Use default tuple fields in conversion since fields from inferred names

are marked not usable in C#7

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Conversion.cs

@@ -650,16 +650,8 @@ private BoundExpression MakeImplicitConversion(BoundExpression rewrittenOperand,
 
             for (int i = 0; i < numElements; i++)
             {
-                var field = srcElementFields[i];
-
-                DiagnosticInfo useSiteInfo = field.GetUseSiteDiagnostic();
-                if ((object)useSiteInfo != null && useSiteInfo.Severity == DiagnosticSeverity.Error)
-                {
-                    Symbol.ReportUseSiteDiagnostic(useSiteInfo, _diagnostics, syntax.Location);
-                }
-                var fieldAccess = MakeTupleFieldAccess(syntax, field, savedTuple, null, LookupResultKind.Empty);
+                var fieldAccess = MakeTupleFieldAccessAndReportUseSiteDiagnostics(savedTuple, syntax, srcElementFields[i]);
                 var convertedFieldAccess = MakeConversionNode(syntax, fieldAccess, elementConversions[i], destElementTypes[i], @checked, explicitCastInCode);
-
                 fieldAccessorsBuilder.Add(convertedFieldAccess);
             }
 

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_DeconstructionAssignmentOperator.cs

@@ -221,18 +221,7 @@ private static bool IsTupleExpression(BoundKind kind)
             var builder = ArrayBuilder<BoundExpression>.GetInstance(numElements);
             for (int i = 0; i < numElements; i++)
             {
-                var field = fields[i];
-
-                // Use default field rather than implicitly named fields since
-                // fields from inferred names are not usable in C# 7.0.
-                field = field.CorrespondingTupleField ?? field;
-
-                DiagnosticInfo useSiteInfo = field.GetUseSiteDiagnostic();
-                if ((object)useSiteInfo != null && useSiteInfo.Severity == DiagnosticSeverity.Error)
-                {
-                    Symbol.ReportUseSiteDiagnostic(useSiteInfo, _diagnostics, expression.Syntax.Location);
-                }
-                var fieldAccess = MakeTupleFieldAccess(expression.Syntax, field, tuple, null, LookupResultKind.Empty);
+                var fieldAccess = MakeTupleFieldAccessAndReportUseSiteDiagnostics(tuple, expression.Syntax, fields[i]);
                 builder.Add(fieldAccess);
             }
             return builder.ToImmutableAndFree();

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Field.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using Microsoft.CodeAnalysis.CSharp.Symbols;
-using System.Diagnostics;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
@@ -90,5 +89,20 @@ public override BoundNode VisitFieldAccess(BoundFieldAccess node)
             // make a field access for the most local access
             return _factory.Field(rewrittenReceiver, underlyingField);
         }
+
+        private BoundExpression MakeTupleFieldAccessAndReportUseSiteDiagnostics(BoundExpression tuple, SyntaxNode syntax, FieldSymbol field)
+        {
+            // Use default field rather than implicitly named fields since
+            // fields from inferred names are not usable in C# 7.0.
+            field = field.CorrespondingTupleField ?? field;
+
+            DiagnosticInfo useSiteInfo = field.GetUseSiteDiagnostic();
+            if ((object)useSiteInfo != null && useSiteInfo.Severity == DiagnosticSeverity.Error)
+            {
+                Symbol.ReportUseSiteDiagnostic(useSiteInfo, _diagnostics, syntax.Location);
+            }
+
+            return MakeTupleFieldAccess(syntax, field, tuple, null, LookupResultKind.Empty);
+        }
     }
 }

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenTupleTest.cs

@@ -3938,6 +3938,29 @@ static class Extension
             verifier7_1.VerifyDiagnostics();
         }
 
+        [WorkItem(21518, "https://github.com/dotnet/roslyn/issues/21518")]
+        [Fact]
+        public void InferredName_Conversion()
+        {
+            var source =
+@"using System.Collections.Generic;
+class C
+{
+    static void F((int, IList<object>) items)
+    {
+    }
+    static void Test()
+    {
+        var items = new List<object>();
+        var group = (1, items);
+        F(group);
+    }
+}";
+            var comp = CreateCompilation(source, parseOptions: TestOptions.Regular.WithLanguageVersion(LanguageVersion.CSharp7),
+                references: new[] { MscorlibRef, ValueTupleRef, SystemRuntimeFacadeRef, SystemCoreRef });
+            comp.VerifyEmitDiagnostics();
+        }
+
         [Fact]
         public void LongTupleWithArgumentEvaluation()
         {

src/Compilers/VisualBasic/Test/Emit/CodeGen/CodeGenTuples.vb

@@ -6277,6 +6277,28 @@ BC37289: Tuple element name 'M' is inferred. Please use language version 15.3 or
             verifier15_3.VerifyDiagnostics()
         End Sub
 
+        <Fact>
+        Public Sub InferredName_Conversion()
+            Dim source = <compilation>
+                             <file>
+Imports System.Collections.Generic
+Class C
+    Shared Sub F(items As (Integer, IList(Of Object)))
+    End Sub
+    Shared Sub Test()
+        Dim items = New List(Of Object)()
+        Dim group = (1, items)
+        F(group)
+    End Sub
+End Class
+    </file>
+                         </compilation>
+            Dim comp = CreateCompilationWithMscorlibAndVBRuntime(source,
+                additionalRefs:={ValueTupleRef, SystemRuntimeFacadeRef, SystemCoreRef},
+                parseOptions:=TestOptions.Regular.WithLanguageVersion(LanguageVersion.VisualBasic15))
+            comp.AssertTheseEmitDiagnostics(<errors/>)
+        End Sub
+
         <Fact>
         Public Sub LongTupleWithArgumentEvaluation()
 
@@ -7564,7 +7586,7 @@ BC36625: Lambda expression cannot be converted to 'Integer' because 'Integer' is
         End Sub
 
         <Fact>
-        Public Sub TupleInferredLambdStrictOn()
+        Public Sub TupleInferredLambdaStrictOn()
 
             Dim comp = CreateCompilationWithMscorlibAndVBRuntime(
 <compilation>
@@ -7600,7 +7622,7 @@ BC36642: Option Strict On requires each lambda expression parameter to be declar
         End Sub
 
         <Fact()>
-        Public Sub TupleInferredLambdStrictOff()
+        Public Sub TupleInferredLambdaStrictOff()
 
             Dim verifier = CompileAndVerify(
 <compilation>