Fix TakeIncrementalSnapshot crash (#38752)

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -5786,6 +5786,9 @@ public override BoundNode VisitAssignmentOperator(BoundAssignmentOperator node)
             var left = node.Left;
             var right = node.Right;
             VisitLValue(left);
+            // we may enter a conditional state for error scenarios on the LHS.
+            Unsplit();
+
             FlowAnalysisAnnotations leftAnnotations = GetLValueAnnotations(left);
             TypeWithAnnotations declaredType = LvalueResultType;
             TypeWithAnnotations leftLValueType = ApplyLValueAnnotations(declaredType, leftAnnotations);

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -104672,6 +104672,66 @@ public class Broken<T> : IEnumerable<IEquatable<T>> where T : class
             comp.VerifyDiagnostics();
         }
 
+        [Fact, WorkItem(37868, "https://github.com/dotnet/roslyn/issues/37868")]
+        public void IsPatternVariableDeclaration_LeftOfAssignmentOperator()
+        {
+            var source = @"
+using System;
+
+class C
+{
+    void Test1()
+    {
+        if (unknown is string b = )
+        {
+            Console.WriteLine(b);
+        }
+    }
+
+    void Test2(bool a)
+    {
+        if (a is bool (b = a))
+        {
+            Console.WriteLine(b);
+        }
+    }
+}
+";
+            var comp = CreateNullableCompilation(source);
+            comp.VerifyDiagnostics(
+                // (8,13): error CS0103: The name 'unknown' does not exist in the current context
+                //         if (unknown is string b = )
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "unknown").WithArguments("unknown").WithLocation(8, 13),
+                // (8,35): error CS1525: Invalid expression term ')'
+                //         if (unknown is string b = )
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, ")").WithArguments(")").WithLocation(8, 35),
+                // (10,31): error CS0165: Use of unassigned local variable 'b'
+                //             Console.WriteLine(b);
+                Diagnostic(ErrorCode.ERR_UseDefViolation, "b").WithArguments("b").WithLocation(10, 31),
+                // (16,23): error CS8129: No suitable 'Deconstruct' instance or extension method was found for type 'bool', with 1 out parameters and a void return type.
+                //         if (a is bool (b = a))
+                Diagnostic(ErrorCode.ERR_MissingDeconstruct, "(b ").WithArguments("bool", "1").WithLocation(16, 23),
+                // (16,24): error CS0103: The name 'b' does not exist in the current context
+                //         if (a is bool (b = a))
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "b").WithArguments("b").WithLocation(16, 24),
+                // (16,26): error CS1026: ) expected
+                //         if (a is bool (b = a))
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "=").WithLocation(16, 26),
+                // (16,30): error CS1525: Invalid expression term ')'
+                //         if (a is bool (b = a))
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, ")").WithArguments(")").WithLocation(16, 30),
+                // (16,30): error CS1002: ; expected
+                //         if (a is bool (b = a))
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, ")").WithLocation(16, 30),
+                // (16,30): error CS1513: } expected
+                //         if (a is bool (b = a))
+                Diagnostic(ErrorCode.ERR_RbraceExpected, ")").WithLocation(16, 30),
+                // (18,31): error CS0103: The name 'b' does not exist in the current context
+                //             Console.WriteLine(b);
+                Diagnostic(ErrorCode.ERR_NameNotInContext, "b").WithArguments("b").WithLocation(18, 31));
+        }
+
+
         [WorkItem(30140, "https://github.com/dotnet/roslyn/issues/30140")]
         [Fact]
         public void NullCoalescingAssignment_UseInExpression()