Add a test that prod_applist_assum reduces the right number of let-ins

engine/termops.mli

@@ -165,7 +165,10 @@ val prod_applist : Evd.evar_map -> constr -> constr list -> constr
 (** In [prod_applist_assum n c args], [c] is supposed to have the
     form [∀Γ.c] with [Γ] of length [m] and possibly with let-ins; it
     returns [c] with the assumptions of [Γ] instantiated by [args] and
-    the local definitions of [Γ] expanded. *)
+    the local definitions of [Γ] expanded.
+    Note that [n] counts both let-ins and prods, while the length of [args]
+    only counts prods. In other words, varying [n] changes how many
+    trailing let-ins are expanded. *)
 val prod_applist_assum : Evd.evar_map -> int -> constr -> constr list -> constr
 
 (** Remove recursively the casts around a term i.e.

test-suite/success/dtauto-let-deps.v

@@ -0,0 +1,24 @@
+(*
+This test is sensitive to changes in which let-ins are expanded when checking
+for dependencies in constructors.
+If the (x := X) is not reduced, Foo1 won't be recognized as a conjunction,
+and if the (y := X) is reduced, Foo2 will be recognized as a conjunction.
+
+This tests the behavior of engine/termops.ml : prod_applist_assum,
+which is currently specified to reduce exactly the parameters.
+
+If dtauto is changed to reduce lets in constructors before checking dependency,
+this test will need to be changed.
+*)
+
+Context (P Q : Type).
+Inductive Foo1 (X : Type) (x := X) := foo1 : let y := X in P -> Q -> Foo1 x.
+Inductive Foo2 (X : Type) (x := X) := foo2 : let y := X in P -> Q -> Foo2 y.
+
+Goal P -> Q -> Foo1 nat.
+solve [dtauto].
+Qed.
+
+Goal P -> Q -> Foo2 nat.
+Fail solve [dtauto].
+Abort.