Merge pull request #7739 from VSadov/fix7388

Fix for unverifiable codegen for conditional access when result constrained to class

src/Compilers/CSharp/Portable/CodeGen/EmitExpression.cs

@@ -2486,15 +2486,18 @@ private void EmitDefaultValue(TypeSymbol type, bool used, CSharpSyntaxNode synta
         {
             if (used)
             {
-                var constantValue = type.GetDefaultValue();
-                if (constantValue != null)
+                // default type parameter values must be emitted as 'initobj' regardless of constraints
+                if (!type.IsTypeParameter())
                 {
-                    _builder.EmitConstantValue(constantValue);
-                }
-                else
-                {
-                    EmitInitObj(type, true, syntaxNode);
+                    var constantValue = type.GetDefaultValue();
+                    if (constantValue != null)
+                    {
+                        _builder.EmitConstantValue(constantValue);
+                        return;
+                    }
                 }
+
+                EmitInitObj(type, true, syntaxNode);
             }
         }
 

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

@@ -31,7 +31,7 @@ private enum ConditionalAccessLoweringKind
 
         // IL gen can generate more compact code for certain conditional accesses 
         // by utilizing stack dup/pop instructions 
-        internal BoundExpression RewriteConditionalAccess(BoundConditionalAccess node, bool used, BoundExpression rewrittenWhenNull = null)
+        internal BoundExpression RewriteConditionalAccess(BoundConditionalAccess node, bool used)
         {
             Debug.Assert(!_inExpressionLambda);
 
@@ -41,7 +41,7 @@ internal BoundExpression RewriteConditionalAccess(BoundConditionalAccess node, b
             // Check trivial case
             if (loweredReceiver.IsDefaultValue())
             {
-                return rewrittenWhenNull ?? _factory.Default(node.Type);
+                return _factory.Default(node.Type);
             }
 
             ConditionalAccessLoweringKind loweringKind;
@@ -154,7 +154,7 @@ internal BoundExpression RewriteConditionalAccess(BoundConditionalAccess node, b
                                  GetNullableMethod(node.Syntax, loweredReceiver.Type, SpecialMember.System_Nullable_T_get_HasValue) :
                                  null,
                         loweredAccessExpression,
-                        rewrittenWhenNull,
+                        null,
                         currentConditionalAccessID,
                         type);
 
@@ -180,7 +180,7 @@ internal BoundExpression RewriteConditionalAccess(BoundConditionalAccess node, b
                         result = RewriteConditionalOperator(node.Syntax,
                             condition,
                             consequence,
-                            rewrittenWhenNull ?? _factory.Default(nodeType),
+                            _factory.Default(nodeType),
                             null,
                             nodeType);
 

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

@@ -6843,5 +6843,56 @@ .locals init (int? V_0)
 }
 ");
         }
+
+        [WorkItem(7388, "https://github.com/dotnet/roslyn/issues/7388")]
+        [Fact]
+        public void ConditionalClassConstrained001()
+        {
+            var source = @"
+using System;
+
+namespace ConsoleApplication9
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            var v = new A<object>();
+            System.Console.WriteLine(A<object>.Test(v));
+        }
+
+        public class A<T> : object where T : class
+        {
+            public T Value { get { return (T)(object)42; }}
+
+            public static T Test(A<T> val)
+            {
+                return val?.Value;
+            }
+        }
+    }
+}
+
+
+";
+            var verifier = CompileAndVerify(source, expectedOutput: @"42");
+
+            verifier.VerifyIL("ConsoleApplication9.Program.A<T>.Test(ConsoleApplication9.Program.A<T>)", @"
+{
+  // Code size       20 (0x14)
+  .maxstack  1
+  .locals init (T V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  brtrue.s   IL_000d
+  IL_0003:  ldloca.s   V_0
+  IL_0005:  initobj    ""T""
+  IL_000b:  ldloc.0
+  IL_000c:  ret
+  IL_000d:  ldarg.0
+  IL_000e:  call       ""T ConsoleApplication9.Program.A<T>.Value.get""
+  IL_0013:  ret
+}");
+        }
+
     }
 }

src/Compilers/VisualBasic/Portable/CodeGen/EmitConversion.vb

@@ -141,7 +141,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.CodeGen
             Dim typeTo = conversion.Type
 
             If conversion.Operand.IsNothingLiteral Then
-                Debug.Assert(typeTo.IsValueType AndAlso Not typeTo.IsTypeParameter)
+                Debug.Assert(typeTo.IsValueType OrElse typeTo.IsTypeParameter)
 
                 If used Then
                     'TODO: used

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

@@ -2436,6 +2436,62 @@ C1
 
         End Sub
 
+        <Fact(), WorkItem(7388, "https://github.com/dotnet/roslyn/issues/7388")>
+        Public Sub ConstrainedToClass()
+
+            Dim compilationDef =
+<compilation>
+    <file name="a.vb"><![CDATA[
+Module Module1
+
+    Sub Main()
+        Dim v As New A(Of Object)
+        System.Console.WriteLine(A(Of Object).test(v))
+    End Sub
+
+End Module
+
+Public Class A(Of T As Class)
+    Public ReadOnly Property Value As T
+        Get
+            Return CType(CObj(42), T)
+        End Get
+    End Property
+
+    Public Shared Function test(val As A(Of T)) As T
+        Return val?.Value
+    End Function
+End Class
+    ]]></file>
+</compilation>
+
+            Dim compilation = CompilationUtils.CreateCompilationWithMscorlibAndVBRuntime(compilationDef, TestOptions.ReleaseExe)
+
+            Dim verifier = CompileAndVerify(compilation, expectedOutput:=
+            <![CDATA[
+42
+]]>)
+
+            verifier.VerifyIL("A(Of T).test(A(Of T))",
+            <![CDATA[
+{
+  // Code size       20 (0x14)
+  .maxstack  1
+  .locals init (T V_0)
+  IL_0000:  ldarg.0
+  IL_0001:  brtrue.s   IL_000d
+  IL_0003:  ldloca.s   V_0
+  IL_0005:  initobj    "T"
+  IL_000b:  ldloc.0
+  IL_000c:  ret
+  IL_000d:  ldarg.0
+  IL_000e:  call       "Function A(Of T).get_Value() As T"
+  IL_0013:  ret
+}
+]]>)
+
+        End Sub
+
         <Fact()>
         Public Sub CodeGen_01()