Fixed recent regression relating to "isinstance" type narrowing when the type of the target is a constrained TypeVar.

server/src/analyzer/typeUtils.ts

@@ -469,8 +469,6 @@ export function partiallySpecializeType(type: Type, contextClassType: ClassType)
 
 // Replaces all of the top-level TypeVars (as opposed to TypeVars
 // used as type arguments in other types) with their concrete form.
-// Unlikely typical TypeVar specialization, this method specializes
-// constrained TypeVars as the first constrained type.
 export function makeTypeVarsConcrete(type: Type): Type {
     return doForSubtypes(type, (subtype) => {
         if (subtype.category === TypeCategory.TypeVar) {
@@ -478,12 +476,6 @@ export function makeTypeVarsConcrete(type: Type): Type {
                 return subtype.boundType;
             }
 
-            // If the type is constrained by more than one type, we can't check
-            // all constrained types, but we can at least check the first one.
-            if (subtype.constraints.length > 0) {
-                return subtype.constraints[0];
-            }
-
             // Normally, we would use UnknownType here, but we need
             // to use Any because unknown types will generate diagnostics
             // in strictly-typed files that cannot be suppressed in

server/src/tests/checker.test.ts

@@ -1338,6 +1338,12 @@ test('GenericTypes24', () => {
     validateResults(analysisResults, 1);
 });
 
+test('GenericTypes25', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['genericTypes25.py']);
+
+    validateResults(analysisResults, 0);
+});
+
 test('Protocol1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['protocol1.py']);
 

server/src/tests/samples/genericTypes25.py

@@ -0,0 +1,21 @@
+# This sample tests type narrowing of generic constrained types.
+
+from typing import AnyStr, List, Sequence, Union
+
+
+Command = Union[AnyStr, Sequence[AnyStr]]
+
+
+def version1(cmd: Command) -> List[str]:
+    if isinstance(cmd, bytes):
+        return [str(cmd, "utf-8")]
+    if isinstance(cmd, str):
+        return [cmd]
+
+    ret: List[str] = []
+    for itm in cmd:
+        if isinstance(itm, str):
+            ret.append(itm)
+        else:
+            ret.append(str(itm, "utf-8"))
+    return ret