beginend_bug.lean:7:2: error:invalid 'apply' tactic, failed to unify
  a = ?M_1
with
  b = c
proof state:
A : Type,
a b c : A,
Hab : a = b,
Hbc : b = c
⊢ a = ?M_1

A : Type,
a b c : A,
Hab : a = b,
Hbc : b = c
⊢ ?M_1 = c
beginend_bug.lean:9:0: error: don't know how to synthesize placeholder
A : Type,
a b c : A,
Hab : a = b,
Hbc : b = c
⊢ a = c
beginend_bug.lean:9:0: error: failed to add declaration 'foo' to environment, value has metavariables
  λ (A : Type) (a b c : A) (Hab : a = b) (Hbc : b = c),
    ?M_1