[17.0] Report errors for structs with missing field initializers and implicit parameterless constructor

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

@@ -3420,22 +3420,30 @@ private BoundNode BindConstructorBody(ConstructorDeclarationSyntax constructor,
 
             bool thisInitializer = initializer?.IsKind(SyntaxKind.ThisConstructorInitializer) == true;
             if (!thisInitializer &&
-                ContainingType.GetMembersUnordered().OfType<SynthesizedRecordConstructor>().Any())
+                hasRecordPrimaryConstructor())
             {
-                var constructorSymbol = (MethodSymbol)this.ContainingMember();
-                if (!constructorSymbol.IsStatic &&
+                if (isInstanceConstructor(out MethodSymbol constructorSymbol) &&
                     !SynthesizedRecordCopyCtor.IsCopyConstructor(constructorSymbol))
                 {
                     // Note: we check the constructor initializer of copy constructors elsewhere
                     Error(diagnostics, ErrorCode.ERR_UnexpectedOrMissingConstructorInitializerInRecord, initializer?.ThisOrBaseKeyword ?? constructor.Identifier);
                 }
             }
 
+            bool isDefaultValueTypeInitializer = thisInitializer
+                && ContainingType.IsDefaultValueTypeConstructor(initializer);
+
+            if (isDefaultValueTypeInitializer &&
+                isInstanceConstructor(out _) &&
+                hasRecordPrimaryConstructor())
+            {
+                Error(diagnostics, ErrorCode.ERR_RecordStructConstructorCallsDefaultConstructor, initializer.ThisOrBaseKeyword);
+            }
+
             // The `: this()` initializer is ignored when it is a default value type constructor
             // and we need to include field initializers into the constructor.
             bool skipInitializer = includesFieldInitializers
-                && thisInitializer
-                && ContainingType.IsDefaultValueTypeConstructor(initializer);
+                && isDefaultValueTypeInitializer;
 
             // Using BindStatement to bind block to make sure we are reusing results of partial binding in SemanticModel
             return new BoundConstructorMethodBody(constructor,
@@ -3447,6 +3455,20 @@ private BoundNode BindConstructorBody(ConstructorDeclarationSyntax constructor,
                                                       null :
                                                       bodyBinder.BindExpressionBodyAsBlock(constructor.ExpressionBody,
                                                                                            constructor.Body == null ? diagnostics : BindingDiagnosticBag.Discarded));
+
+            bool hasRecordPrimaryConstructor() =>
+                ContainingType.GetMembersUnordered().OfType<SynthesizedRecordConstructor>().Any();
+
+            bool isInstanceConstructor(out MethodSymbol constructorSymbol)
+            {
+                if (this.ContainingMember() is MethodSymbol { IsStatic: false } method)
+                {
+                    constructorSymbol = method;
+                    return true;
+                }
+                constructorSymbol = null;
+                return false;
+            }
         }
 
         internal virtual BoundExpressionStatement BindConstructorInitializer(ConstructorInitializerSyntax initializer, BindingDiagnosticBag diagnostics)

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6866,6 +6866,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_LambdaWithAttributesToExpressionTree" xml:space="preserve">
     <value>A lambda expression with attributes cannot be converted to an expression tree</value>
   </data>
+  <data name="ERR_RecordStructConstructorCallsDefaultConstructor" xml:space="preserve">
+    <value>A constructor declared in a 'record struct' with parameter list must have a 'this' initializer that calls the primary constructor or an explicitly declared constructor.</value>
+  </data>
   <data name="WRN_CompileTimeCheckedOverflow" xml:space="preserve">
     <value>The operation may overflow '{0}' at runtime (use 'unchecked' syntax to override)</value>
   </data>

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

@@ -1999,6 +1999,7 @@ internal enum ErrorCode
 
         WRN_CompileTimeCheckedOverflow = 8973,
         WRN_MethGrpToNonDel = 8974,
+        ERR_RecordStructConstructorCallsDefaultConstructor = 8982,
 
         #endregion
 

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

@@ -43,7 +43,7 @@ public static BoundBlock Rewrite(
             if (method.ReturnsVoid || method.IsIterator || method.IsAsyncEffectivelyReturningTask(compilation))
             {
                 // we don't analyze synthesized void methods.
-                if ((method.IsImplicitlyDeclared && !method.IsScriptInitializer) ||
+                if ((method.IsImplicitlyDeclared && !method.IsScriptInitializer && !(method.ContainingType.IsStructType() && method.IsParameterlessConstructor() && !method.IsDefaultValueTypeConstructor(requireZeroInit: true))) ||
                     Analyze(compilation, method, block, diagnostics))
                 {
                     block = AppendImplicitReturn(block, method, originalBodyNested);

src/Compilers/CSharp/Test/Emit/Emit/EditAndContinue/EditAndContinueTests.cs

@@ -4932,19 +4932,19 @@ public void Struct_ImplementSynthesizedConstructor()
 struct S
 {
     int a = 1;
-    int b;
+    int b = 2;
 }
 ";
             var source1 =
 @"
 struct S
 {
     int a = 1;
-    int b;
+    int b = 2;
 
     public S()
     {
-        b = 2;
+        b = 3;
     }
 }
 ";