From 0d92942a42bf6ec1ed29bfe9a66485737cc208d0 Mon Sep 17 00:00:00 2001
From: Eric Traut <eric@traut.com>
Date: Wed, 11 Sep 2024 14:30:16 -0700
Subject: [PATCH] Fixed bug that results in incorrect type narrowing for
 sequence patterns in the negative (fall-through) case when the subject is a
 tuple with an unbounded entry. This addresses #8967. (#8968)

---
 .../src/analyzer/patternMatching.ts            |  5 ++++-
 .../src/tests/samples/matchSequence1.py        | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/packages/pyright-internal/src/analyzer/patternMatching.ts b/packages/pyright-internal/src/analyzer/patternMatching.ts
index c86f7cd7140e..55ae16160e1c 100644
--- a/packages/pyright-internal/src/analyzer/patternMatching.ts
+++ b/packages/pyright-internal/src/analyzer/patternMatching.ts
@@ -109,6 +109,7 @@ interface SequencePatternInfo {
     entryTypes: Type[];
     isIndeterminateLength?: boolean;
     isTuple?: boolean;
+    isUnboundedTuple?: boolean;
 }
 
 interface MappingPatternInfo {
@@ -225,7 +226,7 @@ function narrowTypeBasedOnSequencePattern(
         // contains indeterminate-length entries or the tuple is of indeterminate
         // length.
         if (!isPositiveTest) {
-            if (entry.isIndeterminateLength) {
+            if (entry.isIndeterminateLength || entry.isUnboundedTuple) {
                 canNarrowTuple = false;
             }
 
@@ -1436,6 +1437,7 @@ function getSequencePatternInfo(
                             entryTypes: isDefiniteNoMatch ? [] : typeArgs.map((t) => t.type),
                             isIndeterminateLength: false,
                             isTuple: true,
+                            isUnboundedTuple: tupleIndeterminateIndex >= 0,
                             isDefiniteNoMatch,
                             isPotentialNoMatch,
                         });
@@ -1488,6 +1490,7 @@ function getSequencePatternInfo(
                                 entryTypes: isDefiniteNoMatch ? [] : typeArgs.map((t) => t.type),
                                 isIndeterminateLength: false,
                                 isTuple: true,
+                                isUnboundedTuple: tupleIndeterminateIndex >= 0,
                                 isDefiniteNoMatch,
                             });
                             return;
diff --git a/packages/pyright-internal/src/tests/samples/matchSequence1.py b/packages/pyright-internal/src/tests/samples/matchSequence1.py
index 13ebdcc2b3fd..6a2d3b57255b 100644
--- a/packages/pyright-internal/src/tests/samples/matchSequence1.py
+++ b/packages/pyright-internal/src/tests/samples/matchSequence1.py
@@ -608,6 +608,24 @@ def test_unbounded_tuple_5(subj: tuple[int, Unpack[tuple[str, ...]]]):
             reveal_type(x, expected_text="Never")
 
 
+def test_unbounded_tuple_6(subj: tuple[str, ...]):
+    match subj:
+        case ("a", b, _, _):
+            reveal_type(b, expected_text="str")
+
+        case ("a", b, _, _, _):
+            reveal_type(b, expected_text="str")
+
+        case (_, b, _, _):
+            reveal_type(b, expected_text="str")
+
+        case (_, b, _, _, _):
+            reveal_type(b, expected_text="str")
+
+        case r:
+            reveal_type(r, expected_text="tuple[str, ...]")
+
+
 def test_variadic_tuple(subj: tuple[int, Unpack[Ts]]) -> tuple[Unpack[Ts]]:
     match subj:
         case _, *rest: