Make 'break' outside loop and similar errors blockers (#1714)

The type checker isn't equipped to deal with such errors currently,
and having it ignore these errors silently on the basis that they
should have been caught by semantic analysis is a bit sketchy.

Fixes #1571.

Author: rwbarton

mypy/semanal.py

@@ -1651,11 +1651,11 @@ def visit_for_stmt(self, s: ForStmt) -> None:
 
     def visit_break_stmt(self, s: BreakStmt) -> None:
         if self.loop_depth == 0:
-            self.fail("'break' outside loop", s, True)
+            self.fail("'break' outside loop", s, True, blocker=True)
 
     def visit_continue_stmt(self, s: ContinueStmt) -> None:
         if self.loop_depth == 0:
-            self.fail("'continue' outside loop", s, True)
+            self.fail("'continue' outside loop", s, True, blocker=True)
 
     def visit_if_stmt(self, s: IfStmt) -> None:
         infer_reachability_of_if_statement(s, pyversion=self.pyversion)
@@ -1783,7 +1783,7 @@ def visit_star_expr(self, expr: StarExpr) -> None:
 
     def visit_yield_from_expr(self, e: YieldFromExpr) -> None:
         if not self.is_func_scope():  # not sure
-            self.fail("'yield from' outside function", e)
+            self.fail("'yield from' outside function", e, True, blocker=True)
         else:
             self.function_stack[-1].is_generator = True
         if e.expr:
@@ -2036,7 +2036,7 @@ def visit__promote_expr(self, expr: PromoteExpr) -> None:
 
     def visit_yield_expr(self, expr: YieldExpr) -> None:
         if not self.is_func_scope():
-            self.fail("'yield' outside function", expr)
+            self.fail("'yield' outside function", expr, True, blocker=True)
         else:
             self.function_stack[-1].is_generator = True
         if expr.expr:
@@ -2250,13 +2250,14 @@ def name_not_defined(self, name: str, ctx: Context) -> None:
     def name_already_defined(self, name: str, ctx: Context) -> None:
         self.fail("Name '{}' already defined".format(name), ctx)
 
-    def fail(self, msg: str, ctx: Context, serious: bool = False) -> None:
+    def fail(self, msg: str, ctx: Context, serious: bool = False, *,
+             blocker: bool = False) -> None:
         if (not serious and
                 not self.check_untyped_defs and
                 self.function_stack and
                 self.function_stack[-1].is_dynamic()):
             return
-        self.errors.report(ctx.get_line(), msg)
+        self.errors.report(ctx.get_line(), msg, blocker=blocker)
 
     def note(self, msg: str, ctx: Context) -> None:
         if (not self.check_untyped_defs and

test-data/unit/check-semanal-error.test

@@ -66,3 +66,16 @@ T = TypeVar('T')
 class C:  # Forgot to add type params here
     def __init__(self, t: T) -> None: pass
 c = C(t=3)  # type: C[int]  # E: "C" expects no type arguments, but 1 given
+
+[case testBreakOutsideLoop]
+break # E: 'break' outside loop
+
+[case testContinueOutsideLoop]
+continue # E: 'continue' outside loop
+
+[case testYieldOutsideFunction]
+yield # E: 'yield' outside function
+
+[case testYieldFromOutsideFunction]
+x = 1
+yield from x # E: 'yield from' outside function