New T() should call Activator.CreateInstance unconditionally.

This is consistent with behavior of "new" when used with structs in general - we should call public parameterless constructor if present, which Activator.CreateInstance does.
It is also consistent with VB and F#

***NO_CI***
 (changeset 1330480)

Src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_ObjectCreationExpression.cs

@@ -150,18 +150,14 @@ public override BoundNode VisitNewT(BoundNewT node)
 
         private BoundExpression MakeNewT(CSharpSyntaxNode syntax, TypeParameterSymbol typeParameter)
         {
-            // How "new T()" is rewritten depends on whether T is known to be a value
-            // type, a reference type, or neither (see OperatorRewriter::VisitNEWTYVAR).
-
-            if (typeParameter.IsValueType)
-            {
-                // "new T()" rewritten as: "default(T)".
-                return new BoundDefaultOperator(syntax, type: typeParameter);
-            }
-
-            // For types not known to be value types, "new T()" requires
-            // Activator.CreateInstance<T>().
-
+            // "new T()" is rewritten as: "Activator.CreateInstance<T>()".
+
+            // NOTE: DIFFERENCE FROM DEV12
+            // Dev12 tried to statically optimize this and would emit default(T) if T happens to be struct
+            // However semantics of "new" in C# requires that parameterless constructor be called
+            // if struct defines one.
+            // Since we cannot know if T has a parameterless constructor statically, 
+            // we must call Activator.CreateInstance unconditionally.
             MethodSymbol method;
 
             if (!this.TryGetWellKnownTypeMember(syntax, WellKnownMember.System_Activator__CreateInstance_T, out method))
@@ -186,31 +182,7 @@ private BoundExpression MakeNewT(CSharpSyntaxNode syntax, TypeParameterSymbol ty
                 resultKind: LookupResultKind.Viable,
                 type: typeParameter);
 
-            if (typeParameter.IsReferenceType)
-            {
-                // "new T()" is rewritten as: "Activator.CreateInstance<T>()".
-                return createInstanceCall;
-            }
-            else
-            {
-                // "new T()" is rewritten as: "(null == (object)default(T)) ? Activator.CreateInstance<T>() : default(T)".
-                var defaultT = new BoundDefaultOperator(syntax, type: typeParameter);
-                return new BoundConditionalOperator(
-                    syntax,
-                    MakeNullCheck(
-                        syntax: syntax,
-                        rewrittenExpr: MakeConversion(
-                            syntax: syntax,
-                            rewrittenOperand: defaultT,
-                            conversionKind: ConversionKind.Boxing,
-                            rewrittenType: this.compilation.GetSpecialType(SpecialType.System_Object),
-                            @checked: false),
-                        operatorKind: BinaryOperatorKind.Equal),
-                    createInstanceCall,
-                    defaultT,
-                    constantValueOpt: null,
-                    type: typeParameter);
-            }
+           return createInstanceCall;
         }
 
         public override BoundNode VisitNoPiaObjectCreationExpression(BoundNoPiaObjectCreationExpression node)

Src/Compilers/CSharp/Test/Emit/CodeGen/ObjectAndCollectionInitializerTests.cs

@@ -190,32 +190,22 @@ struct S : I
             var compVerifier = CompileAndVerify(source, emitOptions: EmitOptions.CCI, expectedOutput: expectedOutput);
             compVerifier.VerifyIL("MemberInitializerTest.Foo<T>", @"
 {
-  // Code size       63 (0x3f)
+  // Code size       36 (0x24)
   .maxstack  2
   .locals init (T V_0, //b
-  T V_1,
-  T V_2)
-  IL_0000:  ldloca.s   V_2
-  IL_0002:  initobj    ""T""
-  IL_0008:  ldloc.2
-  IL_0009:  box        ""T""
-  IL_000e:  brfalse.s  IL_001b
-  IL_0010:  ldloca.s   V_2
-  IL_0012:  initobj    ""T""
-  IL_0018:  ldloc.2
-  IL_0019:  br.s       IL_0020
-  IL_001b:  call       ""T System.Activator.CreateInstance<T>()""
-  IL_0020:  stloc.1
-  IL_0021:  ldloca.s   V_1
-  IL_0023:  ldc.i4.1
-  IL_0024:  constrained. ""T""
-  IL_002a:  callvirt   ""void I.X.set""
-  IL_002f:  ldloc.1
-  IL_0030:  stloc.0
-  IL_0031:  ldloca.s   V_0
-  IL_0033:  constrained. ""T""
-  IL_0039:  callvirt   ""byte I.X.get""
-  IL_003e:  ret
+                T V_1)
+  IL_0000:  call       ""T System.Activator.CreateInstance<T>()""
+  IL_0005:  stloc.1
+  IL_0006:  ldloca.s   V_1
+  IL_0008:  ldc.i4.1
+  IL_0009:  constrained. ""T""
+  IL_000f:  callvirt   ""void I.X.set""
+  IL_0014:  ldloc.1
+  IL_0015:  stloc.0
+  IL_0016:  ldloca.s   V_0
+  IL_0018:  constrained. ""T""
+  IL_001e:  callvirt   ""byte I.X.get""
+  IL_0023:  ret
 }");
         }
 
@@ -253,38 +243,28 @@ struct S : I
             var compVerifier = CompileAndVerify(source, emitOptions: EmitOptions.CCI, expectedOutput: expectedOutput);
             compVerifier.VerifyIL("MemberInitializerTest.Foo<T>", @"
 {
-  // Code size       78 (0x4e)
+  // Code size       51 (0x33)
   .maxstack  6
   .locals init (T V_0, //b
-  T V_1,
-  T V_2)
-  IL_0000:  ldloca.s   V_2
-  IL_0002:  initobj    ""T""
-  IL_0008:  ldloc.2
-  IL_0009:  box        ""T""
-  IL_000e:  brfalse.s  IL_001b
-  IL_0010:  ldloca.s   V_2
-  IL_0012:  initobj    ""T""
-  IL_0018:  ldloc.2
-  IL_0019:  br.s       IL_0020
-  IL_001b:  call       ""T System.Activator.CreateInstance<T>()""
-  IL_0020:  stloc.1
-  IL_0021:  ldloca.s   V_1
-  IL_0023:  ldc.i4.s   11
-  IL_0025:  ldc.i4.0
-  IL_0026:  ldc.i4.0
-  IL_0027:  ldc.i4.0
-  IL_0028:  ldc.i4.1
-  IL_0029:  newobj     ""decimal..ctor(int, int, int, bool, byte)""
-  IL_002e:  newobj     ""decimal?..ctor(decimal)""
-  IL_0033:  constrained. ""T""
-  IL_0039:  callvirt   ""void I.X.set""
-  IL_003e:  ldloc.1
-  IL_003f:  stloc.0
-  IL_0040:  ldloca.s   V_0
-  IL_0042:  constrained. ""T""
-  IL_0048:  callvirt   ""decimal? I.X.get""
-  IL_004d:  ret
+                T V_1)
+  IL_0000:  call       ""T System.Activator.CreateInstance<T>()""
+  IL_0005:  stloc.1
+  IL_0006:  ldloca.s   V_1
+  IL_0008:  ldc.i4.s   11
+  IL_000a:  ldc.i4.0
+  IL_000b:  ldc.i4.0
+  IL_000c:  ldc.i4.0
+  IL_000d:  ldc.i4.1
+  IL_000e:  newobj     ""decimal..ctor(int, int, int, bool, byte)""
+  IL_0013:  newobj     ""decimal?..ctor(decimal)""
+  IL_0018:  constrained. ""T""
+  IL_001e:  callvirt   ""void I.X.set""
+  IL_0023:  ldloc.1
+  IL_0024:  stloc.0
+  IL_0025:  ldloca.s   V_0
+  IL_0027:  constrained. ""T""
+  IL_002d:  callvirt   ""decimal? I.X.get""
+  IL_0032:  ret
 }");
         }
 

Src/Compilers/CSharp/Test/Symbol/Symbols/GenericConstraintTests.cs

@@ -1285,21 +1285,12 @@ static void Main()
 C
 S");
             compilation.VerifyIL("C.F1<T>()",
-@"{
-  // Code size       32 (0x20)
+@"
+{
+  // Code size        6 (0x6)
   .maxstack  1
-  .locals init (T V_0)
-  IL_0000:  ldloca.s   V_0
-  IL_0002:  initobj    ""T""
-  IL_0008:  ldloc.0
-  IL_0009:  box        ""T""
-  IL_000e:  brfalse.s  IL_001a
-  IL_0010:  ldloca.s   V_0
-  IL_0012:  initobj    ""T""
-  IL_0018:  ldloc.0
-  IL_0019:  ret
-  IL_001a:  call       ""T System.Activator.CreateInstance<T>()""
-  IL_001f:  ret
+  IL_0000:  call       ""T System.Activator.CreateInstance<T>()""
+  IL_0005:  ret
 }");
             compilation.VerifyIL("C.F2<T>()",
 @"{
@@ -1309,14 +1300,12 @@ .maxstack  1
   IL_0005:  ret
 }");
             compilation.VerifyIL("C.F3<T>()",
-@"{
-  // Code size       10 (0xa)
+@"
+{
+  // Code size        6 (0x6)
   .maxstack  1
-  .locals init (T V_0)
-  IL_0000:  ldloca.s   V_0
-  IL_0002:  initobj    ""T""
-  IL_0008:  ldloc.0
-  IL_0009:  ret
+  IL_0000:  call       ""T System.Activator.CreateInstance<T>()""
+  IL_0005:  ret
 }");
         }
 
@@ -1348,24 +1337,21 @@ class C
 }";
             var compilation = CompileAndVerify(source);
             compilation.VerifyIL("C.M<T, U, V>()",
-@"{
-  // Code size       41 (0x29)
+@"
+{
+  // Code size       31 (0x1f)
   .maxstack  1
-  .locals init (U V_0)
   IL_0000:  newobj     ""A..ctor()""
   IL_0005:  pop
   IL_0006:  newobj     ""B..ctor()""
   IL_000b:  pop
-  IL_000c:  ldloca.s   V_0
-  IL_000e:  initobj    ""U""
-  IL_0014:  ldloc.0
-  IL_0015:  box        ""U""
-  IL_001a:  brtrue.s   IL_0022
-  IL_001c:  call       ""U System.Activator.CreateInstance<U>()""
-  IL_0021:  pop
-  IL_0022:  call       ""V System.Activator.CreateInstance<V>()""
-  IL_0027:  pop
-  IL_0028:  ret
+  IL_000c:  call       ""T System.Activator.CreateInstance<T>()""
+  IL_0011:  pop
+  IL_0012:  call       ""U System.Activator.CreateInstance<U>()""
+  IL_0017:  pop
+  IL_0018:  call       ""V System.Activator.CreateInstance<V>()""
+  IL_001d:  pop
+  IL_001e:  ret
 }");
         }