Default [unreachable] to warn

Summary: No other Python type checker I checked errors on unreachable code by default. (pyright/basedpyright greys it out.) Furthermore, this is neither a type error nor a runtime error. I suggest we downgrade it to a warning.

Reviewed By: stroxler

Differential Revision: D95282312

fbshipit-source-id: ba5b57d41986aa200e91355939f5f2b3752af7b8

Author: rchen152

conformance/third_party/conformance.exp

@@ -1752,7 +1752,7 @@
       "description": "This `yield` expression is unreachable",
       "line": 140,
       "name": "unreachable",
-      "severity": "error",
+      "severity": "warn",
       "stop_column": 13,
       "stop_line": 140
     },
@@ -1763,7 +1763,7 @@
       "description": "This `yield` expression is unreachable",
       "line": 145,
       "name": "unreachable",
-      "severity": "error",
+      "severity": "warn",
       "stop_column": 13,
       "stop_line": 145
     },
@@ -1774,7 +1774,7 @@
       "description": "This `yield` expression is unreachable",
       "line": 190,
       "name": "unreachable",
-      "severity": "error",
+      "severity": "warn",
       "stop_column": 10,
       "stop_line": 190
     }

crates/pyrefly_config/src/error_kind.rs

@@ -344,6 +344,7 @@ impl ErrorKind {
             ErrorKind::UnannotatedParameter => Severity::Ignore,
             ErrorKind::UnannotatedReturn => Severity::Ignore,
             ErrorKind::UnnecessaryComparison => Severity::Warn,
+            ErrorKind::Unreachable => Severity::Warn,
             ErrorKind::UnresolvableDunderAll => Severity::Warn,
             // TODO: up severity to Warn when https://github.com/facebook/pyrefly/issues/1950 is fixed
             ErrorKind::UntypedImport => Severity::Ignore,

website/docs/error-kinds.mdx

@@ -1300,6 +1300,8 @@ def valid_generator():
     yield 2  # This is valid
 ```
 
+The default severity of this diagnostic is `warn`.
+
 ## unresolvable-dunder-all
 
 Default severity: **warn**