Suppress errors for unreachable branches in conditional expressions (#18295)

Fixes #4134
Fixes #9195

Suppress errors when analyzing unreachable conditional expression
branches. Same idea as what's done when analyzing the right-hand operand
of `and`/`or`:

https://github.com/python/mypy/blob/973618a6bfa88398e08dc250c8427b381b3a0fce/mypy/checkexpr.py#L4252-L4256

This PR originally added filters of the same form to the places where
`analyze_cond_branch` is called in
`ExpressionChecker.visit_conditional_expr`. However, since 5 out of the
6 `analyze_cond_branch` call sites now use `filter_errors` for the case
when `map is None`, I decided to move the error filtering logic to
inside `analyze_cond_branch`.

**Given:**
```python
from typing import TypeVar
T = TypeVar("T", int, str)

def foo(x: T) -> T:
    return x + 1 if isinstance(x, int) else x + "a"
```
**Before:**
```none
main.py:5:16: error: Unsupported operand types for + ("str" and "int")  [operator]
main.py:5:49: error: Unsupported operand types for + ("int" and "str")  [operator]
Found 2 errors in 1 file (checked 1 source file)
```
**After:**
```
Success: no issues found in 1 source file
```

Author: brianschubert

mypy/checker.py

@@ -4793,7 +4793,9 @@ def visit_assert_stmt(self, s: AssertStmt) -> None:
         # If this is asserting some isinstance check, bind that type in the following code
         true_map, else_map = self.find_isinstance_check(s.expr)
         if s.msg is not None:
-            self.expr_checker.analyze_cond_branch(else_map, s.msg, None)
+            self.expr_checker.analyze_cond_branch(
+                else_map, s.msg, None, suppress_unreachable_errors=False
+            )
         self.push_type_map(true_map)
 
     def visit_raise_stmt(self, s: RaiseStmt) -> None:

mypy/checkexpr.py

@@ -4249,11 +4249,7 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
         ):
             self.msg.unreachable_right_operand(e.op, e.right)
 
-        # If right_map is None then we know mypy considers the right branch
-        # to be unreachable and therefore any errors found in the right branch
-        # should be suppressed.
-        with self.msg.filter_errors(filter_errors=right_map is None):
-            right_type = self.analyze_cond_branch(right_map, e.right, expanded_left_type)
+        right_type = self.analyze_cond_branch(right_map, e.right, expanded_left_type)
 
         if left_map is None and right_map is None:
             return UninhabitedType()
@@ -5851,12 +5847,15 @@ def analyze_cond_branch(
         node: Expression,
         context: Type | None,
         allow_none_return: bool = False,
+        suppress_unreachable_errors: bool = True,
     ) -> Type:
         with self.chk.binder.frame_context(can_skip=True, fall_through=0):
             if map is None:
                 # We still need to type check node, in case we want to
-                # process it for isinstance checks later
-                self.accept(node, type_context=context, allow_none_return=allow_none_return)
+                # process it for isinstance checks later. Since the branch was
+                # determined to be unreachable, any errors should be suppressed.
+                with self.msg.filter_errors(filter_errors=suppress_unreachable_errors):
+                    self.accept(node, type_context=context, allow_none_return=allow_none_return)
                 return UninhabitedType()
             self.chk.push_type_map(map)
             return self.accept(node, type_context=context, allow_none_return=allow_none_return)

test-data/unit/check-expressions.test

@@ -1506,6 +1506,13 @@ x.append(y) if bool() else x.append(y)
 z = x.append(y) if bool() else x.append(y) # E: "append" of "list" does not return a value (it only ever returns None)
 [builtins fixtures/list.pyi]
 
+[case testConditionalExpressionWithUnreachableBranches]
+from typing import TypeVar
+T = TypeVar("T", int, str)
+def foo(x: T) -> T:
+    return x + 1 if isinstance(x, int) else x + "a"
+[builtins fixtures/isinstancelist.pyi]
+
 -- Special cases
 -- -------------