Don't read the same property twice in pattern matching (#35669)

Don't read the same property twice in pattern matching when accessed through the same underlying object, but references of different types.
Fixes #34933

src/Compilers/CSharp/Portable/BoundTree/BoundDagEvaluation.cs

@@ -9,7 +9,7 @@ partial class BoundDagEvaluation
         public override bool Equals(object obj) => obj is BoundDagEvaluation other && this.Equals(other);
         public virtual bool Equals(BoundDagEvaluation other)
         {
-            return other != (object)null && this.Kind == other.Kind && this.Input.Equals(other.Input) && this.Symbol == other.Symbol;
+            return other != (object)null && this.Kind == other.Kind && this.GetOriginalInput().Equals(other.GetOriginalInput()) && this.Symbol == other.Symbol;
         }
         private Symbol Symbol
         {
@@ -28,8 +28,25 @@ private Symbol Symbol
         }
         public override int GetHashCode()
         {
-            return Hash.Combine(this.Input.GetHashCode(), this.Symbol?.GetHashCode() ?? 0);
+            return Hash.Combine(GetOriginalInput().GetHashCode(), this.Symbol?.GetHashCode() ?? 0);
         }
+
+        /// <summary>
+        /// Returns the original input for this evaluation, stripped of all Type Evaluations.
+        /// 
+        /// A BoundDagTypeEvaluation doesn't change the underlying object being pointed to
+        /// So two evaluations act on the same input so long as they have the same original input.
+        /// </summary>
+        private BoundDagTemp GetOriginalInput()
+        {
+            var input = this.Input;
+            while (input.Source is BoundDagTypeEvaluation source)
+            {
+                input = source.Input;
+            }
+            return input;
+        }
+
         public static bool operator ==(BoundDagEvaluation left, BoundDagEvaluation right)
         {
             return (left is null) ? right is null : left.Equals(right);

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

@@ -3673,6 +3673,15 @@ private TypeWithState GetAdjustedResult(TypeWithState type, int slot)
             return type;
         }
 
+        /// <summary>
+        /// Gets the corresponding member for a symbol from initial binding to match an updated receiver type in NullableWalker.
+        /// For instance, this will map from List&lt;string~&gt;.Add(string~) to List&lt;string?&gt;.Add(string?) in the following example:
+        /// <example>
+        /// string s = null;
+        /// var list = new[] { s }.ToList();
+        /// list.Add(null);
+        /// </example>
+        /// </summary>
         private static Symbol AsMemberOfType(TypeSymbol type, Symbol symbol)
         {
             Debug.Assert((object)symbol != null);