An input requires analysis using a different slot when suppressed (#39697)

Fixes #38586

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

@@ -142,7 +142,7 @@ public override BoundNode VisitITuplePattern(BoundITuplePattern node)
         protected override LocalState VisitSwitchStatementDispatch(BoundSwitchStatement node)
         {
             // first, learn from any null tests in the patterns
-            int slot = MakeSlot(node.Expression);
+            int slot = node.Expression.IsSuppressed ? GetOrCreatePlaceholderSlot(node.Expression) : MakeSlot(node.Expression);
             if (slot > 0)
             {
                 var originalInputType = node.Expression.Type;
@@ -493,7 +493,7 @@ public override BoundNode VisitUnconvertedSwitchExpression(BoundUnconvertedSwitc
         private void VisitSwitchExpressionCore(BoundSwitchExpression node, bool inferType)
         {
             // first, learn from any null tests in the patterns
-            int slot = MakeSlot(node.Expression);
+            int slot = node.Expression.IsSuppressed ? GetOrCreatePlaceholderSlot(node.Expression) : MakeSlot(node.Expression);
             if (slot > 0)
             {
                 var originalInputType = node.Expression.Type;

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

@@ -120001,5 +120001,63 @@ public static void M()
                 Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "s4.field").WithLocation(21, 9)
                 );
         }
+
+        [Fact]
+        [WorkItem(38586, "https://github.com/dotnet/roslyn/issues/38586")]
+        public void SuppressSwitchExpressionInput()
+        {
+            var source =
+@"#nullable enable
+public class C {
+    public int M0(C a)  => a switch { C _ => 0 }; // ok
+    public int M1(C? a) => a switch { C _ => 0 }; // warns
+    public int M2(C? a) => a! switch { C _ => 0 }; // ok
+
+    public int M3(C a)  => (1, a) switch { (_, C _) => 0 }; // ok
+    public int M4(C? a) => (1, a) switch { (_, C _) => 0 }; // warns
+    public int M5(C? a) => (1, a!) switch { (_, C _) => 0 }; // ok
+
+    public void M6(C? a, bool b)
+    {
+        if (a == null) return;
+        switch (a!)
+        {
+            case C _:
+                break;
+            case null: // does not affect knowledge of 'a'
+                break;
+        }
+
+        a.ToString();
+    }
+
+    public void M7(C? a, bool b)
+    {
+        if (a == null) return;
+        switch (a)
+        {
+            case C _:
+                break;
+            case null: // affects knowledge of a
+                break;
+        }
+
+        a.ToString(); // warns
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (4,30): warning CS8655: The switch expression does not handle some null inputs (it is not exhaustive).
+                //     public int M1(C? a) => a switch { C _ => 0 }; // warns
+                Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveForNull, "switch").WithLocation(4, 30),
+                // (8,35): warning CS8655: The switch expression does not handle some null inputs (it is not exhaustive).
+                //     public int M4(C? a) => (1, a) switch { (_, C _) => 0 }; // warns
+                Diagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveForNull, "switch").WithLocation(8, 35),
+                // (36,9): warning CS8602: Dereference of a possibly null reference.
+                //         a.ToString(); // warns
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "a").WithLocation(36, 9)
+                );
+        }
     }
 }