Improved consistency of unreachable code. Previously, unreachable code was not supported for if or else suites when the condition type was narrowed to Never. This addresses https://github.com/microsoft/pylance-release/issues/6028. (#8190)

packages/pyright-internal/src/analyzer/codeFlowEngine.ts

@@ -95,7 +95,7 @@ export interface CodeFlowAnalyzer {
 }
 
 export interface CodeFlowEngine {
-    createCodeFlowAnalyzer: (typeAtStart: TypeResult | undefined) => CodeFlowAnalyzer;
+    createCodeFlowAnalyzer: () => CodeFlowAnalyzer;
     isFlowNodeReachable: (flowNode: FlowNode, sourceFlowNode?: FlowNode, ignoreNoReturn?: boolean) => boolean;
     narrowConstrainedTypeVar: (flowNode: FlowNode, typeVar: TypeVarType) => Type | undefined;
     printControlFlowGraph: (
@@ -173,11 +173,7 @@ export function getCodeFlowEngine(
     // Creates a new code flow analyzer that can be used to narrow the types
     // of the expressions within an execution context. Each code flow analyzer
     // instance maintains a cache of types it has already determined.
-    // The caller should pass a typeAtStart value because the code flow
+    function createCodeFlowAnalyzer(): CodeFlowAnalyzer {
-    // analyzer may cache types based on this value, but the typeAtStart
-    // may vary depending on the context in which the code flow analysis
-    // is performed.
-    function createCodeFlowAnalyzer(typeAtStart: TypeResult | undefined): CodeFlowAnalyzer {
         const flowNodeTypeCacheSet = new Map<string, CodeFlowTypeCache>();
 
         function getFlowNodeTypeCacheForReference(referenceKey: string) {
@@ -513,10 +509,6 @@ export function getCodeFlowEngine(
                                     }
                                 }
 
-                                if (flowTypeResult && !isFlowNodeReachable(flowNode)) {
-                                    flowTypeResult = undefined;
-                                }
-
                                 return setCacheEntry(curFlowNode, flowTypeResult?.type, !!flowTypeResult?.isIncomplete);
                             }
 
@@ -1226,8 +1218,6 @@ export function getCodeFlowEngine(
                     curFlowNode.flags &
                     (FlowFlags.VariableAnnotation |
                         FlowFlags.Assignment |
-                        FlowFlags.TrueCondition |
-                        FlowFlags.FalseCondition |
                         FlowFlags.WildcardImport |
                         FlowFlags.NarrowForPattern |
                         FlowFlags.ExhaustedMatch)
@@ -1235,15 +1225,19 @@ export function getCodeFlowEngine(
                     const typedFlowNode = curFlowNode as
                         | FlowVariableAnnotation
                         | FlowAssignment
-                        | FlowCondition
                         | FlowWildcardImport
-                        | FlowCondition
                         | FlowExhaustedMatch;
                     curFlowNode = typedFlowNode.antecedent;
                     continue;
                 }
 
-                if (curFlowNode.flags & (FlowFlags.TrueNeverCondition | FlowFlags.FalseNeverCondition)) {
+                if (
+                    curFlowNode.flags &
+                    (FlowFlags.TrueCondition |
+                        FlowFlags.FalseCondition |
+                        FlowFlags.TrueNeverCondition |
+                        FlowFlags.FalseNeverCondition)
+                ) {
                     const conditionalFlowNode = curFlowNode as FlowCondition;
                     if (conditionalFlowNode.reference) {
                         // Make sure the reference type has a declared type. If not,
@@ -1365,7 +1359,7 @@ export function getCodeFlowEngine(
 
         // Protect against infinite recursion.
         if (isReachableRecursionSet.has(flowNode.id)) {
-            return true;
+            return false;
         }
         isReachableRecursionSet.add(flowNode.id);
 

packages/pyright-internal/src/analyzer/typeEvaluator.ts

@@ -19876,7 +19876,7 @@ export function createTypeEvaluator(
         }
 
         // Allocate a new code flow analyzer.
-        const analyzer = codeFlowEngine.createCodeFlowAnalyzer(typeAtStart);
+        const analyzer = codeFlowEngine.createCodeFlowAnalyzer();
         if (entries) {
             entries.push({ typeAtStart, codeFlowAnalyzer: analyzer });
         } else {
@@ -22181,7 +22181,7 @@ export function createTypeEvaluator(
             const prevTypeCache = returnTypeInferenceTypeCache;
             returnTypeInferenceContextStack.push({
                 functionNode,
-                codeFlowAnalyzer: codeFlowEngine.createCodeFlowAnalyzer(/* typeAtStart */ undefined),
+                codeFlowAnalyzer: codeFlowEngine.createCodeFlowAnalyzer(),
             });
 
             try {

packages/pyright-internal/src/tests/checker.test.ts

@@ -165,7 +165,7 @@ test('With2', () => {
 test('With3', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['with3.py']);
 
-    TestUtils.validateResults(analysisResults, 5);
+    TestUtils.validateResults(analysisResults, 6);
 });
 
 test('With4', () => {

packages/pyright-internal/src/tests/samples/constrainedTypeVar13.py

@@ -49,21 +49,21 @@ class Class1(Generic[_T1, _T2, _T3, _P, Unpack[_Ts]]):
             # This should generate an error.
             return [0]
 
-        if cond:
+        if cond or 3 > 2:
             if isinstance(val1, str):
                 # This should generate an error.
                 return [0]
             else:
                 return [0]
 
-        if cond:
+        if cond or 3 > 2:
             if isinstance(val3, B):
                 return [B()]
             else:
                 # This should generate an error.
                 return [C()]
 
-        if cond:
+        if cond or 3 > 2:
             if not isinstance(val3, B) and not isinstance(val3, C):
                 return [A()]
 

packages/pyright-internal/src/tests/samples/tryExcept4.py

@@ -1,7 +1,10 @@
 # This sample validates that the exception type provided
 # within a raise statement is valid.
 
-a: bool = True
+from random import random
+
+
+a: bool = True if random() > 0.5 else False
 
 
 class CustomException1(BaseException):
@@ -11,10 +14,10 @@ class CustomException1(BaseException):
 
 # This should generate an error because CustomException1
 # requires an argument to instantiate.
-if a:
+if a or 2 > 1:
     raise CustomException1
 
-if a:
+if a or 2 > 1:
     raise CustomException1(3)
 
 
@@ -24,5 +27,5 @@ class CustomException2:
 
 # This should generate an error because
 # the exception doesn't derive from BaseException.
-if a:
+if a or 2 > 1:
     raise CustomException2

packages/pyright-internal/src/tests/samples/unreachable1.py

@@ -1,8 +1,8 @@
 # This sample tests the detection and reporting of unreachable code.
 
-from abc import abstractmethod
 import os
 import sys
+from abc import abstractmethod
 from typing import NoReturn
 
 
@@ -109,3 +109,19 @@ def func10():
     return
     # This should be marked unreachable.
     b = e.errno
+
+
+def func11(obj: str) -> list:
+    if isinstance(obj, str):
+        return []
+    else:
+        # This should be marked as unreachable.
+        return obj
+
+
+def func12(obj: str) -> list:
+    if isinstance(obj, str):
+        return []
+
+    # This should be marked as unreachable.
+    return obj

packages/pyright-internal/src/tests/samples/with3.py

@@ -17,8 +17,7 @@ def test1() -> None:
             raise RuntimeError()
         return
 
-    # This should generate an error because
+    # This should generate an error.
-    # the code is not unreachable.
     c = "hi" + 3
 
     with memoryview(x):

packages/pyright-internal/src/tests/typeEvaluator1.test.ts

@@ -26,7 +26,7 @@ import * as TestUtils from './testUtils';
 test('Unreachable1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['unreachable1.py']);
 
-    TestUtils.validateResults(analysisResults, 0, 0, 2, 1, 4);
+    TestUtils.validateResults(analysisResults, 0, 0, 2, 1, 6);
 });
 
 test('Builtins1', () => {