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

def Lean.Meta.addPPExplicitToExposeDiff (a : Lean.Expr) (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 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 a simple heuristic, we should extend it as we find other counterintuitive error messages.

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

partial def Lean.Meta.addPPExplicitToExposeDiff.hasExplicitDiff? (xs : Array Lean.Expr) (as : Array Lean.Expr) (bs : Array Lean.Expr) : Lean.MetaM (Option (Array Lean.Expr × Array Lean.Expr))

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

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

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

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

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

Throw an exception if e is not type correct.

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

Return true if e is type correct.