Fix missing error context for unpacking assignment involving star expression (#19258)

Fixes #19257

Author: brianschubert

mypy/checker.py

@@ -3840,7 +3840,7 @@ def check_assignment_to_multiple_lvalues(
                 if rvalue_needed > 0:
                     rvalues = (
                         rvalues[0:iterable_start]
-                        + [TempNode(iterable_type) for i in range(rvalue_needed)]
+                        + [TempNode(iterable_type, context=rval) for _ in range(rvalue_needed)]
                         + rvalues[iterable_end + 1 :]
                     )
 

test-data/unit/check-lists.test

@@ -94,3 +94,12 @@ def foo(x: object) -> None:
         [reveal_type(x) for x in [1, 2, 3]]  # N: Revealed type is "builtins.int"
 
 [builtins fixtures/isinstancelist.pyi]
+
+[case testUnpackAssignmentWithStarExpr]
+a: A
+b: list[B]
+if int():
+    (a,) = [*b]  # E: Incompatible types in assignment (expression has type "B", variable has type "A")
+
+class A: pass
+class B: pass

test-data/unit/check-tuples.test

@@ -618,6 +618,15 @@ u, v, w = r, s = 1, 1 # E: Need more than 2 values to unpack (3 expected)
 d, e = f, g, h = 1, 1 # E: Need more than 2 values to unpack (3 expected)
 [builtins fixtures/tuple.pyi]
 
+[case testUnpackAssignmentWithStarExpr]
+a: A
+b: list[B]
+if int():
+    (a,) = (*b,)  # E: Incompatible types in assignment (expression has type "B", variable has type "A")
+
+class A: pass
+class B: pass
+
 
 -- Assignment to starred expressions
 -- ---------------------------------