Analyze and check default arguments to lambdas (#7306)

This will allow us to report errors in them and also is needed to
support them in mypyc.

Author: msullivan

mypy/checker.py

@@ -926,19 +926,7 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
 
                 # Type check initialization expressions.
                 body_is_trivial = self.is_trivial_body(defn.body)
-                for arg in item.arguments:
-                    if arg.initializer is None:
-                        continue
-                    if body_is_trivial and isinstance(arg.initializer, EllipsisExpr):
-                        continue
-                    name = arg.variable.name()
-                    msg = 'Incompatible default for '
-                    if name.startswith('__tuple_arg_'):
-                        msg += "tuple argument {}".format(name[12:])
-                    else:
-                        msg += 'argument "{}"'.format(name)
-                    self.check_simple_assignment(arg.variable.type, arg.initializer,
-                        context=arg, msg=msg, lvalue_name='argument', rvalue_name='default')
+                self.check_default_args(item, body_is_trivial)
 
             # Type check body in a new scope.
             with self.binder.top_frame_context():
@@ -978,6 +966,21 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
 
             self.binder = old_binder
 
+    def check_default_args(self, item: FuncItem, body_is_trivial: bool) -> None:
+        for arg in item.arguments:
+            if arg.initializer is None:
+                continue
+            if body_is_trivial and isinstance(arg.initializer, EllipsisExpr):
+                continue
+            name = arg.variable.name()
+            msg = 'Incompatible default for '
+            if name.startswith('__tuple_arg_'):
+                msg += "tuple argument {}".format(name[12:])
+            else:
+                msg += 'argument "{}"'.format(name)
+            self.check_simple_assignment(arg.variable.type, arg.initializer,
+                context=arg, msg=msg, lvalue_name='argument', rvalue_name='default')
+
     def is_forward_op_method(self, method_name: str) -> bool:
         if self.options.python_version[0] == 2 and method_name == '__div__':
             return True

mypy/checkexpr.py

@@ -3065,6 +3065,7 @@ def find_typeddict_context(self, context: Optional[Type]) -> Optional[TypedDictT
 
     def visit_lambda_expr(self, e: LambdaExpr) -> Type:
         """Type check lambda expression."""
+        self.chk.check_default_args(e, body_is_trivial=False)
         inferred_type, type_override = self.infer_lambda_type_using_context(e)
         if not inferred_type:
             self.chk.return_types.append(AnyType(TypeOfAny.special_form))

mypy/semanal.py

@@ -557,7 +557,7 @@ def analyze_func_def(self, defn: FuncDef) -> None:
                     defn.type = defn.type.copy_modified(ret_type=NoneType())
             self.prepare_method_signature(defn, self.type)
 
-        # Analyze function signature and initializers first.
+        # Analyze function signature
         with self.tvar_scope_frame(self.tvar_scope.method_frame()):
             if defn.type:
                 self.check_classvar_in_signature(defn.type)
@@ -577,10 +577,8 @@ def analyze_func_def(self, defn: FuncDef) -> None:
                 if isinstance(defn, FuncDef):
                     assert isinstance(defn.type, CallableType)
                     defn.type = set_callable_name(defn.type, defn)
-            for arg in defn.arguments:
-                if arg.initializer:
-                    arg.initializer.accept(self)
 
+        self.analyze_arg_initializers(defn)
         self.analyze_function_body(defn)
         if defn.is_coroutine and isinstance(defn.type, CallableType) and not self.deferred:
             if defn.is_async_generator:
@@ -868,6 +866,13 @@ def add_function_to_symbol_table(self, func: Union[FuncDef, OverloadedFuncDef])
         func._fullname = self.qualified_name(func.name())
         self.add_symbol(func.name(), func, func)
 
+    def analyze_arg_initializers(self, defn: FuncItem) -> None:
+        with self.tvar_scope_frame(self.tvar_scope.method_frame()):
+            # Analyze default arguments
+            for arg in defn.arguments:
+                if arg.initializer:
+                    arg.initializer.accept(self)
+
     def analyze_function_body(self, defn: FuncItem) -> None:
         is_method = self.is_class_scope()
         with self.tvar_scope_frame(self.tvar_scope.method_frame()):
@@ -3722,6 +3727,7 @@ def analyze_comp_for_2(self, expr: Union[GeneratorExpr,
         expr.sequences[0].accept(self)
 
     def visit_lambda_expr(self, expr: LambdaExpr) -> None:
+        self.analyze_arg_initializers(expr)
         self.analyze_function_body(expr)
 
     def visit_conditional_expr(self, expr: ConditionalExpr) -> None:

test-data/unit/check-functions.test

@@ -2549,3 +2549,9 @@ def f() -> int: ...
 [file p/d.py]
 import p
 def f() -> int: ...
+
+[case testLambdaDefaultTypeErrors]
+lambda a=nonsense: a  # E: Name 'nonsense' is not defined
+lambda a=(1 + 'asdf'): a  # E: Unsupported operand types for + ("int" and "str")
+def f(x: int = i):  # E: Name 'i' is not defined
+    i = 42