Fix checks for auto-getter where malformed IsReadOnlyAttribute is present

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertyAccessorSymbol.cs

@@ -431,10 +431,14 @@ internal override bool IsDeclaredReadOnly
                     return true;
                 }
 
-                // We can't emit the synthesized attribute for netmodules, so in this case we consider auto-getters **not** implicitly readonly.
-                if (DeclaringCompilation.Options.OutputKind == OutputKind.NetModule &&
-                    DeclaringCompilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_IsReadOnlyAttribute).IsErrorType())
+                // If we have IsReadOnly..ctor, we can use the attribute. Otherwise, we need to NOT be a netmodule and the type must not already exist in order to synthesize it.
+                var isReadOnlyAttributeUsable = DeclaringCompilation.GetWellKnownTypeMember(WellKnownMember.System_Runtime_CompilerServices_IsReadOnlyAttribute__ctor) != null ||
+                    (DeclaringCompilation.Options.OutputKind != OutputKind.NetModule &&
+                     DeclaringCompilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_IsReadOnlyAttribute) is MissingMetadataTypeSymbol);
+
+                if (!isReadOnlyAttributeUsable)
                 {
+                    // if the readonly attribute isn't usable, don't implicitly make auto-getters readonly.
                     return false;
                 }
 

src/Compilers/CSharp/Test/Semantic/Semantics/ReadOnlyStructsTests.cs

@@ -1484,17 +1484,17 @@ public struct S
     public int P1 { get; private set; }
 }
 ";
-            var moduleComp = CreateCompilation(csharp, options: TestOptions.DebugModule, targetFramework: TargetFramework.Mscorlib45);
-            moduleComp.VerifyDiagnostics(
-                // (12,21): error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.IsReadOnlyAttribute..ctor'
-                //     public int P1 { get; private set; }
-                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "get").WithArguments("System.Runtime.CompilerServices.IsReadOnlyAttribute", ".ctor").WithLocation(12, 21));
+            var moduleMetadata = CreateCompilation(csharp, options: TestOptions.DebugModule, targetFramework: TargetFramework.Mscorlib45).EmitToImageReference();
+            var moduleComp = CreateCompilation("", new[] { moduleMetadata });
+            var moduleGetterAttributes = moduleComp.GetMember<PropertySymbol>("S.P1").GetMethod.GetAttributes();
+            Assert.Equal(1, moduleGetterAttributes.Length);
+            Assert.Equal("CompilerGeneratedAttribute", moduleGetterAttributes[0].AttributeClass.Name);
 
-            var dllComp = CreateCompilation(csharp, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib45);
-            dllComp.VerifyDiagnostics(
-                // (12,21): error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.IsReadOnlyAttribute..ctor'
-                //     public int P1 { get; private set; }
-                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "get").WithArguments("System.Runtime.CompilerServices.IsReadOnlyAttribute", ".ctor").WithLocation(12, 21));
+            var dllMetadata = CreateCompilation(csharp, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib45).EmitToImageReference();
+            var dllComp = CreateCompilation("", new[] { dllMetadata });
+            var dllGetterAttributes = dllComp.GetMember<PropertySymbol>("S.P1").GetMethod.GetAttributes();
+            Assert.Equal(1, dllGetterAttributes.Length);
+            Assert.Equal("CompilerGeneratedAttribute", dllGetterAttributes[0].AttributeClass.Name);
         }
 
         [Fact]