Suppress second error message with := and [truthy-bool] (#15941)

Closes #15685
CC @ikonst

Co-authored-by: Ilya Priven <[email protected]>

Author: sobolevn

mypy/checker.py

@@ -5762,7 +5762,9 @@ def combine_maps(list_maps: list[TypeMap]) -> TypeMap:
             else_map = {}
         return if_map, else_map
 
-    def find_isinstance_check(self, node: Expression) -> tuple[TypeMap, TypeMap]:
+    def find_isinstance_check(
+        self, node: Expression, *, in_boolean_context: bool = True
+    ) -> tuple[TypeMap, TypeMap]:
         """Find any isinstance checks (within a chain of ands).  Includes
         implicit and explicit checks for None and calls to callable.
         Also includes TypeGuard and TypeIs functions.
@@ -5773,15 +5775,24 @@ def find_isinstance_check(self, node: Expression) -> tuple[TypeMap, TypeMap]:
         If either of the values in the tuple is None, then that particular
         branch can never occur.
 
+        If `in_boolean_context=True` is passed, it means that we handle
+        a walrus expression. We treat rhs values
+        in expressions like `(a := A())` specially:
+        for example, some errors are suppressed.
+
         May return {}, {}.
         Can return None, None in situations involving NoReturn.
         """
-        if_map, else_map = self.find_isinstance_check_helper(node)
+        if_map, else_map = self.find_isinstance_check_helper(
+            node, in_boolean_context=in_boolean_context
+        )
         new_if_map = self.propagate_up_typemap_info(if_map)
         new_else_map = self.propagate_up_typemap_info(else_map)
         return new_if_map, new_else_map
 
-    def find_isinstance_check_helper(self, node: Expression) -> tuple[TypeMap, TypeMap]:
+    def find_isinstance_check_helper(
+        self, node: Expression, *, in_boolean_context: bool = True
+    ) -> tuple[TypeMap, TypeMap]:
         if is_true_literal(node):
             return {}, None
         if is_false_literal(node):
@@ -6050,7 +6061,9 @@ def has_no_custom_eq_checks(t: Type) -> bool:
             if else_assignment_map is not None:
                 else_map.update(else_assignment_map)
 
-            if_condition_map, else_condition_map = self.find_isinstance_check(node.value)
+            if_condition_map, else_condition_map = self.find_isinstance_check(
+                node.value, in_boolean_context=False
+            )
 
             if if_condition_map is not None:
                 if_map.update(if_condition_map)
@@ -6112,7 +6125,10 @@ def has_no_custom_eq_checks(t: Type) -> bool:
         # Restrict the type of the variable to True-ish/False-ish in the if and else branches
         # respectively
         original_vartype = self.lookup_type(node)
-        self._check_for_truthy_type(original_vartype, node)
+        if in_boolean_context:
+            # We don't check `:=` values in expressions like `(a := A())`,
+            # because they produce two error messages.
+            self._check_for_truthy_type(original_vartype, node)
         vartype = try_expanding_sum_type_to_union(original_vartype, "builtins.bool")
 
         if_type = true_only(vartype)

test-data/unit/check-errorcodes.test

@@ -892,6 +892,12 @@ if a:
 any_or_object: Union[object, Any]
 if any_or_object:
     pass
+
+if (my_foo := Foo()):  # E: "__main__.my_foo" has type "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    pass
+
+if my_a := (a or Foo()):  # E: "__main__.Foo" returns "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    pass
 [builtins fixtures/list.pyi]
 
 [case testTruthyFunctions]

test-data/unit/check-python38.test

@@ -297,8 +297,7 @@ def f(x: int = (c := 4)) -> int:
         z2: NT  # E: Variable "NT" is not valid as a type \
                 # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
 
-    if Alias := int:  # E: Function "Alias" could always be true in boolean context \
-                      # E: Function "int" could always be true in boolean context
+    if Alias := int:  # E: Function "Alias" could always be true in boolean context
         z3: Alias  # E: Variable "Alias" is not valid as a type \
                    # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases