Changelog and test for coq#17789

doc/changelog/04-tactics/17789-change-uniq.rst

@@ -0,0 +1,5 @@
+- **Changed:**
+  instances declared with flag:`Typeclasses Unique Instances` do not allow backtracking even when the goal contains evars
+  (`#17789 <https://github.com/coq/coq/pull/17789>`_,
+  fixes `#6714 <https://github.com/coq/coq/issues/6714>`_,
+  by Jan-Oliver Kaiser).

test-suite/bugs/bug_6714.v

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+Set Typeclasses Unique Instances.
+
+Class Foo (x y : bool) := {}.
+Class Bar (x y : bool) := {}.
+Class Bax (x : bool) := {}.
+
+Instance bar_1 : Bar true false := {}.
+Instance bar_2 : Bar true true := {}.
+
+Instance bax_1 : Bax false := {}.
+
+Instance foo x y : Bar x y -> Bax y -> Foo x y := {}.
+
+Typeclasses eauto := debug.
+
+Fail Definition bla : Foo true _ := _.
+
+ (*
+Debug: Calling typeclass resolution with flags: depth = ∞,unique = false,do_split = true,fail = false
+Debug: 1: looking for (Foo true ?y) with backtracking
+Debug: 1.1: simple apply foo on (Foo true ?y), 2 subgoal(s)
+Debug: 1.1-1 : (Bar true ?y)
+Debug: 1.1-1: looking for (Bar true ?y) with backtracking
+Debug: 1.1-1.1: exact bar_2 on (Bar true ?y), 0 subgoal(s)
+Debug: 1.1-2 : (Bax true)
+Debug: 1.1-2: looking for (Bax true) without backtracking
+Debug: 1.1-2: no match for (Bax true), 0 possibilities
+Debug: 1.1-1.2: exact bar_1 on (Bar true ?y), 0 subgoal(s)
+Debug: 1.1-2 : (Bax false)
+Debug: 1.1-2: looking for (Bax false) without backtracking
+Debug: 1.1-2.1: exact bax_1 on (Bax false), 0 subgoal(s)
+
+produces bla =
+foo true false bar_1 bax_1 : Foo true false
+     : Foo true false
+*)