ILSpy/ICSharpCode.Decompiler/CSharp/OutputVisitor/GenericGrammarAmbiguityVisitor.cs

// Copyright (c) 2020 Daniel Grunwald
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
// to whom the Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
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using System;
using System.Diagnostics;
using System.Linq;

using ICSharpCode.Decompiler.CSharp.Syntax;

namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
{
	/// <summary>
	/// Used to test for the "F(G&lt;A,B&gt;(7));" grammar ambiguity.
	/// </summary>
	class GenericGrammarAmbiguityVisitor : DepthFirstAstVisitor<bool>
	{
		/// <summary>
		/// Resolves ambiguities in the specified syntax tree.
		/// This method must be called after the InsertParenthesesVisitor, because the ambiguity depends on whether the
		/// final `>` in the possible-type-argument is followed by an opening parenthesis.
		/// </summary>
		public static void ResolveAmbiguities(AstNode rootNode)
		{
			foreach (var node in rootNode.Descendants.OfType<BinaryOperatorExpression>())
			{
				if (CausesAmbiguityWithGenerics(node))
				{
					node.ReplaceWith(n => new ParenthesizedExpression(n));
				}
			}
		}

		public static bool CausesAmbiguityWithGenerics(BinaryOperatorExpression binaryOperatorExpression)
		{
			if (binaryOperatorExpression.Operator != BinaryOperatorType.LessThan)
				return false;

			var v = new GenericGrammarAmbiguityVisitor();
			v.genericNestingLevel = 1;

			for (AstNode node = binaryOperatorExpression.Right; node != null; node = node.GetNextNode())
			{
				if (node.AcceptVisitor(v))
					return v.ambiguityFound;
			}
			return false;
		}

		int genericNestingLevel;
		bool ambiguityFound;

		protected override bool VisitChildren(AstNode node)
		{
			// unhandled node: probably not syntactically valid in a typename

			// These are preconditions for all recursive Visit() calls.
			Debug.Assert(genericNestingLevel > 0);
			Debug.Assert(!ambiguityFound);

			// The return value merely indicates whether to stop visiting.
			return true; // stop visiting, no ambiguity found
		}

		public override bool VisitBinaryOperatorExpression(BinaryOperatorExpression binaryOperatorExpression)
		{
			if (binaryOperatorExpression.Left.AcceptVisitor(this))
				return true;
			Debug.Assert(genericNestingLevel > 0);
			switch (binaryOperatorExpression.Operator)
			{
				case BinaryOperatorType.LessThan:
					genericNestingLevel += 1;
					break;
				case BinaryOperatorType.GreaterThan:
					genericNestingLevel--;
					break;
				case BinaryOperatorType.ShiftRight when genericNestingLevel >= 2:
					genericNestingLevel -= 2;
					break;
				case BinaryOperatorType.UnsignedShiftRight when genericNestingLevel >= 3:
					genericNestingLevel -= 3;
					break;
				default:
					return true; // stop visiting, no ambiguity found
			}
			if (genericNestingLevel == 0)
			{
				// Of the all tokens that might follow `>` and trigger the ambiguity to be resolved in favor of generics,
				// `(` is the only one that might start an expression.
				ambiguityFound = binaryOperatorExpression.Right is ParenthesizedExpression;
				return true; // stop visiting
			}
			return binaryOperatorExpression.Right.AcceptVisitor(this);
		}

		public override bool VisitIdentifierExpression(IdentifierExpression identifierExpression)
		{
			// identifier could also be valid in a type argument
			return false; // keep visiting
		}

		public override bool VisitTypeReferenceExpression(TypeReferenceExpression typeReferenceExpression)
		{
			return false; // keep visiting
		}

		public override bool VisitMemberReferenceExpression(MemberReferenceExpression memberReferenceExpression)
		{
			// MRE could also be valid in a type argument
			return memberReferenceExpression.Target.AcceptVisitor(this);
		}

	}
}