Attach warnings to non-PVP compatible uses of signatures.

Summary:
If you use an inherited signature from another package in your own code,
the only valid PVP bound you can specify for this package is an *exact*
version bound.  This is because the signature is used both covariantly
(it provides declarations for import) and contravariantly (it specifies
what is required).  However, this is a bit distressing if you want to
use a PVP-style bound that allows for upgrading a package.  So there is
a dichotomy:

    1. Any signatures that come from packages with exact bounds
    (this includes, in particular, signature packages, who are
    included solely to make declarations available), can be
    used without problem by modules, but

    2. Any signatures that come from packages that are version
    bounded (i.e., any package that also provides modules) must
    NOT be used, because if they were used, they could break
    under a PVP policy that allows relaxations in the needed
    requirements.

To help users avoid situation (2), I've added a warning to all
signature declarations that come solely from (2).  This is not
perfect; you might still end up relying on some type identity
specified by a signature in a version-bounded package, but it
should help catch major errors.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2906

4 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/backpack/should_compile/bkp10.stderr b/testsuite/tests/backpack/should_compile/bkp10.stderr
index 350670e6d4..13c33f363f 100644
--- a/testsuite/tests/backpack/should_compile/bkp10.stderr
+++ b/testsuite/tests/backpack/should_compile/bkp10.stderr
@@ -4,3 +4,7 @@
 [2 of 2] Processing q
   [1 of 2] Compiling H2[sig]          ( q/H2.hsig, nothing )
   [2 of 2] Compiling B                ( q/B.hs, nothing )
+
+bkp10.bkp:13:18: warning: [-Wdeprecations (in -Wdefault)]
+    In the use of type constructor or class ‘S’ (imported from H2):
+    "Inherited requirements from non-signature libraries (libraries with modules) should not be used, as this mode of use is not compatible with PVP-style version bounds.  Instead, copy the declaration to the local hsig file or move the signature to a library of its own and add that library as a dependency."
diff --git a/testsuite/tests/backpack/should_compile/bkp11.stderr b/testsuite/tests/backpack/should_compile/bkp11.stderr
index a804563b2d..ca45b493d3 100644
--- a/testsuite/tests/backpack/should_compile/bkp11.stderr
+++ b/testsuite/tests/backpack/should_compile/bkp11.stderr
@@ -5,3 +5,11 @@
 [2 of 2] Processing q
   [1 of 2] Compiling H[sig]           ( q/H.hsig, nothing )
   [2 of 2] Compiling B                ( q/B.hs, nothing )
+
+bkp11.bkp:16:14: warning: [-Wdeprecations (in -Wdefault)]
+    In the use of type constructor or class ‘S’ (imported from H):
+    "Inherited requirements from non-signature libraries (libraries with modules) should not be used, as this mode of use is not compatible with PVP-style version bounds.  Instead, copy the declaration to the local hsig file or move the signature to a library of its own and add that library as a dependency."
+
+bkp11.bkp:16:19: warning: [-Wdeprecations (in -Wdefault)]
+    In the use of type constructor or class ‘T’ (imported from H):
+    "Inherited requirements from non-signature libraries (libraries with modules) should not be used, as this mode of use is not compatible with PVP-style version bounds.  Instead, copy the declaration to the local hsig file or move the signature to a library of its own and add that library as a dependency."
diff --git a/testsuite/tests/backpack/should_compile/bkp24.stderr b/testsuite/tests/backpack/should_compile/bkp24.stderr
index ddafe4150b..fbde703f49 100644
--- a/testsuite/tests/backpack/should_compile/bkp24.stderr
+++ b/testsuite/tests/backpack/should_compile/bkp24.stderr
@@ -11,6 +11,10 @@
 [4 of 5] Processing q
   [1 of 2] Compiling B[sig]           ( q/B.hsig, nothing )
   [2 of 2] Compiling Q                ( q/Q.hs, nothing )
+
+bkp24.bkp:23:24: warning: [-Wdeprecations (in -Wdefault)]
+    In the use of type constructor or class ‘B’ (imported from B):
+    "Inherited requirements from non-signature libraries (libraries with modules) should not be used, as this mode of use is not compatible with PVP-style version bounds.  Instead, copy the declaration to the local hsig file or move the signature to a library of its own and add that library as a dependency."
 [5 of 5] Processing r
   Instantiating r
   [1 of 2] Including q[B=b:B]
diff --git a/testsuite/tests/backpack/should_compile/bkp36.stderr b/testsuite/tests/backpack/should_compile/bkp36.stderr
index 45ade1412f..c891ab476b 100644
--- a/testsuite/tests/backpack/should_compile/bkp36.stderr
+++ b/testsuite/tests/backpack/should_compile/bkp36.stderr
@@ -7,3 +7,7 @@
 [3 of 3] Processing q
   [1 of 2] Compiling B[sig]           ( q/B.hsig, nothing )
   [2 of 2] Compiling Q                ( q/Q.hs, nothing )
+
+bkp36.bkp:20:16: warning: [-Wdeprecations (in -Wdefault)]
+    In the use of type constructor or class ‘T’ (imported from B):
+    "Inherited requirements from non-signature libraries (libraries with modules) should not be used, as this mode of use is not compatible with PVP-style version bounds.  Instead, copy the declaration to the local hsig file or move the signature to a library of its own and add that library as a dependency."