Typeless expressions should contribute nullability to lambda return (#47581)

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

@@ -505,8 +505,6 @@ private BoundExpression CreateMethodGroupConversion(SyntaxNode syntax, BoundExpr
                 _ => throw ExceptionUtilities.UnexpectedValue(source),
             };
             BoundMethodGroup group = FixMethodGroupWithTypeOrValue(originalGroup, conversion, diagnostics);
-            BoundExpression? receiverOpt = group.ReceiverOpt;
-            MethodSymbol? method = conversion.Method;
             bool hasErrors = false;
 
             if (MethodGroupConversionHasErrors(syntax, conversion, group.ReceiverOpt, conversion.IsExtensionMethod, destination, diagnostics))

src/Compilers/CSharp/Portable/Binder/Semantics/BestTypeInferrer.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Linq;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.PooledObjects;
 
@@ -14,11 +15,17 @@ internal static class BestTypeInferrer
     {
         public static NullableAnnotation GetNullableAnnotation(ArrayBuilder<TypeWithAnnotations> types)
         {
+#if DEBUG
+            var example = types.FirstOrDefault(t => t.HasType);
+#endif
+
             var result = NullableAnnotation.NotAnnotated;
             foreach (var type in types)
             {
-                Debug.Assert(type.HasType);
-                Debug.Assert(type.Equals(types[0], TypeCompareKind.AllIgnoreOptions));
+#if DEBUG
+                Debug.Assert(!type.HasType || type.Equals(example, TypeCompareKind.AllIgnoreOptions));
+#endif
+
                 // This uses the covariant merging rules.
                 result = result.Join(type.NullableAnnotation);
             }

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -2842,11 +2842,9 @@ protected override void VisitStatement(BoundStatement statement)
 
         public override BoundNode? VisitUnconvertedObjectCreationExpression(BoundUnconvertedObjectCreationExpression node)
         {
-            var discardedDiagnostics = new DiagnosticBag();
-            var expr = _binder!.BindObjectCreationForErrorRecovery(node, discardedDiagnostics);
-            discardedDiagnostics.Free();
-            Visit(expr);
-            SetResultType(node, TypeWithState.Create(expr.Type, NullableFlowState.NotNull));
+            // This method is only involved in method inference with unbound lambdas.
+            // The diagnostics on arguments are reported by VisitObjectCreationExpression.
+            SetResultType(node, TypeWithState.Create(null, NullableFlowState.NotNull));
             return null;
         }
 
@@ -4108,8 +4106,6 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
             BoundExpression originalConsequence,
             BoundExpression originalAlternative)
         {
-            Debug.Assert(node.Type is object);
-
             VisitCondition(condition);
             var consequenceState = this.StateWhenTrue;
             var alternativeState = this.StateWhenFalse;
@@ -4127,7 +4123,7 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
                 (alternativeLValue, alternativeRValue) = visitConditionalRefOperand(alternativeState, originalAlternative);
                 Join(ref this.State, ref consequenceState);
 
-                TypeSymbol refResultType = node.Type.SetUnknownNullabilityForReferenceTypes();
+                TypeSymbol? refResultType = node.Type?.SetUnknownNullabilityForReferenceTypes();
                 if (IsNullabilityMismatch(consequenceLValue, alternativeLValue))
                 {
                     // l-value types must match
@@ -4204,8 +4200,8 @@ private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResu
             NullableFlowState resultState;
             if (resultType is null)
             {
-                resultType = node.Type.SetUnknownNullabilityForReferenceTypes();
-                resultState = NullableFlowState.NotNull;
+                resultType = node.Type?.SetUnknownNullabilityForReferenceTypes();
+                resultState = consequenceRValue.State.Join(alternativeRValue.State);
 
                 var resultTypeWithState = TypeWithState.Create(resultType, resultState);
 
@@ -6541,9 +6537,7 @@ private bool HasTopLevelNullabilityConversion(TypeWithAnnotations source, TypeWi
                 case ConversionKind.ObjectCreation:
                 case ConversionKind.SwitchExpression:
                 case ConversionKind.ConditionalExpression:
-                    // These are not represented as a separate conversion in the bound tree.
-                    // Instead they are folded into the operand.
-                    throw ExceptionUtilities.UnexpectedValue(conversion.Kind);
+                    return operandType;
 
                 case ConversionKind.ExplicitUserDefined:
                 case ConversionKind.ImplicitUserDefined:
@@ -8795,9 +8789,9 @@ private TypeWithState InferResultNullabilityOfBinaryLogicalOperator(BoundExpress
 
         public override BoundNode? VisitDefaultLiteral(BoundDefaultLiteral node)
         {
-            // Can occur in error scenarios
+            // Can occur in error scenarios and lambda scenarios
             var result = base.VisitDefaultLiteral(node);
-            SetUnknownResultNullability(node);
+            SetResultType(node, TypeWithState.Create(node.Type, NullableFlowState.MaybeDefault));
             return result;
         }
 

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

@@ -566,6 +566,7 @@ public override BoundNode VisitConvertedSwitchExpression(BoundConvertedSwitchExp
 
         public override BoundNode VisitUnconvertedSwitchExpression(BoundUnconvertedSwitchExpression node)
         {
+            // This method is only involved in method inference with unbound lambdas.
             VisitSwitchExpressionCore(node, inferType: true);
             return null;
         }
@@ -632,21 +633,26 @@ private void VisitSwitchExpressionCore(BoundSwitchExpression node, bool inferTyp
 
             TypeSymbol inferredType =
                 (inferType ? BestTypeInferrer.InferBestType(placeholders, _conversions, ref useSiteDiagnostics) : null)
-                    ?? node.Type?.SetUnknownNullabilityForReferenceTypes()
-                    ?? new ExtendedErrorTypeSymbol(this.compilation, "", arity: 0, errorInfo: null, unreported: false);
+                    ?? node.Type?.SetUnknownNullabilityForReferenceTypes();
 
             var inferredTypeWithAnnotations = TypeWithAnnotations.Create(inferredType);
 
             // Convert elements to best type to determine element top-level nullability and to report nested nullability warnings
+            if (inferredType is not null)
+            {
                 for (int i = 0; i < numSwitchArms; i++)
                 {
                     var expression = expressions[i];
                     resultTypes[i] = VisitConversion(conversionOpt: null, expression, conversions[i], inferredTypeWithAnnotations, resultTypes[i], checkConversion: true,
                         fromExplicitCast: false, useLegacyWarnings: false, AssignmentKind.Assignment, reportRemainingWarnings: true, reportTopLevelWarnings: false);
                 }
+            }
 
             var inferredState = BestTypeInferrer.GetNullableState(resultTypes);
             var resultType = TypeWithState.Create(inferredType, inferredState);
+
+            if (inferredType is not null)
+            {
                 inferredTypeWithAnnotations = resultType.ToTypeWithAnnotations(compilation);
                 if (resultType.State == NullableFlowState.MaybeDefault)
                 {
@@ -661,6 +667,7 @@ private void VisitSwitchExpressionCore(BoundSwitchExpression node, bool inferTyp
                     _ = VisitConversion(conversionOpt, conversionOperand: nodeForSyntax, conversions[i], targetTypeWithNullability: inferredTypeWithAnnotations, operandType: resultTypes[i],
                         checkConversion: true, fromExplicitCast: false, useLegacyWarnings: false, AssignmentKind.Assignment, reportRemainingWarnings: false, reportTopLevelWarnings: true);
                 }
+            }
 
             conversions.Free();
             resultTypes.Free();

src/Compilers/CSharp/Portable/Symbols/TypeWithAnnotations.cs

@@ -782,7 +782,7 @@ static NullableFlowState getFlowState(TypeSymbol type, NullableAnnotation annota
             {
                 if (type is null)
                 {
-                    return annotation.IsAnnotated() ? NullableFlowState.MaybeNull : NullableFlowState.NotNull;
+                    return annotation.IsAnnotated() ? NullableFlowState.MaybeDefault : NullableFlowState.NotNull;
                 }
                 if (type.IsPossiblyNullableReferenceTypeTypeParameter())
                 {

src/Compilers/CSharp/Test/Symbol/Symbols/Source/NullablePublicAPITests.cs

@@ -1268,7 +1268,7 @@ void M()
             Assert.Equal(notNull, leftInfo.Nullability);
             Assert.Equal(notNull, leftInfo.ConvertedNullability);
             Assert.Equal(@null, rightInfo.Nullability);
-            Assert.Equal(notNull, rightInfo.ConvertedNullability);
+            Assert.Equal(@null, rightInfo.ConvertedNullability);
         }
 
         [Fact]