Collection expressions: report specific error for spread with no GetE…

…numerator() method (#70717)

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

@@ -764,6 +764,7 @@ internal bool TryGetCollectionIterationType(ExpressionSyntax syntax, TypeSymbol
                 syntax,
                 ref collectionExpr,
                 isAsync: false,
+                isSpread: false,
                 BindingDiagnosticBag.Discarded,
                 out iterationType,
                 builder: out _);

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

@@ -4758,7 +4758,7 @@ BoundNode bindSpreadElement(SpreadElementSyntax syntax, BindingDiagnosticBag dia
             {
                 var expression = BindRValueWithoutTargetType(syntax.Expression, diagnostics);
                 ForEachEnumeratorInfo.Builder builder;
-                bool hasErrors = !GetEnumeratorInfoAndInferCollectionElementType(syntax, syntax.Expression, ref expression, isAsync: false, diagnostics, inferredType: out _, out builder) ||
+                bool hasErrors = !GetEnumeratorInfoAndInferCollectionElementType(syntax, syntax.Expression, ref expression, isAsync: false, isSpread: true, diagnostics, inferredType: out _, out builder) ||
                     builder.IsIncomplete;
                 if (hasErrors)
                 {

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

@@ -192,7 +192,7 @@ internal override BoundStatement BindForEachDeconstruction(BindingDiagnosticBag
             BoundExpression collectionExpr = originalBinder.GetBinder(_syntax.Expression).BindRValueWithoutTargetType(_syntax.Expression, diagnostics);
 
             TypeWithAnnotations inferredType;
-            bool hasErrors = !GetEnumeratorInfoAndInferCollectionElementType(_syntax, _syntax.Expression, ref collectionExpr, IsAsync, diagnostics, out inferredType, builder: out _);
+            bool hasErrors = !GetEnumeratorInfoAndInferCollectionElementType(_syntax, _syntax.Expression, ref collectionExpr, isAsync: IsAsync, isSpread: false, diagnostics, out inferredType, builder: out _);
 
             ExpressionSyntax variables = ((ForEachVariableStatementSyntax)_syntax).Variable;
 
@@ -225,7 +225,7 @@ private BoundForEachStatement BindForEachPartsWorker(BindingDiagnosticBag diagno
 
             ForEachEnumeratorInfo.Builder builder;
             TypeWithAnnotations inferredType;
-            bool hasErrors = !GetEnumeratorInfoAndInferCollectionElementType(_syntax, _syntax.Expression, ref collectionExpr, IsAsync, diagnostics, out inferredType, out builder);
+            bool hasErrors = !GetEnumeratorInfoAndInferCollectionElementType(_syntax, _syntax.Expression, ref collectionExpr, isAsync: IsAsync, isSpread: false, diagnostics, out inferredType, out builder);
 
             // These occur when special types are missing or malformed, or the patterns are incompletely implemented.
             hasErrors |= builder.IsIncomplete;
@@ -610,7 +610,7 @@ internal TypeWithAnnotations InferCollectionElementType(BindingDiagnosticBag dia
             // Use the right binder to avoid seeing iteration variable
             BoundExpression collectionExpr = this.GetBinder(collectionSyntax).BindValue(collectionSyntax, diagnostics, BindValueKind.RValue);
 
-            GetEnumeratorInfoAndInferCollectionElementType(_syntax, collectionSyntax, ref collectionExpr, IsAsync, diagnostics, out TypeWithAnnotations inferredType, builder: out _);
+            GetEnumeratorInfoAndInferCollectionElementType(_syntax, collectionSyntax, ref collectionExpr, isAsync: IsAsync, isSpread: false, diagnostics, out TypeWithAnnotations inferredType, builder: out _);
             return inferredType;
         }
     }
@@ -661,11 +661,14 @@ internal bool GetEnumeratorInfoAndInferCollectionElementType(
             ExpressionSyntax collectionSyntax,
             ref BoundExpression collectionExpr,
             bool isAsync,
+            bool isSpread,
             BindingDiagnosticBag diagnostics,
             out TypeWithAnnotations inferredType,
             out ForEachEnumeratorInfo.Builder builder)
         {
-            bool gotInfo = GetEnumeratorInfo(syntax, collectionSyntax, ref collectionExpr, isAsync, diagnostics, out builder);
+            Debug.Assert(!isAsync || !isSpread);
+
+            bool gotInfo = GetEnumeratorInfo(syntax, collectionSyntax, ref collectionExpr, isAsync, isSpread, diagnostics, out builder);
 
             if (!gotInfo)
             {
@@ -749,8 +752,17 @@ private BoundExpression UnwrapCollectionExpressionIfNullable(BoundExpression col
         /// <param name="collectionExpr">The expression over which to iterate.</param>
         /// <param name="diagnostics">Populated with binding diagnostics.</param>
         /// <returns>Partially populated (all but conversions) or null if there was an error.</returns>
-        private bool GetEnumeratorInfo(SyntaxNode syntax, ExpressionSyntax collectionSyntax, ref BoundExpression collectionExpr, bool isAsync, BindingDiagnosticBag diagnostics, out ForEachEnumeratorInfo.Builder builder)
+        private bool GetEnumeratorInfo(
+            SyntaxNode syntax,
+            ExpressionSyntax collectionSyntax,
+            ref BoundExpression collectionExpr,
+            bool isAsync,
+            bool isSpread,
+            BindingDiagnosticBag diagnostics,
+            out ForEachEnumeratorInfo.Builder builder)
         {
+            Debug.Assert(!isAsync || !isSpread);
+
             BoundExpression originalCollectionExpr = collectionExpr;
 
             EnumeratorResult found = GetEnumeratorInfoCore(syntax, collectionSyntax, ref collectionExpr, isAsync, diagnostics, out builder);
@@ -776,9 +788,14 @@ private bool GetEnumeratorInfo(SyntaxNode syntax, ExpressionSyntax collectionSyn
             // Retry with a different assumption about whether the foreach is async
             bool wrongAsync = GetEnumeratorInfoCore(syntax, collectionSyntax, ref originalCollectionExpr, !isAsync, BindingDiagnosticBag.Discarded, builder: out _) == EnumeratorResult.Succeeded;
 
-            var errorCode = wrongAsync
-                ? (isAsync ? ErrorCode.ERR_AwaitForEachMissingMemberWrongAsync : ErrorCode.ERR_ForEachMissingMemberWrongAsync)
-                : (isAsync ? ErrorCode.ERR_AwaitForEachMissingMember : ErrorCode.ERR_ForEachMissingMember);
+            ErrorCode errorCode = (wrongAsync, isAsync, isSpread) switch
+            {
+                (true, true, _) => ErrorCode.ERR_AwaitForEachMissingMemberWrongAsync,
+                (true, false, _) => ErrorCode.ERR_ForEachMissingMemberWrongAsync,
+                (false, true, _) => ErrorCode.ERR_AwaitForEachMissingMember,
+                (false, false, true) => ErrorCode.ERR_SpreadMissingMember,
+                (false, false, false) => ErrorCode.ERR_ForEachMissingMember,
+            };
 
             diagnostics.Add(errorCode, collectionSyntax.Location,
                 collectionExprType, isAsync ? WellKnownMemberNames.GetAsyncEnumeratorMethodName : WellKnownMemberNames.GetEnumeratorMethodName);

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -2758,6 +2758,9 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
   <data name="ERR_AwaitForEachMissingMemberWrongAsync" xml:space="preserve">
     <value>Asynchronous foreach statement cannot operate on variables of type '{0}' because '{0}' does not contain a public instance or extension definition for '{1}'. Did you mean 'foreach' rather than 'await foreach'?</value>
   </data>
+  <data name="ERR_SpreadMissingMember" xml:space="preserve">
+    <value>Spread operator '..' cannot operate on variables of type '{0}' because '{0}' does not contain a public instance or extension definition for '{1}'</value>
+  </data>
   <data name="ERR_PossibleAsyncIteratorWithoutYield" xml:space="preserve">
     <value>The body of an async-iterator method must contain a 'yield' statement.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2280,6 +2280,7 @@ internal enum ErrorCode
         ERR_CollectionExpressionImmutableArray = 9210,
 
         ERR_InvalidExperimentalDiagID = 9211,
+        ERR_SpreadMissingMember = 9212,
 
         #endregion
 

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -2409,6 +2409,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.WRN_CollectionExpressionRefStructSpreadMayAllocate:
                 case ErrorCode.ERR_CollectionExpressionImmutableArray:
                 case ErrorCode.ERR_InvalidExperimentalDiagID:
+                case ErrorCode.ERR_SpreadMissingMember:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement