2025-11-24 17:49:46 +00:00
Meta feat: dedicated fix operator for well-founded recursion on Nat (#7965) 2025-12-01 12:51:55 +00:00
Parser refactor: String functions foldr, all, any, contains to go trough String.Slice (#11357) 2025-11-25 15:42:43 +00:00
ParserCompiler chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
PrettyPrinter refactor: deprecate String.posOf and variants in favor of unified String.find (#11276) 2025-11-23 18:39:53 +00:00
Server chore: minor String API improvements (#11439) 2025-12-01 11:44:14 +00:00
Util feat: document that backward options may disappear (#11304) 2025-11-24 17:49:46 +00:00
Widget chore: rename String.Range to Lean.Syntax.Range (#10852) 2025-10-21 07:32:25 +00:00
AddDecl.lean test: avoid testing colliding private inductives (#11041) 2025-11-01 11:47:52 +00:00
Attributes.lean chore: make declMetaExt persistent for shake (#11201) 2025-11-16 20:11:56 +00:00
AuxRecursor.lean feat: sparse casesOn constructions (#11072) 2025-11-05 15:49:11 +00:00
BuiltinDocAttr.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
Class.lean refactor: update and consolidate attribute-related error messages (#9495) 2025-07-26 02:03:18 +00:00
Compiler.lean chore: remove public section from end of files (#10684) 2025-10-06 13:30:48 +00:00
CoreM.lean feat: catch and provide context for misuse of NNG-style induction pattern (#11347) 2025-11-25 18:44:40 +00:00
Data.lean chore: remove public section from end of files (#10684) 2025-10-06 13:30:48 +00:00
Declaration.lean chore: >6 month old deprecations (#10969) 2025-10-26 22:48:41 +00:00
DeclarationRange.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
DefEqAttrib.lean perf: clarify and granularize access to async env ext state (#9587) 2025-08-02 17:01:08 +00:00
DocString.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
Elab.lean feat: with_weak_namespace command (#11338) 2025-11-25 02:37:40 +00:00
EnvExtension.lean feat: shake import minimizer aware of the module system and arbitrary elaboration dependencies (#10575) 2025-09-28 16:00:00 +00:00
Environment.lean chore: CI: enable leak sanitizer again (#11339) 2025-11-27 18:32:35 +00:00
ErrorExplanation.lean chore: rename String.ValidPos to String.Pos (#11240) 2025-11-24 16:40:21 +00:00
ErrorExplanations.lean fix: add missing import (#11441) 2025-12-01 11:02:03 +00:00
Exception.lean feat: hint about inaccessible private declaration on dot notation failure (#10803) 2025-10-17 09:31:56 +00:00
Expr.lean fix: move the monad argument for ForIn, ForIn', and ForM (#10204) 2025-11-25 12:20:37 +00:00
ExtraModUses.lean chore: make declMetaExt persistent for shake (#11201) 2025-11-16 20:11:56 +00:00
HeadIndex.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
Hygiene.lean chore: do not set unused Option.Decl.group (#11307) 2025-11-21 16:44:38 +00:00
ImportingFlag.lean chore: miscellaneous documentation typos (#10009) 2025-08-20 21:39:03 +00:00
InternalExceptionId.lean chore: reorganize Init imports around strings (#10289) 2025-09-07 17:09:14 +00:00
KeyedDeclsAttribute.lean fix: some ExtraModUses (#10620) 2025-10-03 15:50:40 +00:00
LabelAttribute.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
Level.lean fix: move the monad argument for ForIn, ForIn', and ForM (#10204) 2025-11-25 12:20:37 +00:00
LibrarySuggestions.lean fix: make library suggestions available in module files (#11373) 2025-11-26 05:39:27 +00:00
Linter.lean chore: remove public section from end of files (#10684) 2025-10-06 13:30:48 +00:00
LoadDynlib.lean refactor: use String.Slice in String.take and variants (#11180) 2025-11-18 16:13:48 +00:00
LocalContext.lean fix: move the monad argument for ForIn, ForIn', and ForM (#10204) 2025-11-25 12:20:37 +00:00
Log.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
Message.lean refactor: use String.split instead of String.splitOn or String.splitToList (#11250) 2025-11-19 09:35:19 +00:00
Meta.lean feat: @[method_specs] to generate specification theorems from class instances (#10302) 2025-09-15 11:17:06 +00:00
MetavarContext.lean fix: remove superfluous Monad instances from some spec lemmas (#10564) (#10618) 2025-09-29 15:02:43 +00:00
Modifiers.lean chore: more module system fixes and refinements for finishing batteries port (#10819) 2025-10-21 08:19:50 +00:00
MonadEnv.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
Namespace.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
Parser.lean feat: hexnum parser (#10716) 2025-10-08 21:12:03 +00:00
ParserCompiler.lean chore: make declMetaExt persistent for shake (#11201) 2025-11-16 20:11:56 +00:00
PrettyPrinter.lean chore: do not set unused Option.Decl.group (#11307) 2025-11-21 16:44:38 +00:00
PrivateName.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
ProjFns.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
ReducibilityAttrs.lean feat: allow setting reducibilityCoreExt in async contexts (#11301) 2025-11-21 09:23:14 +00:00
Replay.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
ReservedNameAction.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
ResolveName.lean chore: more module system fixes and refinements for finishing batteries port (#10819) 2025-10-21 08:19:50 +00:00
Runtime.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
ScopedEnvExtension.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
Server.lean feat: revamp server logging (#10787) 2025-10-28 16:26:59 +00:00
Setup.lean fix: symbol clashes between packages (#11082) 2025-11-19 02:24:44 +00:00
Shell.lean chore: revert "refactor: port shell option processing to Lean" (#11378) 2025-11-26 09:28:48 +00:00
Structure.lean chore: remove redundant imports in core (#10750) 2025-10-16 20:27:46 +00:00
SubExpr.lean refactor: use String.split instead of String.splitOn or String.splitToList (#11250) 2025-11-19 09:35:19 +00:00
Syntax.lean fix: move the monad argument for ForIn, ForIn', and ForM (#10204) 2025-11-25 12:20:37 +00:00
ToExpr.lean feat: support for Rat scientific literals (#10961) 2025-10-26 02:05:26 +00:00
ToLevel.lean refactor: module-ize Lean (#9330) 2025-07-25 12:02:51 +00:00
Util.lean feat: instantiate tactic parameter optimizer (#10916) 2025-10-23 01:22:33 +00:00
Widget.lean chore: remove public section from end of files (#10684) 2025-10-06 13:30:48 +00:00