unikernel/duniverse/ppxlib/test/driver/compiler-pp/run.t

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+The --use-compiler-pp flag can be used when using the driver's source code
+output, either directly when generating a .corrected file or to force
+printing the AST as source using the installed compiler's printer.
+
+Our driver has a deriver and an extension that produces a pattern-matching with
+named existentials.
+
+This feature has been introduced in 4.13 so the syntax is unsupported before that.
+
+If we run the driver in source output mode, without the `--use-compiler-pp` flag,
+it will successfully print out the source using the 4.13 syntax. If we're running
+on an older compiler, like we are for this test, that can be troublesome.
+
+If instead we use the flag, this will force the migration thus causing an error as
+named existentials can't be migrated down to 4.12.
+
+Let's consider the following file:
+
+  $ cat > test.ml << EOF
+  > [%%named_existentials]
+  > EOF
+
+Running the driver will generate a function with a single pattern matching in it:
+
+  $ ./driver.exe test.ml
+  let f x = match x with | Constructor (type a) _ -> ()
+
+Now if we run it with `--use-compiler-pp`, we should get the migration error:
+
+  $ ./driver.exe --use-compiler-pp test.ml
+  File "test.ml", line 1, characters 0-22:
+  1 | [%%named_existentials]
+      ^^^^^^^^^^^^^^^^^^^^^^
+  Error: migration error: existentials in pattern-matching is not supported before OCaml 4.13
+  [1]
+
+This should also work for correction based code gen:
+
+  $ cat > test_inline.ml << EOF
+  > type t = int
+  > [@@deriving_inline named_existentials]
+  > [@@@end]
+  > EOF
+
+If we run the driver without `--use-compiler-pp`:
+
+  $ ./driver.exe test_inline.ml -diff-cmd -
+  type t = int[@@deriving_inline named_existentials]
+  [@@@end ]
+  $ cat test_inline.ml.ppx-corrected
+  type t = int
+  [@@deriving_inline named_existentials]
+  let _ = fun (_ : t) -> ()
+  let f x = match x with | Constructor (type a) _ -> ()
+  let _ = f
+  [@@@end]
+
+and with the flag:
+
+  $ ./driver.exe test_inline.ml -diff-cmd - --use-compiler-pp
+  File "test_inline.ml", lines 1-2, characters 0-38:
+  1 | type t = int
+  2 | [@@deriving_inline named_existentials]
+  Error: migration error: existentials in pattern-matching is not supported before OCaml 4.13
+  [1]