Fix generate method in switch arms.

src/EditorFeatures/CSharpTest/Diagnostics/GenerateMethod/GenerateMethodTests.cs

@@ -9130,5 +9130,81 @@ internal static void Method()
 }
                         ");
         }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateMethod)]
+        public async Task TestInSwitchExpression1()
+        {
+            await TestInRegularAndScriptAsync(
+@"
+using System;
+
+class Class
+{
+    string Method(int i)
+    {
+        return i switch
+        {
+            0 => [|Goo|](),
+        };
+    }
+}",
+@"
+using System;
+
+class Class
+{
+    string Method(int i)
+    {
+        return i switch
+        {
+            0 => Goo(),
+        };
+    }
+
+    private string Goo()
+    {
+        throw new NotImplementedException();
+    }
+}");
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateMethod)]
+        public async Task TestInSwitchExpression2()
+        {
+            await TestInRegularAndScriptAsync(
+@"
+using System;
+
+class Class
+{
+    void Method(int i)
+    {
+        var v = i switch
+        {
+            0 => """",
+            1 => [|Goo|](),
+        };
+    }
+}",
+@"
+using System;
+
+class Class
+{
+    void Method(int i)
+    {
+        var v = i switch
+        {
+            0 => """",
+            1 => Goo(),
+        };
+    }
+
+    private string Goo()
+    {
+        throw new NotImplementedException();
+    }
+}");
+        }
     }
 }

src/Workspaces/SharedUtilitiesAndExtensions/Workspace/CSharp/LanguageServices/CSharpTypeInferenceService.TypeInferrer.cs

@@ -179,6 +179,7 @@ protected override IEnumerable<TypeInferenceInfo> InferTypesWorker_DoNotCallDire
                     RefExpressionSyntax refExpression => InferTypeInRefExpression(refExpression),
                     ReturnStatementSyntax returnStatement => InferTypeForReturnStatement(returnStatement),
                     SubpatternSyntax subpattern => InferTypeInSubpattern(subpattern, node),
+                    SwitchExpressionArmSyntax arm => InferTypeInSwitchExpressionArm(arm),
                     SwitchLabelSyntax switchLabel => InferTypeInSwitchLabel(switchLabel),
                     SwitchStatementSyntax switchStatement => InferTypeInSwitchStatement(switchStatement),
                     ThrowExpressionSyntax throwExpression => InferTypeInThrowExpression(throwExpression),
@@ -2058,6 +2059,29 @@ private ISymbol GetDeclaredMemberSymbolFromOriginalSemanticModel(SyntaxNode decl
                     : null;
             }
 
+            private IEnumerable<TypeInferenceInfo> InferTypeInSwitchExpressionArm(
+                SwitchExpressionArmSyntax arm)
+            {
+                if (arm.Parent is SwitchExpressionSyntax switchExpression)
+                {
+                    // see if we can figure out an appropriate type from a prior arm.
+                    var armIndex = switchExpression.Arms.IndexOf(arm);
+                    if (armIndex > 0)
+                    {
+                        var previousArm = switchExpression.Arms[armIndex - 1];
+                        var priorArmTypes = GetTypes(previousArm.Expression, objectAsDefault: false);
+                        if (priorArmTypes.Any())
+                            return priorArmTypes;
+                    }
+
+                    // if a prior arm gave us nothing useful, or we're the first arm, then try to infer looking at
+                    // what type gets inferred for the switch expression itself.
+                    return InferTypes(switchExpression);
+                }
+
+                return SpecializedCollections.EmptyEnumerable<TypeInferenceInfo>();
+            }
+
             private IEnumerable<TypeInferenceInfo> InferTypeInSwitchLabel(
                 SwitchLabelSyntax switchLabel, SyntaxToken? previousToken = null)
             {