doc: elabissue for weird elab error

tests/elabissues/let_destruct_inside_forall.lean

@@ -0,0 +1,37 @@
+axiom Foo : Type
+axiom foo : Foo
+
+/- This works -/
+def works : Prop :=
+-- ∀ (x : Foo),
+let ⟨foo₁, foo₂⟩ := (foo, foo);
+false
+
+/- Uncommenting breaks it -/
+def fails₁ : Prop :=
+∀ (x : Foo),
+let ⟨foo₁, foo₂⟩ := (foo, foo);
+false
+-- let_destruct_inside_forall.lean:11:4: error: invalid match/convoy expression, expected type is not known
+
+/- All the following variations fail as well with the same message -/
+def fails₂ : Prop :=
+∀ (x : Foo),
+let (⟨foo₁, foo₂⟩ : Foo × Foo) := (foo, foo);
+foo₁ = foo₂
+
+def fails₃ : Prop :=
+∀ (x : Foo),
+let (⟨foo₁, foo₂⟩ : Foo × Foo) := (foo, foo);
+false
+
+def fails₄ : Prop :=
+∀ (x : Foo),
+let (⟨foo₁, foo₂⟩ : Foo × Foo) := ((foo, foo) : Foo × Foo);
+false
+
+def fails₅ : Prop :=
+∀ (x : Foo),
+let p : Foo × Foo := (foo, foo);
+let ⟨foo₁, foo₂⟩ := p;
+foo₁ = foo₂