Do not pass target typed expressions to BestTypeInferrer

Author: RikkiGibson

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

@@ -83,11 +83,7 @@ private static bool IsTargetTypedExpression(BoundExpression node)
             HashSet<TypeSymbol> candidateTypes = new HashSet<TypeSymbol>(comparer);
             foreach (BoundExpression expr in exprs)
             {
-                if (IsTargetTypedExpression(expr))
-                {
-                    continue;
-                }
-
+                Debug.Assert(!IsTargetTypedExpression(expr));
                 TypeSymbol? type = expr.GetTypeOrFunctionType();
 
                 if (type is { })
@@ -152,7 +148,8 @@ private static bool IsTargetTypedExpression(BoundExpression node)
             {
                 var conversionsWithoutNullability = conversions.WithNullability(false);
 
-                if (!IsTargetTypedExpression(expr1) && expr1.Type is { } type1)
+                Debug.Assert(!IsTargetTypedExpression(expr1));
+                if (expr1.Type is { } type1)
                 {
                     if (type1.IsErrorType())
                     {
@@ -166,7 +163,8 @@ private static bool IsTargetTypedExpression(BoundExpression node)
                     }
                 }
 
-                if (!IsTargetTypedExpression(expr2) && expr2.Type is { } type2)
+                Debug.Assert(!IsTargetTypedExpression(expr1));
+                if (expr2.Type is { } type2)
                 {
                     if (type2.IsErrorType())
                     {

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

@@ -4613,7 +4613,11 @@ internal static TypeWithAnnotations BestTypeForLambdaReturns(
             {
                 var (returnExpr, resultType, _) = returns[i];
                 resultTypes.Add(resultType);
-                placeholdersBuilder.Add(CreatePlaceholderIfNecessary(returnExpr, resultType));
+
+                if (!IsTargetTypedExpression(returnExpr))
+                {
+                    placeholdersBuilder.Add(CreatePlaceholderIfNecessary(returnExpr, resultType));
+                }
             }
 
             var discardedUseSiteInfo = CompoundUseSiteInfo<AssemblySymbol>.Discarded;
@@ -5817,6 +5821,14 @@ void makeAndAdjustReceiverSlot(BoundExpression receiver)
             {
                 resultType = null;
             }
+            else if (IsTargetTypedExpression(consequence))
+            {
+                resultType = alternativeRValue.Type;
+            }
+            else if (IsTargetTypedExpression(alternative))
+            {
+                resultType = consequenceRValue.Type;
+            }
             else
             {
                 // Determine nested nullability using BestTypeInferrer.

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

@@ -898,8 +898,11 @@ private void VisitSwitchExpressionCore(BoundSwitchExpression node, bool inferTyp
                 resultTypes.Add(armType);
                 Join(ref endState, ref this.State);
 
-                // Build placeholders for inference in order to preserve annotations.
-                placeholderBuilder.Add(CreatePlaceholderIfNecessary(expression, armType.ToTypeWithAnnotations(compilation)));
+                if (!IsTargetTypedExpression(expression))
+                {
+                    // Build placeholders for inference in order to preserve annotations.
+                    placeholderBuilder.Add(CreatePlaceholderIfNecessary(expression, armType.ToTypeWithAnnotations(compilation)));
+                }
             }
 
             SetState(endState);

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

@@ -5385,5 +5385,7 @@ public string[] M2(bool b)
                 Assert.Equal(PublicNullableAnnotation.NotAnnotated, type.ElementNullableAnnotation);
             }
         }
+
+        // TODO2: test collection-exprs in lambda returns to exercise all BestTypeInferrer usages in NullableWalker
     }
 }