Infer Exception type when typing throw expression (#19828)

src/EditorFeatures/Test2/IntelliSense/CSharpCompletionCommandHandlerTests.vb

@@ -536,6 +536,44 @@ class Variable
             End Using
         End Function
 
+        <WpfFact, Trait(Traits.Feature, Traits.Features.Completion)>
+        <WorkItem(17256, "https://github.com/dotnet/roslyn/issues/17256")>
+        Public Async Function TestThrowExpression() As Task
+            Using state = TestState.CreateCSharpTestState(
+                  <Document><![CDATA[
+using System;
+class C
+{
+    public object Foo()
+    {
+        return null ?? throw new$$
+    }
+}]]></Document>)
+
+                state.SendTypeChars(" ")
+                Await state.AssertSelectedCompletionItem(displayText:="Exception", isHardSelected:=True)
+            End Using
+        End Function
+
+        <WpfFact, Trait(Traits.Feature, Traits.Features.Completion)>
+        <WorkItem(17256, "https://github.com/dotnet/roslyn/issues/17256")>
+        Public Async Function TestThrowStatement() As Task
+            Using state = TestState.CreateCSharpTestState(
+                  <Document><![CDATA[
+using System;
+class C
+{
+    public object Foo()
+    {
+        throw new$$
+    }
+}]]></Document>)
+
+                state.SendTypeChars(" ")
+                Await state.AssertSelectedCompletionItem(displayText:="Exception", isHardSelected:=True)
+            End Using
+        End Function
+
         <WpfFact, Trait(Traits.Feature, Traits.Features.Completion)>
         <WorkItem(13527, "https://github.com/dotnet/roslyn/issues/13527")>
         Public Async Function TestSymbolInTupleLiteral() As Task
@@ -3338,4 +3376,4 @@ class C
             End Function
         End Class
     End Class
-End Namespace
\ No newline at end of file
+End Namespace

src/Workspaces/CSharp/Portable/LanguageServices/CSharpTypeInferenceService.TypeInferrer.cs

@@ -163,6 +163,7 @@ private IEnumerable<TypeInferenceInfo> GetTypesSimple(ExpressionSyntax expressio
                     case SimpleLambdaExpressionSyntax simpleLambdaExpression: return InferTypeInSimpleLambdaExpression(simpleLambdaExpression);
                     case SwitchLabelSyntax switchLabel: return InferTypeInSwitchLabel(switchLabel);
                     case SwitchStatementSyntax switchStatement: return InferTypeInSwitchStatement(switchStatement);
+                    case ThrowExpressionSyntax throwExpression: return InferTypeInThrowExpression(throwExpression);
                     case ThrowStatementSyntax throwStatement: return InferTypeInThrowStatement(throwStatement);
                     case UsingStatementSyntax usingStatement: return InferTypeInUsingStatement(usingStatement);
                     case WhileStatementSyntax whileStatement: return InferTypeInWhileStatement(whileStatement);
@@ -1971,6 +1972,17 @@ private ISymbol GetDeclaredMemberSymbolFromOriginalSemanticModel(SemanticModel c
                 return SpecializedCollections.SingletonEnumerable(new TypeInferenceInfo(this.Compilation.GetSpecialType(SpecialType.System_Int32)));
             }
 
+            private IEnumerable<TypeInferenceInfo> InferTypeInThrowExpression(ThrowExpressionSyntax throwExpression, SyntaxToken? previousToken = null)
+            {
+                // If we have a position, it has to be after the 'throw' keyword.
+                if (previousToken.HasValue && previousToken.Value != throwExpression.ThrowKeyword)
+                {
+                    return SpecializedCollections.EmptyEnumerable<TypeInferenceInfo>();
+                }
+
+                return SpecializedCollections.SingletonEnumerable(new TypeInferenceInfo(this.Compilation.ExceptionType()));
+            }
+
             private IEnumerable<TypeInferenceInfo> InferTypeInThrowStatement(ThrowStatementSyntax throwStatement, SyntaxToken? previousToken = null)
             {
                 // If we have a position, it has to be after the 'throw' keyword.