Preserve can_be_true and can_be_false when copying types (#7991)

This pull request modifies the `last_known_value` eraser so it
always copies over the previously inferred value of the
`can_be_true` and `can_be_false` fields.

This isn't entirely typesafe -- for example, we can sometimes
accidentally infer the wrong result if we try reassigning variables
or call a mutating function :

```
    def bad1(a: List[str], b: bool) -> bool:
        x = a and b
        a = ["foo"]
        y: bool
        return x or y   # Should be an error

    def bad2(a: List[str], b: bool) -> bool:
        x = a and b
        a.append("foo")
        y: bool
        return x or y   # Should be an error
```

But this was apparently the old behavior/something mypy wasn't
previously able to detect, so I guess this is fine for now.

I also decided against modifying every `copy_modified` method in
all the type classes because of this reason: I wanted to limit
the spread of this potentially misleading additional inferred info.

Fixes #7986, probably.

mypy/erasetype.py

@@ -142,10 +142,13 @@ class LastKnownValueEraser(TypeTranslator):
     def visit_instance(self, t: Instance) -> Type:
         if not t.last_known_value and not t.args:
             return t
-        return t.copy_modified(
+        new_t = t.copy_modified(
             args=[a.accept(self) for a in t.args],
             last_known_value=None,
         )
+        new_t.can_be_true = t.can_be_true
+        new_t.can_be_false = t.can_be_false
+        return new_t
 
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:
         # Type aliases can't contain literal values, because they are

test-data/unit/check-expressions.test

@@ -343,6 +343,15 @@ b = bool()
 reveal_type(s and b or b)  # N: Revealed type is 'builtins.bool'
 [builtins fixtures/bool.pyi]
 
+[case testRestrictedBoolAndOrWithGenerics]
+from typing import List
+
+def f(a: List[str], b: bool) -> bool:
+    x = a and b
+    y: bool
+    return reveal_type(x or y)  # N: Revealed type is 'builtins.bool'
+[builtins fixtures/list.pyi]
+
 [case testNonBooleanOr]
 c, d, b = None, None, None # type: (C, D, bool)
 if int():