eq_class_error.lean:15:10: error: don't know how to synthesize placeholder
decidable_eq_foo : Π f₁ f₂, decidable (f₁ = f₂)
⊢ decidable (b = b)
eq_class_error.lean:9:11: error: failed to synthesize placeholder

⊢ Π f₁ f₂,
    decidable (f₁ = f₂)