Fix dubious warnings

Author: martijnbastiaan

HACKING.md

@@ -135,6 +135,33 @@ a big win. You need about 2GB of RAM for each thread.
 cabal run clash-testsuite -- --auto-detect-tools -j$(nproc)
 ```
 
+#### Turning Clash warnings into test failures: `-fclash-werror`
+
+Some compiler bugs only show up as a warning on stderr, which by itself does not
+fail a test. `-fclash-werror` makes Clash throw a `ClashException` instead of
+printing a warning, so a test that adds it to its `clashFlags` fails as soon as
+Clash starts warning:
+
+```haskell
+runTest "PopCountNoInteger" def{clashFlags=["-fclash-werror"]}
+```
+
+Unlike GHC's `-Werror`, `-fclash-werror` only affects Clash's own warnings
+(GHC's `-Werror` also sets it). Warnings this covers include:
+
+* `Dubious primitive instantiation ...` — e.g. an `Integer` reaching the
+  netlist. Useful for pinning down code that should be `Integer`-free.
+* `Unmatchable constant as case subject ...` — a constant the primitive
+  evaluator has no reduction rule for, so `caseCon` cannot fold the
+  case-expression. Only reported when invariants are checked, which
+  [`commonArgs`](tests/src/Test/Tasty/Clash.hs) already does for every test via
+  `-fclash-debug DebugSilent`.
+
+Note that this cannot be turned on testsuite-wide: a number of existing tests
+legitimately provoke these warnings (`Numbers/Bits` exercises `Bits Integer`,
+and `TopEntity/T1074` has a type-equality dictionary as a case subject, which is
+benign but irreducible). Enable it per test instead.
+
 #### `-p` (pattern filter)
 
 `-p PATTERN` is a [tasty pattern][tasty-patterns] that restricts which tests

changelog/2026-08-05T12_00_00+02_00_zero_width_bitvector_concat_folding

@@ -0,0 +1 @@
+FIXED: `(++#)` on `BitVector`s was only constant folded when *both* operands were literals. A zero-width operand is often not a literal but `removedArg` (`reduceSplitHandler` produces one for `split#` with a zero-width half), so the evaluator got stuck. In a constant case subject this made `caseCon` report `Unmatchable constant as case subject` and left a redundant multiplexer in the generated HDL. A zero-width operand no longer has to be a literal.

changelog/2026-08-05T12_05_00+02_00_integer_log_base_out_of_domain

@@ -0,0 +1 @@
+FIXED: `GHC.Num.Integer.integerLogBase#` and `GHC.Num.Natural.naturalLogBase#` were not constant folded outside the domain of `Clash.Util.flogBase` (`y <= 0`), even though GHC's `integerLogBase#` is total. Such calls appear in (dead) `KnownNat` evidence for ill-defined applications of e.g. `GHC.TypeLits.Extra.CLog`, and produced an `Unmatchable constant as case subject` report. Clash now mirrors the values GHC computes at runtime.

changelog/2026-08-05T12_10_00+02_00_popcount_no_integer

@@ -0,0 +1 @@
+FIXED: `popCount`, `countLeadingZeros` and `countTrailingZeros` on `BitVector` (and hence on `Unsigned`, `Signed` and `Index`, which delegate to it) went through `Integer`, which made Clash emit a `Dubious primitive instantiation for GHC.Num.Integer.integerToInt#` warning and put an `Integer` in the generated HDL. They now use `Clash.Sized.Internal.Index.fromEnum#`, which has a black box of its own.

changelog/2026-08-05T12_15_00+02_00_fclash_werror

@@ -0,0 +1 @@
+ADDED: `-fclash-werror` turns Clash's own warnings into errors. Unlike GHC's `-Werror` it does not make GHC's warnings fatal, which makes it usable in tests: a test that should not provoke any Clash warning can pass `-fclash-werror` and will fail as soon as it does.

changelog/2026-08-05T12_20_00+02_00_unmatchable_constant_is_a_warning

@@ -0,0 +1 @@
+CHANGED: The `Unmatchable constant as case subject` report (emitted when invariants are checked, e.g. under `-fclash-debug DebugSilent`) is now a proper warning instead of a trace, so `-Werror` / `-fclash-werror` turns it into an error. It also names the binder being normalized.

changelog/2026-08-05T12_25_00+02_00_dubious_primitive_warning_context

@@ -0,0 +1 @@
+CHANGED: `Dubious primitive instantiation` warnings now name the component, the binder the primitive's result is assigned to, and the primitive's arguments (resolving variable references against sibling let-bindings). They are also deduplicated per *(component, primitive)* pair instead of per primitive, so a single run reveals every offending site rather than only the first. Designs with many offending components therefore emit more warnings than before.

clash-ghc/src-bin-9.10.1/Clash/Main.hs

@@ -271,7 +271,10 @@ main' postLoadMode units dflags0 args flagWarnings startAction clashOpts = do
   -- Propagate some GHC flags to Clash
   liftIO . modifyIORef' clashOpts $ \opts ->
     opts
-      { opt_werror = EnumSet.member Opt_WarnIsError (generalFlags dflags3)
+      -- Note the '||': -fclash-werror has already been parsed at this point,
+      -- and GHC's -Werror should not undo it.
+      { opt_werror = opt_werror opts
+                       || EnumSet.member Opt_WarnIsError (generalFlags dflags3)
       , opt_ghcDebugLevel = debugLevel dflags3
       }
 

clash-ghc/src-bin-9.10.2/Clash/Main.hs

@@ -271,7 +271,10 @@ main' postLoadMode units dflags0 args flagWarnings startAction clashOpts = do
   -- Propagate some GHC flags to Clash
   liftIO . modifyIORef' clashOpts $ \opts ->
     opts
-      { opt_werror = EnumSet.member Opt_WarnIsError (generalFlags dflags3)
+      -- Note the '||': -fclash-werror has already been parsed at this point,
+      -- and GHC's -Werror should not undo it.
+      { opt_werror = opt_werror opts
+                       || EnumSet.member Opt_WarnIsError (generalFlags dflags3)
       , opt_ghcDebugLevel = debugLevel dflags3
       }
 

clash-ghc/src-bin-9.12/Clash/Main.hs

@@ -287,7 +287,10 @@ main' postLoadMode units dflags0 args flagWarnings startAction clashOpts = do
 
   -- Propagate -Werror to Clash
   liftIO . modifyIORef' clashOpts $ \opts ->
-    opts { opt_werror = EnumSet.member Opt_WarnIsError (generalFlags dflags3) }
+    -- Note the '||': -fclash-werror has already been parsed at this point, and
+    -- GHC's -Werror should not undo it.
+    opts { opt_werror = opt_werror opts
+                          || EnumSet.member Opt_WarnIsError (generalFlags dflags3) }
 
   let dflags4 = if backendNeedsFullWays bcknd &&
                    not (gopt Opt_ExternalInterpreter dflags3)