Only report unreachability once

Author: A5rocks

mypy/binder.py

@@ -66,6 +66,7 @@ def __init__(self, id: int, conditional_frame: bool = False) -> None:
         self.unreachable = False
         self.conditional_frame = conditional_frame
         self.suppress_unreachable_warnings = False
+        self.unreachable_warning_emitted = False
 
     def __repr__(self) -> str:
         return f"Frame({self.id}, {self.types}, {self.unreachable}, {self.conditional_frame})"
@@ -186,6 +187,9 @@ def unreachable(self) -> None:
     def suppress_unreachable_warnings(self) -> None:
         self.frames[-1].suppress_unreachable_warnings = True
 
+    def emitted_unreachable_warning(self) -> None:
+        self.frames[-1].unreachable_warning_emitted = True
+
     def get(self, expr: Expression) -> Type | None:
         key = literal_hash(expr)
         assert key is not None, "Internal error: binder tried to get non-literal"
@@ -202,6 +206,9 @@ def is_unreachable(self) -> bool:
     def is_unreachable_warning_suppressed(self) -> bool:
         return any(f.suppress_unreachable_warnings for f in self.frames)
 
+    def is_unreachable_warning_emitted(self) -> bool:
+        return any(f.unreachable_warning_emitted for f in self.frames)
+
     def cleanse(self, expr: Expression) -> None:
         """Remove all references to a Node from the binder."""
         key = literal_hash(expr)

mypy/checker.py

@@ -467,6 +467,7 @@ def check_first_pass(self) -> None:
                                 reported_unreachable = True
                             elif not self.is_noop_for_reachability(d):
                                 self.msg.unreachable_statement(d)
+                                self.binder.emitted_unreachable_warning()
                                 reported_unreachable = True
                         self.accept(d)
 
@@ -3060,6 +3061,7 @@ def visit_block(self, b: Block) -> None:
                     reported_unreachable = True
                 elif not self.is_noop_for_reachability(s):
                     self.msg.unreachable_statement(s)
+                    self.binder.emitted_unreachable_warning()
                     reported_unreachable = True
             self.accept(s)
 
@@ -3069,6 +3071,7 @@ def should_report_unreachable_issues(self) -> bool:
             and self.options.warn_unreachable
             and not self.current_node_deferred
             and not self.binder.is_unreachable_warning_suppressed()
+            and not self.binder.is_unreachable_warning_emitted()
         )
 
     def is_noop_for_reachability(self, s: Statement) -> bool:

test-data/unit/check-isinstance.test

@@ -393,7 +393,7 @@ while 2:
         x.b    # E: "A" has no attribute "b"
         break
         _ = "unreachable"  # E: Statement is unreachable
-    x.b  # E: Statement is unreachable
+    x.b
 
 [case testUnionTryFinally6]
 class A: pass
@@ -733,9 +733,8 @@ while bool():
     if isinstance(x, int):
         x + 1
         break
-    # TODO: only report unreachability once
     elif isinstance(x, str):  # E: Statement is unreachable
-        _ = "unreachable"  # E: Statement is unreachable
+        _ = "unreachable"
         x + 'a'
         break
     _ = "unreachable"  # E: Statement is unreachable