EPC20: don't warn when type defines implicit conversion to string (#317)

src/ErrorProne.NET.CoreAnalyzers.Tests/CoreAnalyzers/DefaultToStringConversionAnalyzerTests.cs

@@ -268,6 +268,40 @@ class Test {
     void T(Task t) => Foo(t);
     void Foo(object o) {}
 }
+";
+            await Verify.VerifyAsync(test);
+        }
+
+        [Test]
+        public async Task NoWarn_When_Type_Has_Implicit_Conversion_To_String()
+        {
+            // Issue #317: a type that defines 'public static implicit operator string'
+            // opts into its own string representation, so EPC20 should not fire.
+            var test = @"
+public class AmazonFile
+{
+    public static implicit operator string(AmazonFile f) => ""x"";
+}
+class Test {
+    string Concat(AmazonFile f) => ""x"" + f;
+    string Interp(AmazonFile f) => $""{f}"";
+    string Format(AmazonFile f) => string.Format(""{0}"", f);
+}
+";
+            await Verify.VerifyAsync(test);
+        }
+
+        [Test]
+        public async Task Warn_When_Implicit_Conversion_Returns_Something_Other_Than_String()
+        {
+            var test = @"
+public class AmazonFile
+{
+    public static implicit operator int(AmazonFile f) => 0;
+}
+class Test {
+    string Concat(AmazonFile f) => ""x"" + [|f|];
+}
 ";
             await Verify.VerifyAsync(test);
         }

src/ErrorProne.NET.CoreAnalyzers/CoreAnalyzers/DefaultToStringImplementationUsageAnalyzer.cs

@@ -50,6 +50,13 @@ protected override bool TryCreateDiagnostic(Compilation compilation, ITypeSymbol
                 return false;
             }
 
+            if (HasImplicitConversionToString(type))
+            {
+                // The type opts into a custom string representation via 'public static implicit operator string',
+                // so a default ToString impl is not what will actually be used. See issue #317.
+                return false;
+            }
+
             if (NoToStringOverride(type, out var typeWithNoToString))
             {
                 diagnostic = Diagnostic.Create(Rule, location, typeWithNoToString);
@@ -58,6 +65,20 @@ protected override bool TryCreateDiagnostic(Compilation compilation, ITypeSymbol
             return diagnostic != null;
         }
 
+        private static bool HasImplicitConversionToString(ITypeSymbol type)
+        {
+            foreach (var member in type.GetMembers(WellKnownMemberNames.ImplicitConversionName))
+            {
+                if (member is IMethodSymbol { MethodKind: MethodKind.Conversion } method &&
+                    method.ReturnType.SpecialType == SpecialType.System_String)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         private static bool NoToStringOverride(ITypeSymbol type, [NotNullWhen(true)]out ITypeSymbol? typeWithNoToString)
         {
             typeWithNoToString = null;