ci: Enable -Werror=compat and -Werror=default

.github/workflows/test.yml

@@ -78,7 +78,8 @@ jobs:
         run: cabal check
       - name: Configure
         shell: bash
-        run: cabal configure --enable-tests
+        # See doc/dev.md for development practices around warnings.
+        run: cabal configure --enable-tests --ghc-options='-Werror=compat -Werror=default'
       - name: Build
         shell: bash
         run: cabal build

doc/dev.md

@@ -0,0 +1,15 @@
+# Developer documentation
+
+## Warnings
+
+`parameterized-utils` is a research-grade codebase, so correctness in all cases is not *always* the highest priority.
+In particular, as developers, we must weigh the cost of development practices that prioritize correctness, uniformity, or other concerns against our desire to encourage relatively rapid prototyping of interesting functionality.
+When forming judgements about appropriate practices, it is worth remembering that `parameterized-utils` is also upstream of a substantial number of projects, such as What4, Crucible, and Macaw.
+
+The developers currently judge that the warnings included in GHC's `-Wdefault` are conservative enough to be worth fixing in most cases.
+Accordingly, the current default development practice is to fix instances of `-Wdefault` in new code.
+However, we also trust one anothers' judgement to override this practice in any particular case.
+When doing so, developers are encouraged to disable individual warnings on a per-module basis, ideally accompanied by a comment including some justification for why the warnings were not (and/or should not be) fixed.
+To prevent warnings from slipping in unnoticed and thus unexamined, we enable `-Werror=default` in CI.
+
+We also enable `-Werror=compat` in order to gradually prepare for breaking changes in GHC.

src/Data/Parameterized/ClassesC.hs

@@ -20,6 +20,9 @@ Note that there is still some ambiguity around naming conventions, see
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE TypeOperators #-}
 
+-- See https://github.com/GaloisInc/parameterized-utils/issues/149
+{-# OPTIONS_GHC -Wno-trustworthy-safe #-}
+
 module Data.Parameterized.ClassesC
   ( TestEqualityC(..)
   , OrdC(..)