diff --git a/src/Lean/Compiler/Decl.lean b/src/Lean/Compiler/Decl.lean
index 9794c7eb85..be1db97fa5 100644
--- a/src/Lean/Compiler/Decl.lean
+++ b/src/Lean/Compiler/Decl.lean
@@ -42,6 +42,48 @@ def macroInline (e : Expr) : CoreM Expr :=
     let val ← Core.instantiateValueLevelParams (← getConstInfo declName) us
     return .visit <| val.beta e.getAppArgs
 
+private def normalizeAlt (e : Expr) (numParams : Nat) : MetaM Expr :=
+  Meta.lambdaTelescope e fun xs body => do
+    if xs.size == numParams then
+      return e
+    else if xs.size > numParams then
+      let body ← Meta.mkLambdaFVars xs[numParams:] body
+      let body ← Meta.withLetDecl (← mkFreshUserName `_k) (← Meta.inferType body) body fun x => Meta.mkLetFVars #[x] x
+      Meta.mkLambdaFVars xs[:numParams] body
+    else
+      Meta.forallBoundedTelescope (← Meta.inferType e) (numParams - xs.size) fun ys _ =>
+        Meta.mkLambdaFVars (xs ++ ys) (mkAppN e ys)
+
+/--
+Inline auxiliary `matcher` applications.
+-/
+partial def inlineMatchers (e : Expr) : CoreM Expr :=
+  Meta.MetaM.run' <| Meta.transform e fun e => do
+    let .const declName us := e.getAppFn | return .visit e
+    let some info ← Meta.getMatcherInfo? declName | return .visit e
+    let numArgs := e.getAppNumArgs
+    if numArgs > info.arity then
+      return .visit e
+    else if numArgs < info.arity then
+      Meta.forallBoundedTelescope (← Meta.inferType e) (info.arity - numArgs) fun xs _ =>
+        return .visit (← Meta.mkLambdaFVars xs (mkAppN e xs))
+    else
+      let mut args := e.getAppArgs
+      let numAlts := info.numAlts
+      let altNumParams := info.altNumParams
+      let rec inlineMatcher (i : Nat) (args : Array Expr) (letFVars : Array Expr) : MetaM Expr := do
+        if i < numAlts then
+          let altIdx := i + info.getFirstAltPos
+          let numParams := altNumParams[i]!
+          let alt ← normalizeAlt args[altIdx]! numParams
+          Meta.withLetDecl (← mkFreshUserName `_alt) (← Meta.inferType alt) alt fun altFVar =>
+            inlineMatcher (i+1) (args.set! altIdx altFVar) (letFVars.push altFVar)
+        else
+          let info ← getConstInfo declName
+          let value := (← Core.instantiateValueLevelParams info us).beta args
+          Meta.mkLetFVars letFVars value
+      return .visit (← inlineMatcher 0 args #[])
+
 /--
 Replace nested occurrences of `unsafeRec` names with the safe ones.
 -/
@@ -73,11 +115,10 @@ def toDecl (declName : Name) : CoreM Decl := do
     let value ← Meta.lambdaTelescope value fun xs body => do Meta.mkLambdaFVars xs (← Meta.etaExpand body)
     let value ← replaceUnsafeRecNames value
     let value ← macroInline value
+    let value ← inlineMatchers value
     let value ← toLCNF value
     return { name := declName, type, value }
 
-
-
 /--
 Join points are let bound variables with an _jp name prefix.
 They always need to satisfy two conditions: