This is not the Kernel type checker, but an auxiliary method for checking whether terms produced by tactics and isDefEq are type correct.

def Lean.Meta.addPPExplicitToExposeDiff (a b : Lean.Expr) : Lean.MetaM (Lean.Expr × Lean.Expr)

Given two expressions a and b, this method tries to annotate terms with pp.explicit := true and other pp options to expose "implicit" differences. For example, suppose a and b are of the form

@HashMap Nat Nat eqInst hasInst1
@HashMap Nat Nat eqInst hasInst2

By default, the pretty printer formats both of them as HashMap Nat Nat. So, counterintuitive error messages such as

error: application type mismatch
  HashMap.insert m
argument
  m
has type
  HashMap Nat Nat
but is expected to have type
  HashMap Nat Nat

would be produced. By adding pp.explicit := true, we can generate the more informative error

error: application type mismatch
  HashMap.insert m
argument
  m
has type
  @HashMap Nat Nat eqInst hasInst1
but is expected to have type
  @HashMap Nat Nat eqInst hasInst2

Remark: this method implements simple heuristics; we should extend it as we find other counterintuitive error messages.

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partial def Lean.Meta.addPPExplicitToExposeDiff.visit (a b : Lean.Expr) : Lean.MetaM (Lean.Expr × Lean.Expr)

def Lean.Meta.throwLetTypeMismatchMessage {α : Type} (fvarId : Lean.FVarId) : Lean.MetaM α

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def Lean.Meta.mkHasTypeButIsExpectedMsg (givenType expectedType : Lean.Expr) : Lean.MetaM Lean.MessageData

Return error message "has type{givenType}\nbut is expected to have type{expectedType}"

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def Lean.Meta.throwAppTypeMismatch {α : Type} (f a : Lean.Expr) : Lean.MetaM α

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def Lean.Meta.checkApp (f a : Lean.Expr) : Lean.MetaM Unit

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def Lean.Meta.checkProj (structName : Lean.Name) (idx : Nat) (e : Lean.Expr) : Lean.MetaM Unit

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def Lean.Meta.check (e : Lean.Expr) : Lean.MetaM Unit

Throw an exception if e is not type correct.

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def Lean.Meta.isTypeCorrect (e : Lean.Expr) : Lean.MetaM Bool

Return true if e is type correct.

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• Lean.Meta.isTypeCorrect e = tryCatch (do Lean.Meta.check e pure true) fun (x : Lean.Exception) => pure false