Mark code after breakless statically true loops as unreachable

pyrefly/lib/binding/scope.rs

@@ -4343,6 +4343,7 @@ impl<'a> BindingsBuilder<'a> {
         is_while_true: bool,
         loop_definitely_runs: bool,
     ) {
+        let loop_raises = self.scopes.terminated_by_raise();
         let finished_loop = self.scopes.finish_loop();
         let (breaks, other_exits): (Vec<Flow>, Vec<Flow>) = finished_loop
             .exits
@@ -4381,8 +4382,13 @@ impl<'a> BindingsBuilder<'a> {
         // also know that breaks are the only way to exit, so we drop the current flow,
         // which is actually unreachable.
         //
-        // TODO(stroxler): in the `is_while_true` case, empty breaks might have implications
-        // for flow termination and/or `NoReturn` behaviors, we should investigate.
+        if is_while_true && breaks.is_empty() {
+            self.scopes.mark_flow_termination(if loop_raises {
+                TerminationKind::Raise
+            } else {
+                TerminationKind::Jump
+            });
+        }
         if let Some(base) = base_if_breaks {
             if is_while_true {
                 self.merge_flow(base, breaks, other_range, MergeStyle::Exclusive)

pyrefly/lib/binding/stmt.rs

@@ -1401,6 +1401,7 @@ impl<'a> BindingsBuilder<'a> {
                 );
             }
             Stmt::While(mut x) => {
+                let was_unreachable = self.scopes.is_definitely_unreachable();
                 self.setup_loop(x.range, &SmallSet::new());
                 // Note that it is important we ensure *after* we set up the loop, so that both the
                 // narrowing and type checking are aware that the test might be impacted by changes
@@ -1451,6 +1452,16 @@ impl<'a> BindingsBuilder<'a> {
                     is_while_true,
                     is_while_true,
                 );
+                if is_while_true
+                    && !test_is_environment_independent
+                    && self.scopes.has_terminated()
+                    && !was_unreachable
+                {
+                    // Another Python environment can skip the loop.
+                    self.scopes.set_definitely_unreachable(false);
+                    self.scopes
+                        .mark_flow_termination(TerminationKind::StaticTest);
+                }
             }
             Stmt::If(mut x) => {
                 let is_definitely_unreachable = self.scopes.is_definitely_unreachable();

pyrefly/lib/test/delayed_inference.rs

@@ -45,7 +45,7 @@ from typing import assert_type, Literal
 x = 5
 while True:
   x = x + 1
-y = x
+y = x  # E: This code is unreachable
 assert_type(y, int)
 "#,
 );

pyrefly/lib/test/flow_branching.rs

@@ -432,6 +432,7 @@ c = C()
 while True:
     if True:
         c = C()
+        break
 
 assert_type(c, C)
     "#,
@@ -989,7 +990,7 @@ for x in []:
 pass  # E: Expected an indented block
 
 while True:
-pass  # E: Expected an indented block
+pass  # E: Expected an indented block  # E: This code is unreachable
 "#,
 );
 

pyrefly/lib/test/flow_looping.rs

@@ -958,3 +958,49 @@ for item in items:
 last  # E: `last` may be uninitialized
 "#,
 );
+
+testcase!(
+    test_unreachable_after_breakless_while_true,
+    r#"
+def retry(flag: bool) -> None:
+    while True:
+        if flag:
+            return
+    raise RuntimeError("unreachable")  # E: This code is unreachable
+
+def spin() -> None:
+    while True:
+        pass
+    raise RuntimeError("unreachable")  # E: This code is unreachable
+"#,
+);
+
+testcase!(
+    test_while_environment_dependent_fallthrough,
+    r#"
+import sys
+
+def configured_true() -> int:
+    while sys.version_info >= (3, 0):
+        pass
+    return 1
+
+def configured_false() -> int:
+    while sys.version_info < (3, 0):
+        pass
+    return 1
+"#,
+);
+
+testcase!(
+    test_while_true_raise_can_be_suppressed,
+    r#"
+from contextlib import suppress
+
+def f() -> int:
+    with suppress(ValueError):
+        while True:
+            raise ValueError()
+    return 1
+"#,
+);

pyrefly/lib/test/lsp/hover_type.rs

@@ -478,7 +478,7 @@ while True:
 y = x
 #   ^
 "#;
-    let report = get_batched_lsp_operations_report(&[("main", code)], get_test_report);
+    let report = get_batched_lsp_operations_report_allow_error(&[("main", code)], get_test_report);
     assert_eq!(
         r#"
 # main.py

pyrefly/lib/test/paramspec.rs

@@ -487,7 +487,7 @@ def run[**P, R](func: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:
             return func(*args,**kwargs)
         except Exception:
             pass
-    raise NotImplementedError("Unreachable")
+    raise NotImplementedError("Unreachable")  # E: This code is unreachable
   "#,
 );