From 03e7af9588442925ecdf6e166be50b71487765ad Mon Sep 17 00:00:00 2001
From: Jan Jones <janjones@microsoft.com>
Date: Tue, 1 Sep 2026 10:06:39 +0200
Subject: [PATCH 1/2] Visit left operand conversion in a fast path of nullable
 analysis

---
 .../Portable/FlowAnalysis/NullableWalker.cs     | 17 +++++++++++++++++
 .../Semantics/NullableReferenceTypesTests.cs    | 16 ++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
index ed81df1a5cee2..4cd27376935f2 100644
--- a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
+++ b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
@@ -5565,6 +5565,23 @@ bool learnFromConditionalAccessOrBoolConstant()
                 else if (binary.Left.ConstantValueOpt is { IsBoolean: true } leftConstant)
                 {
                     Unsplit();
+
+                    // The left operand was visited without its stripped conversion.
+                    if (binary.Left is BoundConversion leftConversionNode && leftConversionNode != leftOperand)
+                    {
+                        Debug.Assert(leftConversionNode.Type is not null);
+                        VisitConversion(
+                            leftConversionNode,
+                            leftOperand,
+                            leftConversion,
+                            TypeWithAnnotations.Create(leftConversionNode.Type),
+                            leftResult,
+                            checkConversion: true,
+                            fromExplicitCast: false,
+                            useLegacyWarnings: false,
+                            AssignmentKind.Argument);
+                    }
+
                     Visit(binary.Right);
                     UseRvalueOnly(binary.Right);
                     if (IsConditionalState && isEquals(binary) != leftConstant.BooleanValue)
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
index 96cf7f4132270..9793e26f4dc81 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
@@ -51746,6 +51746,22 @@ public void M4(C? c, string? x)
                 );
         }
 
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/84642")]
+        public void EqualsBoolConstant_DefaultLiteral()
+        {
+            var source = """
+                class C
+                {
+                    static bool M()
+                    {
+                        return default != true;
+                    }
+                }
+                """;
+
+            CreateNullableCompilation(source).VerifyEmitDiagnostics();
+        }
+
         [Fact]
         public void EqualsBoolConstant_UserDefinedOperator_BoolRight()
         {

From f21e1e7e73093f9846d5e2ddd625f4da809efa46 Mon Sep 17 00:00:00 2001
From: Jan Jones <janjones@microsoft.com>
Date: Thu, 3 Sep 2026 09:36:45 +0200
Subject: [PATCH 2/2] Remove unnecessary assert

---
 src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
index 4cd27376935f2..ec15414de3d2d 100644
--- a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
+++ b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
@@ -5569,7 +5569,6 @@ bool learnFromConditionalAccessOrBoolConstant()
                     // The left operand was visited without its stripped conversion.
                     if (binary.Left is BoundConversion leftConversionNode && leftConversionNode != leftOperand)
                     {
-                        Debug.Assert(leftConversionNode.Type is not null);
                         VisitConversion(
                             leftConversionNode,
                             leftOperand,