Extensions: improve diagnostics for operators

src/Compilers/CSharp/Portable/Binder/Binder.OperatorResolutionForReporting.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Symbols;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.PooledObjects;
+
+namespace Microsoft.CodeAnalysis.CSharp;
+
+internal partial class Binder
+{
+    /// <summary>
+    /// This type collects different kinds of results from operator scenarios and provides a unified way to report diagnostics.
+    /// It collects the first non-empty result for extensions and non-extensions separately.
+    /// This follows a similar logic to ResolveMethodGroupInternal and OverloadResolutionResult.ReportDiagnostics
+    /// </summary>
+    private struct OperatorResolutionForReporting
+    {
+        private object? _nonExtensionResult;
+        private object? _extensionResult;
+
+        [Conditional("DEBUG")]
+        private void AssertInvariant()
+        {
+            Debug.Assert(_nonExtensionResult is null or OverloadResolutionResult<MethodSymbol> or BinaryOperatorOverloadResolutionResult or UnaryOperatorOverloadResolutionResult);
+            Debug.Assert(_extensionResult is null or OverloadResolutionResult<MethodSymbol> or BinaryOperatorOverloadResolutionResult or UnaryOperatorOverloadResolutionResult);
+        }
+
+        /// <returns>Returns true if the result was set and <see cref="OperatorResolutionForReporting"/> took ownership of the result.</returns>
+        private bool SaveResult(object result, ref object? savedResult)
+        {
+            if (savedResult is null)
+            {
+                savedResult = result;
+                AssertInvariant();
+                return true;
+            }
+
+            return false;
+        }
+
+        /// <returns>Returns true if the result was set and <see cref="OperatorResolutionForReporting"/> took ownership of the result.</returns>
+        public bool SaveResult(OverloadResolutionResult<MethodSymbol> result, bool isExtension)
+        {
+            if (result.ResultsBuilder.IsEmpty)
+            {
+                return false;
+            }
+
+            return SaveResult(result, ref isExtension ? ref _extensionResult : ref _nonExtensionResult);
+        }
+
+        /// <returns>Returns true if the result was set and <see cref="OperatorResolutionForReporting"/> took ownership of the result.</returns>
+        public bool SaveResult(BinaryOperatorOverloadResolutionResult result, bool isExtension)
+        {
+            if (result.Results.IsEmpty)
+            {
+                return false;
+            }
+
+            return SaveResult(result, ref isExtension ? ref _extensionResult : ref _nonExtensionResult);
+        }
+
+        /// <returns>Returns true if the result was set and <see cref="OperatorResolutionForReporting"/> took ownership of the result.</returns>
+        public bool SaveResult(UnaryOperatorOverloadResolutionResult result, bool isExtension)
+        {
+            if (result.Results.IsEmpty)
+            {
+                return false;
+            }
+
+            return SaveResult(result, ref isExtension ? ref _extensionResult : ref _nonExtensionResult);
+        }
+
+        /// <summary>
+        /// Follows a very simplified version of OverloadResolutionResult.ReportDiagnostics which can be expanded in the future if needed.
+        /// </summary>
+        internal bool TryReportDiagnostics(SyntaxNode node, Binder binder, object leftDisplay, object? rightDisplay, BindingDiagnosticBag diagnostics)
+        {
+            object? resultToUse = pickResultToUse(_nonExtensionResult, _extensionResult);
+            if (resultToUse is null)
+            {
+                return false;
+            }
+
+            var results = ArrayBuilder<(MethodSymbol?, OperatorAnalysisResultKind)>.GetInstance();
+            populateResults(results, resultToUse);
+
+            bool reported = tryReportDiagnostics(node, binder, results, leftDisplay, rightDisplay, diagnostics);
+            results.Free();
+
+            return reported;
+
+            static bool tryReportDiagnostics(
+                SyntaxNode node,
+                Binder binder,
+                ArrayBuilder<(MethodSymbol? member, OperatorAnalysisResultKind resultKind)> results,
+                object leftDisplay,
+                object? rightDisplay,
+                BindingDiagnosticBag diagnostics)
+            {
+                assertNone(results, OperatorAnalysisResultKind.Undefined);
+
+                if (hadAmbiguousBestMethods(results, node, binder, diagnostics))
+                {
+                    return true;
+                }
+
+                if (results.Any(m => m.resultKind == OperatorAnalysisResultKind.Applicable))
+                {
+                    return false;
+                }
+
+                assertNone(results, OperatorAnalysisResultKind.Applicable);
+
+                if (results.Any(m => m.resultKind == OperatorAnalysisResultKind.Worse))
+                {
+                    return false;
+                }
+
+                assertNone(results, OperatorAnalysisResultKind.Worse);
+
+                Debug.Assert(results.All(r => r.resultKind == OperatorAnalysisResultKind.Inapplicable));
+
+                // There is much room to improve diagnostics on inapplicable candidates, but for now we just report the candidate if there is a single one.
+                if (results.Count == 1 && results[0].member is { } inapplicableMember)
+                {
+                    var toReport = nodeToReport(node);
+                    if (rightDisplay is null)
+                    {
+                        // error: Operator cannot be applied to operand of type '{0}'. The closest inapplicable candidate is '{1}'
+                        Error(diagnostics, ErrorCode.ERR_SingleInapplicableUnaryOperator, toReport, leftDisplay, inapplicableMember);
+                    }
+                    else
+                    {
+                        // error: Operator cannot be applied to operands of type '{0}' and '{1}'. The closest inapplicable candidate is '{2}'
+                        Error(diagnostics, ErrorCode.ERR_SingleInapplicableBinaryOperator, toReport, leftDisplay, rightDisplay, inapplicableMember);
+                    }
+
+                    return true;
+                }
+
+                return false;
+            }
+
+            static object? pickResultToUse(object? nonExtensionResult, object? extensionResult)
+            {
+                if (nonExtensionResult is null)
+                {
+                    return extensionResult;
+                }
+
+                if (extensionResult is null)
+                {
+                    return nonExtensionResult;
+                }
+
+                bool useNonExtension = getBestKind(nonExtensionResult) >= getBestKind(extensionResult);
+                return useNonExtension ? nonExtensionResult : extensionResult;
+            }
+
+            static OperatorAnalysisResultKind getBestKind(object result)
+            {
+                OperatorAnalysisResultKind bestKind = OperatorAnalysisResultKind.Undefined;
+
+                switch (result)
+                {
+                    case OverloadResolutionResult<MethodSymbol> r1:
+                        foreach (var res in r1.ResultsBuilder)
+                        {
+                            var kind = mapKind(res.Result.Kind);
+                            if (kind > bestKind)
+                            {
+                                bestKind = kind;
+                            }
+                        }
+                        break;
+
+                    case BinaryOperatorOverloadResolutionResult r2:
+                        foreach (var res in r2.Results)
+                        {
+                            if (res.Signature.Method is null)
+                            {
+                                // Skip built-in operators
+                                continue;
+                            }
+
+                            if (res.Kind > bestKind)
+                            {
+                                bestKind = res.Kind;
+                            }
+                        }
+                        break;
+
+                    case UnaryOperatorOverloadResolutionResult r3:
+                        foreach (var res in r3.Results)
+                        {
+                            if (res.Signature.Method is null)
+                            {
+                                // Skip built-in operators
+                                continue;
+                            }
+
+                            if (res.Kind > bestKind)
+                            {
+                                bestKind = res.Kind;
+                            }
+                        }
+                        break;
+
+                    default:
+                        throw ExceptionUtilities.UnexpectedValue(result);
+                }
+
+                return bestKind;
+            }
+
+            static bool hadAmbiguousBestMethods(ArrayBuilder<(MethodSymbol?, OperatorAnalysisResultKind)> results, SyntaxNode node, Binder binder, BindingDiagnosticBag diagnostics)
+            {
+                if (!tryGetTwoBest(results, out var first, out var second))
+                {
+                    return false;
+                }
+
+                Error(diagnostics, ErrorCode.ERR_AmbigOperator, nodeToReport(node), first, second);
+                return true;
+            }
+
+            static SyntaxNodeOrToken nodeToReport(SyntaxNode node)
+            {
+                return node switch
+                {
+                    AssignmentExpressionSyntax assignment => assignment.OperatorToken,
+                    BinaryExpressionSyntax binary => binary.OperatorToken,
+                    PrefixUnaryExpressionSyntax prefix => prefix.OperatorToken,
+                    PostfixUnaryExpressionSyntax postfix => postfix.OperatorToken,
+                    _ => node
+                };
+            }
+
+            [Conditional("DEBUG")]
+            static void assertNone(ArrayBuilder<(MethodSymbol? member, OperatorAnalysisResultKind resultKind)> results, OperatorAnalysisResultKind kind)
+            {
+                Debug.Assert(results.All(r => r.resultKind != kind));
+            }
+
+            static bool tryGetTwoBest(ArrayBuilder<(MethodSymbol?, OperatorAnalysisResultKind)> results, [NotNullWhen(true)] out MethodSymbol? first, [NotNullWhen(true)] out MethodSymbol? second)
+            {
+                first = null;
+                second = null;
+                bool foundFirst = false;
+
+                foreach (var (member, resultKind) in results)
+                {
+                    if (member is null)
+                    {
+                        continue;
+                    }
+
+                    if (resultKind == OperatorAnalysisResultKind.Applicable)
+                    {
+                        if (!foundFirst)
+                        {
+                            first = member;
+                            foundFirst = true;
+                        }
+                        else
+                        {
+                            Debug.Assert(first is not null);
+                            second = member;
+                            return true;
+                        }
+                    }
+                }
+
+                return false;
+            }
+
+            static void populateResults(ArrayBuilder<(MethodSymbol?, OperatorAnalysisResultKind)> results, object? result)
+            {
+                switch (result)
+                {
+                    case OverloadResolutionResult<MethodSymbol> result1:
+                        foreach (var res in result1.ResultsBuilder)
+                        {
+                            OperatorAnalysisResultKind kind = mapKind(res.Result.Kind);
+
+                            results.Add((res.Member, kind));
+                        }
+                        break;
+
+                    case BinaryOperatorOverloadResolutionResult result2:
+                        foreach (var res in result2.Results)
+                        {
+                            results.Add((res.Signature.Method, res.Kind));
+                        }
+                        break;
+
+                    case UnaryOperatorOverloadResolutionResult result3:
+                        foreach (var res in result3.Results)
+                        {
+                            results.Add((res.Signature.Method, res.Kind));
+                        }
+                        break;
+
+                    default:
+                        throw ExceptionUtilities.UnexpectedValue(result);
+                }
+            }
+
+            static OperatorAnalysisResultKind mapKind(MemberResolutionKind kind)
+            {
+                return kind switch
+                {
+                    MemberResolutionKind.ApplicableInExpandedForm => OperatorAnalysisResultKind.Applicable,
+                    MemberResolutionKind.ApplicableInNormalForm => OperatorAnalysisResultKind.Applicable,
+                    MemberResolutionKind.Worse => OperatorAnalysisResultKind.Worse,
+                    MemberResolutionKind.Worst => OperatorAnalysisResultKind.Worse,
+                    _ => OperatorAnalysisResultKind.Inapplicable,
+                };
+            }
+        }
+
+        internal void Free()
+        {
+            free(ref _nonExtensionResult);
+            free(ref _extensionResult);
+
+            static void free(ref object? result)
+            {
+                switch (result)
+                {
+                    case null:
+                        return;
+                    case OverloadResolutionResult<MethodSymbol> result1:
+                        result1.Free();
+                        break;
+                    case BinaryOperatorOverloadResolutionResult result2:
+                        result2.Free();
+                        break;
+                    case UnaryOperatorOverloadResolutionResult result3:
+                        result3.Free();
+                        break;
+                }
+
+                result = null;
+            }
+        }
+    }
+}