Lean 4

Motive is not type correct (Lean 4)

rw tried to replace a term, but other parts of the goal depend on that exact term, so the rewritten goal would no longer type-check.

What it looks like

Main.lean:9:6: error: Tactic `rewrite` failed: motive is not type correct:
  fun _a => …
Error: …

Explanation: The rewrite tactic rewrites an expression 'e' using an equality 'a = b' by the following process. …

What does "Motive is not type correct (Lean 4)" mean?

rw abstracts every occurrence of the left-hand side into a function called the motive, then rewrites using congruence. If an occurrence is needed for something else to type-check — a proof that mentions it, or an index — the motive does not type-check and rw stops. The message says rewrite even when you wrote rw. Lean 4.23 and later print Tactic `rewrite` failed: motive is not type correct:; Lean 4.22 and earlier print tactic 'rewrite' failed, motive is not type correct:. Lean's own explanation follows, suggesting the occs option, simp or conv.

What causes "Motive is not type correct (Lean 4)"?

The term you rewrite also appears inside a type — a proof argument, a Fin n bound, an index proof or a Decidable instance.
Dependent types: rewriting n in a goal about v : Vector α n.
Every occurrence is rewritten when only some can safely change.

How do I confirm the cause?

Read the motive (fun _a => …): each _a marks an occurrence rw tried to replace — find the one inside a type or a proof.
Try simp only [h]; if it works, a dependency was the problem.

How do I fix "Motive is not type correct (Lean 4)"?

Use simp only [h] instead of rw [h] — simp can handle many of these dependencies.
Rewrite only some occurrences: rw (occs := .pos [1]) [h], or nth_rewrite in Mathlib.
Use conv to rewrite inside one specific part of the goal.
If h : x = e with x a local variable, subst h replaces it everywhere at once.

How do I stop it happening again?

State lemmas so the term you rewrite does not appear inside dependent arguments.
Prefer simp for rewriting in dependent positions and under binders.

Got a different error? Paste it into the Explain This Error tool → Identifies the family and the fix — runs locally, nothing uploaded.