Univs: fix bug #5180

In the kernel's generic conversion, backtrack on UniverseInconsistency
for the unfolding heuristic (single backtracking point in reduction).
This exception can be raised in the univ_compare structure to produce
better error messages when the generic conversion function is called
from higher level code in reductionops.ml, which itself is called during
unification in evarconv.ml.
Inside the kernel, the infer and check variants of conversion never
raise UniverseInconsistency though, so this does not change the behavior
of the kernel.

1 files changed, 64 insertions, 0 deletions

diff --git a/test-suite/bugs/closed/5180.v b/test-suite/bugs/closed/5180.v
new file mode 100644
index 0000000000..261092ee6d
--- /dev/null
+++ b/test-suite/bugs/closed/5180.v
@@ -0,0 +1,64 @@
+Universes a b c ω ω'.
+Definition Typeω := Type@{ω}.
+Definition Type2 : Typeω := Type@{c}.
+Definition Type1 : Type2 := Type@{b}.
+Definition Type0 : Type1 := Type@{a}.
+
+Set Universe Polymorphism.
+Set Printing Universes.
+
+Definition Typei' (n : nat)
+  := match n return Type@{ω'} with
+     | 0 => Type0
+     | 1 => Type1
+     | 2 => Type2
+     | _ => Typeω
+     end.
+Definition TypeOfTypei' {n} (x : Typei' n) : Type@{ω'}
+  := match n return Typei' n -> Type@{ω'} with
+     | 0 | 1 | 2 | _ => fun x => x
+     end x.
+Definition Typei (n : nat) : Typei' (S n)
+  := match n return Typei' (S n) with
+     | 0 => Type0
+     | 1 => Type1
+     | _ => Type2
+     end.
+Definition TypeOfTypei {n} (x : TypeOfTypei' (Typei n)) : Type@{ω'}
+  := match n return TypeOfTypei' (Typei n) -> Type@{ω'} with
+     | 0 | 1 | _ => fun x => x
+     end x.
+Check Typei 0 : Typei 1.
+Check Typei 1 : Typei 2.
+
+Definition lift' {n} : TypeOfTypei' (Typei n) -> TypeOfTypei' (Typei (S n))
+  := match n return TypeOfTypei' (Typei n) -> TypeOfTypei' (Typei (S n)) with
+     | 0 | 1 | 2 | _ => fun x => (x : Type)
+     end.
+Definition lift'' {n} : TypeOfTypei' (Typei n) -> TypeOfTypei' (Typei (S n))
+  := match n return TypeOfTypei' (Typei n) -> TypeOfTypei' (Typei (S n)) with
+     | 0 | 1 | 2 | _ => fun x => x
+     end. (* The command has indeed failed with message:
+In environment
+n : nat
+x : TypeOfTypei' (Typei 0)
+The term "x" has type "TypeOfTypei' (Typei 0)" while it is expected to have type
+ "TypeOfTypei' (Typei 1)" (universe inconsistency: Cannot enforce b = a because  a < b).
+ *)
+Check (fun x : TypeOfTypei' (Typei 0) => TypeOfTypei' (Typei 1)).
+
+Definition lift''' {n} : TypeOfTypei' (Typei n) -> TypeOfTypei' (Typei (S n)).
+  refine match n return TypeOfTypei' (Typei n) -> TypeOfTypei' (Typei (S n))  with
+         | 0 | 1 | 2 | _ => fun x => _
+         end.
+  exact x.
+  Undo.
+  (* The command has indeed failed with message:
+In environment
+n : nat
+x : TypeOfTypei' (Typei 0)
+The term "x" has type "TypeOfTypei' (Typei 0)" while it is expected to have type
+ "TypeOfTypei' (Typei 1)" (universe inconsistency: Cannot enforce b = a because  a < b).
+ *)
+  all:compute in *.
+  all:exact x.
\ No newline at end of file