Merge pull request #16605 from AlekseyTs/Issue16478

Restore detection of an ambiguous method type argument inference.

src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/MethodTypeInference.cs

@@ -2501,6 +2501,11 @@ private bool Fix(int iParam, ref HashSet<DiagnosticInfo> useSiteDiagnostics)
 
                     best = MergeTupleNames(MergeDynamic(best, candidate, _conversions.CorLibrary), candidate);
                 }
+                else
+                {
+                    // best candidate is not unique
+                    return false;
+                }
 
                 OuterBreak:
                 ;

src/Compilers/CSharp/Test/Semantic/Semantics/OverloadResolutionTests.cs

@@ -9068,5 +9068,100 @@ public enum Something
             // should be NO errors.
             CompileAndVerify(source, expectedOutput: @"False");
         }
+
+        [Fact, WorkItem(16478, "https://github.com/dotnet/roslyn/issues/16478")]
+        public void AmbiguousInference_01()
+        {
+            string source =
+@"
+using System;
+using System.Collections.Generic;
+
+public class Test
+{
+    public static void Assert<T>(T a, T b)
+    {
+        Console.WriteLine(""Non collection"");
+    }
+
+    public static void Assert<T>(IEnumerable<T> a, IEnumerable<T> b)
+    {
+        Console.WriteLine(""Collection"");
+    }
+    
+    public static void Main()
+    {
+        string[] a = new[] { ""A"" };
+        StringValues b = new StringValues();
+
+        Assert(a, b);
+        Assert(b, a);
+    }
+    
+    private class StringValues : List<string>
+    {
+        public static implicit operator StringValues(string[] values)
+        {
+            return new StringValues();
+        }
+        
+        public static implicit operator string[] (StringValues value)
+        {
+            return new string[0];
+        }
+    }
+}";
+            CompileAndVerify(source, expectedOutput:
+@"Collection
+Collection");
+        }
+
+        [Fact, WorkItem(16478, "https://github.com/dotnet/roslyn/issues/16478")]
+        public void AmbiguousInference_02()
+        {
+            string source =
+@"
+using System;
+using System.Collections.Generic;
+
+public class Test
+{
+    public static void Assert<T>(T a, T b)
+    {
+        Console.WriteLine(""Non collection"");
+    }
+    
+    public static void Main()
+    {
+        string[] a = new[] { ""A"" };
+        StringValues b = new StringValues();
+
+        Assert(a, b);
+        Assert(b, a);
+    }
+    
+    private class StringValues : List<string>
+    {
+        public static implicit operator StringValues(string[] values)
+        {
+            return new StringValues();
+        }
+        
+        public static implicit operator string[] (StringValues value)
+        {
+            return new string[0];
+        }
+    }
+}";
+            var comp = CreateCompilationWithMscorlibAndSystemCore(source);
+            comp.VerifyDiagnostics(
+                // (17,9): error CS0411: The type arguments for method 'Test.Assert<T>(T, T)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
+                //         Assert(a, b);
+                Diagnostic(ErrorCode.ERR_CantInferMethTypeArgs, "Assert").WithArguments("Test.Assert<T>(T, T)").WithLocation(17, 9),
+                // (18,9): error CS0411: The type arguments for method 'Test.Assert<T>(T, T)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
+                //         Assert(b, a);
+                Diagnostic(ErrorCode.ERR_CantInferMethTypeArgs, "Assert").WithArguments("Test.Assert<T>(T, T)").WithLocation(18, 9)
+                );
+        }
     }
 }

src/Compilers/VisualBasic/Test/Semantic/Semantics/OverloadResolution.vb

@@ -5775,5 +5775,103 @@ BC30057: Too many arguments to 'Public Sub M1()'.
 </expected>)
         End Sub
 
+        <Fact>
+        <WorkItem(16478, "https://github.com/dotnet/roslyn/issues/16478")>
+        Public Sub AmbiguousInference_01()
+
+            Dim compilationDef =
+<compilation>
+    <file name="a.vb">
+Imports System
+Imports System.Collections.Generic
+
+Public Class Test
+    Public Shared Sub Assert(Of T)(a As T, b As T)
+        Console.WriteLine("Non collection")
+    End Sub
+
+    Public Shared Sub Assert(Of T)(a As IEnumerable(Of T), b As IEnumerable(Of T))
+        Console.WriteLine("Collection")
+    End Sub
+
+    Public Shared Sub Main()
+        Dim a = {"A"}
+        Dim b = New StringValues()
+
+        Assert(a, b)
+        Assert(b, a)
+    End Sub
+
+    Private Class StringValues
+        Inherits List(Of String)
+        Public Shared Widening Operator CType(values As String()) As StringValues
+            Return New StringValues()
+        End Operator
+
+        Public Shared Widening Operator CType(value As StringValues) As String()
+            Return {}
+        End Operator
+    End Class
+End Class
+    </file>
+</compilation>
+
+            Dim compilation = CompilationUtils.CreateCompilationWithMscorlibAndVBRuntime(compilationDef)
+
+            CompileAndVerify(compilationDef, expectedOutput:=
+"Collection
+Collection")
+        End Sub
+
+        <Fact>
+        <WorkItem(16478, "https://github.com/dotnet/roslyn/issues/16478")>
+        Public Sub AmbiguousInference_02()
+
+            Dim compilationDef =
+<compilation>
+    <file name="a.vb">
+Imports System
+Imports System.Collections.Generic
+
+Public Class Test
+    Public Shared Sub Assert(Of T)(a As T, b As T)
+        Console.WriteLine("Non collection")
+    End Sub
+
+    Public Shared Sub Main()
+        Dim a = {"A"}
+        Dim b = New StringValues()
+
+        Assert(a, b)
+        Assert(b, a)
+    End Sub
+
+    Private Class StringValues
+        Inherits List(Of String)
+        Public Shared Widening Operator CType(values As String()) As StringValues
+            Return New StringValues()
+        End Operator
+
+        Public Shared Widening Operator CType(value As StringValues) As String()
+            Return {}
+        End Operator
+    End Class
+End Class
+    </file>
+</compilation>
+
+            Dim compilation = CompilationUtils.CreateCompilationWithMscorlibAndVBRuntime(compilationDef)
+
+            CompilationUtils.AssertTheseDiagnostics(compilation,
+<expected>
+BC36651: Data type(s) of the type parameter(s) in method 'Public Shared Sub Assert(Of T)(a As T, b As T)' cannot be inferred from these arguments because more than one type is possible. Specifying the data type(s) explicitly might correct this error.
+        Assert(a, b)
+        ~~~~~~
+BC36651: Data type(s) of the type parameter(s) in method 'Public Shared Sub Assert(Of T)(a As T, b As T)' cannot be inferred from these arguments because more than one type is possible. Specifying the data type(s) explicitly might correct this error.
+        Assert(b, a)
+        ~~~~~~
+</expected>)
+        End Sub
+
     End Class
 End Namespace