Skip the veryfasttree dependency entirely on macOS arm64

pyproject.toml

@@ -30,7 +30,14 @@ dependencies = [
     "typer>=0.12",
     "lxml>=5.0",
     "pyfamsa>=0.5",
-    "veryfasttree>=4.0",
+    # No macOS arm64 wheel exists for this (as of 4.0.4.1) -- installing it
+    # there falls back to a from-source build that's known to fail (an
+    # upstream OpenMP-detection bug). CANDy already defaults to fasttree
+    # instead on exactly this platform (see config.default_tree_tool), so
+    # skip the dependency there entirely rather than failing every install.
+    # Still installed on Intel Mac (a real x86_64 wheel exists) and under
+    # Rosetta translation (ditto) -- neither of those needs a source build.
+    "veryfasttree>=4.0; sys_platform != 'darwin' or platform_machine != 'arm64'",
     "platformdirs>=4.0",
 ]
 

src/candy/phylogenetics/veryfasttree_builder.py

@@ -3,7 +3,16 @@
 import logging
 from pathlib import Path
 
-import veryfasttree
+try:
+    import veryfasttree
+except ImportError as exc:  # pragma: no cover -- exercised only where the wheel is genuinely absent
+    raise ImportError(
+        "The 'veryfasttree' package is not installed. On macOS arm64 (Apple Silicon), this is "
+        "deliberate -- veryfasttree has no native wheel there and CANDy defaults to --tree-tool "
+        "fasttree instead (see the README's Apple Silicon setup section). To force veryfasttree "
+        "anyway, `pip install veryfasttree` yourself -- it will attempt a from-source build, which "
+        "is known to fail on stock macOS due to an upstream OpenMP-detection bug."
+    ) from exc
 
 logger = logging.getLogger(__name__)