Add new test cases for failures of type-specific unification methods

These test cases will also have improved error messages in the next
non-test commit.  They come from the work done to address reviewer
comments for ocaml#10407.

testsuite/tests/typing-gadts/test.ml

@@ -1263,3 +1263,111 @@ Line 7, characters 22-36:
 Error: This function should have type int -> int
        but its first argument is not labelled
 |}]
+
+(* Check got/expected when the order changes *)
+module M = struct
+  type t
+end
+type (_,_) eq = Refl: ('a,'a) eq
+let f (x:M.t) (y: (M.t, int -> int) eq) =
+  let Refl = y in
+  if true then fun x -> x + 1 else x
+[%%expect{|
+module M : sig type t end
+type (_, _) eq = Refl : ('a, 'a) eq
+Line 7, characters 35-36:
+7 |   if true then fun x -> x + 1 else x
+                                       ^
+Error: This expression has type M.t = int -> int
+       but an expression was expected of type int -> int
+       This instance of int -> int is ambiguous:
+       it would escape the scope of its equation
+|}]
+
+module M = struct
+  type t
+end
+type (_,_) eq = Refl: ('a,'a) eq
+let f w (x:M.t) (y: (M.t, <m:int>) eq) =
+  let Refl = y in
+  let z = if true then x else w in
+  z#m
+[%%expect{|
+module M : sig type t end
+type (_, _) eq = Refl : ('a, 'a) eq
+Line 8, characters 2-3:
+8 |   z#m
+      ^
+Error: This expression has type M.t
+       It has no method m
+Hint: Did you mean m?
+|}]
+
+(* Check got/expected when the order changes *)
+module M = struct
+  type t
+end
+type (_,_) eq = Refl: ('a,'a) eq
+let f w (x:M.t) (y: (M.t, <m:int>) eq) =
+  let Refl = y in
+  let z = if true then w else x in
+  z#m
+[%%expect{|
+module M : sig type t end
+type (_, _) eq = Refl : ('a, 'a) eq
+Line 8, characters 2-3:
+8 |   z#m
+      ^
+Error: This expression has type M.t
+       It has no method m
+Hint: Did you mean m?
+|}]
+
+type (_,_) eq = Refl: ('a,'a) eq
+module M = struct
+  type t = C : (<m:int; ..> as 'a) * ('a, <m:int; b:bool>) eq -> t
+end
+let f (C (x,y) : M.t) =
+  let g w =
+    let Refl = y in
+    let z = if true then w else x in
+    z#b
+  in ()
+[%%expect{|
+type (_, _) eq = Refl : ('a, 'a) eq
+module M :
+  sig
+    type t =
+        C : (< m : int; .. > as 'a) * ('a, < b : bool; m : int >) eq -> t
+  end
+Line 9, characters 4-5:
+9 |     z#b
+        ^
+Error: This expression has type < m : int; .. >
+       It has no method b
+|}]
+
+(* Check got/expected when the order changes *)
+type (_,_) eq = Refl: ('a,'a) eq
+module M = struct
+  type t = C : (<m:int; ..> as 'a) * ('a, <m:int; b:bool>) eq -> t
+end
+let f (C (x,y) : M.t) =
+  let g w =
+    let Refl = y in
+    let z = if true then x else w in
+    z#b
+  in ()
+[%%expect{|
+type (_, _) eq = Refl : ('a, 'a) eq
+module M :
+  sig
+    type t =
+        C : (< m : int; .. > as 'a) * ('a, < b : bool; m : int >) eq -> t
+  end
+Line 9, characters 4-5:
+9 |     z#b
+        ^
+Error: This expression has type < m : int; .. >
+       It has no method b
+|}]

testsuite/tests/typing-misc/labels.ml

@@ -34,6 +34,34 @@ Error: This function should have type unit -> unit
        but its first argument is labelled ?opt
 |}];;
 
+(* filter_arrow *)
+
+let (f : x:int -> int) = fun y -> y
+[%%expect{|
+Line 1, characters 25-35:
+1 | let (f : x:int -> int) = fun y -> y
+                             ^^^^^^^^^^
+Error: This function should have type x:int -> int
+       but its first argument is not labelled
+|}];;
+
+let (f : int -> int) = fun ~y -> y
+[%%expect{|
+Line 1, characters 23-34:
+1 | let (f : int -> int) = fun ~y -> y
+                           ^^^^^^^^^^^
+Error: This function should have type int -> int
+       but its first argument is labelled ~y
+|}];;
+
+let (f : x:int -> int) = fun ~y -> y
+[%%expect{|
+Line 1, characters 25-36:
+1 | let (f : x:int -> int) = fun ~y -> y
+                             ^^^^^^^^^^^
+Error: This function should have type x:int -> int
+       but its first argument is labelled ~y
+|}];;
 
 (* More examples *)
 

testsuite/tests/typing-objects/errors.ml

@@ -34,3 +34,21 @@ Error: The class type object method x : 'a * float end
        The method x has type 'a * float but is expected to have type int
        Type 'a * float is not compatible with type int
 |}]
+
+let foo = 42#m;;
+[%%expect{|
+Line 1, characters 10-12:
+1 | let foo = 42#m;;
+              ^^
+Error: This expression has type int
+       It has no method m
+|}]
+
+let foo = object (self) method foo = self#bar end;;
+[%%expect{|
+Line 1, characters 37-41:
+1 | let foo = object (self) method foo = self#bar end;;
+                                         ^^^^
+Error: This expression has type < foo : 'a >
+       It has no method bar
+|}]