ci: verify default V3 dispatch on macOS

Author: medvednikov

.github/workflows/macos_ci.yml

@@ -41,10 +41,9 @@ jobs:
       VTEST_SHOW_LONGEST_BY_RUNTIME: 3
       VTEST_SHOW_LONGEST_BY_COMPTIME: 3
       VTEST_SHOW_LONGEST_BY_TOTALTIME: 3
-      # V3 is the default compiler on macOS; disable the V3->V1 fallback for the
-      # whole job so a V3 regression fails CI directly — including when it stops
-      # ci/macos_ci.vsh itself (compiled by `v run`) from building. The setenv at
-      # the end of that script is too late to guard its own compilation.
+      # V3 is the default compiler on macOS; disable the V3->V1 fallback for every
+      # eligible V3 build so a regression fails CI directly. The `.vsh` runner is
+      # intentionally compiled by V1; the canary below verifies V3 dispatch itself.
       V_MACOS_V3_NO_FALLBACK: 1
     steps:
       - uses: actions/checkout@v7
@@ -56,14 +55,22 @@ jobs:
           restore-keys: |
             brew-${{ matrix.os }}-
       - name: Build V
-        # `make`'s post-bootstrap `v run` helpers (e.g. .github/problem-matchers/
-        # register_all.vsh) are not buildable by V3 yet and legitimately fall back to V1;
-        # without clearing the job-wide no-fallback guard for this bootstrap step that
-        # fallback would become a hard build failure. Later steps keep the guard, so V3 is
-        # still enforced where it must build the program.
+        # `make` runs bootstrap helpers before the V3 canary; `.vsh` helpers intentionally
+        # use V1, and other helpers may need the compatibility fallback. Clear the guard
+        # for this bootstrap step; later eligible builds and the canary retain it.
         env:
           V_MACOS_V3_NO_FALLBACK: 0
         run: make -j4 && ./v symlink
+      - name: Assert V3 is the default compiler (fail if V1 is silently used)
+        run: |
+          set -uo pipefail
+          printf 'fn main() { println(6 * 7) }\n' > "$RUNNER_TEMP/v3_canary.v"
+          V_MACOS_V3_NO_FALLBACK=1 ./v -v run "$RUNNER_TEMP/v3_canary.v" > "$RUNNER_TEMP/v3_canary.out" 2>&1 || true
+          cat "$RUNNER_TEMP/v3_canary.out"
+          grep -q 'V3 compiler in process' "$RUNNER_TEMP/v3_canary.out" || { echo '::error::V3 was NOT dispatched on macOS — the program was compiled by V1'; exit 1; }
+          grep -qx '42' "$RUNNER_TEMP/v3_canary.out" || { echo '::error::the V3-compiled canary did not build/run correctly'; exit 1; }
+          if grep -qiE 'retrying with .-old-compiler|compatibility fallback|used the stable compiler' "$RUNNER_TEMP/v3_canary.out"; then echo '::error::V3 silently fell back to V1'; exit 1; fi
+          echo 'OK: V3 compiled and ran this program in-process on macOS, with the fallback disabled'
       - name: Pkg-config regressions (Clang, no static executable)
         run: |
           set -euo pipefail