Dealing with more than one parameterless constructor

src/Compilers/CSharp/Portable/Symbols/MemberSymbolExtensions.cs

@@ -403,12 +403,33 @@ internal static bool IncludeFieldInitializersInBody(this MethodSymbol methodSymb
 
         internal static bool IsDefaultValueTypeConstructor(this NamedTypeSymbol type, ConstructorInitializerSyntax initializerSyntax)
         {
-            if (initializerSyntax.ArgumentList.Arguments.Count > 0)
+            if (initializerSyntax.ArgumentList.Arguments.Count > 0 || !type.IsValueType)
             {
                 return false;
             }
-            var constructor = type.InstanceConstructors.SingleOrDefault(m => m.ParameterCount == 0);
-            return constructor?.IsDefaultValueTypeConstructor(requireZeroInit: true) == true;
+
+            // If exactly one parameterless constructor, return whether it's the default value type constructor
+            // Otherwise, return false
+            bool foundParameterlessCtor = false;
+            bool result = false;
+            foreach (var constructor in type.InstanceConstructors)
+            {
+                if (constructor.ParameterCount != 0)
+                {
+                    continue;
+                }
+
+                if (foundParameterlessCtor)
+                {
+                    // finding more than one parameterless constructor is an error scenario (reported elsewhere)
+                    return false;
+                }
+
+                foundParameterlessCtor = true;
+                result = constructor.IsDefaultValueTypeConstructor(requireZeroInit: true);
+            }
+
+            return result;
         }
 
         /// <summary>

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

@@ -1258,5 +1258,111 @@ .maxstack  1
   IL_0006:  ret
 }");
         }
+
+        [Fact, WorkItem(55797, "https://github.com/dotnet/roslyn/issues/55797")]
+        public void TwoParameterlessConstructors()
+        {
+            string source = @"
+public class C
+{
+    public C() : Garbage()
+    {
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (4,12): error CS0501: 'C.C()' must declare a body because it is not marked abstract, extern, or partial
+                //     public C() : Garbage()
+                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "C").WithArguments("C.C()").WithLocation(4, 12),
+                // (4,18): error CS1018: Keyword 'this' or 'base' expected
+                //     public C() : Garbage()
+                Diagnostic(ErrorCode.ERR_ThisOrBaseExpected, "Garbage").WithLocation(4, 18),
+                // (4,18): error CS1002: ; expected
+                //     public C() : Garbage()
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "Garbage").WithLocation(4, 18),
+                // (4,18): error CS1520: Method must have a return type
+                //     public C() : Garbage()
+                Diagnostic(ErrorCode.ERR_MemberNeedsType, "Garbage").WithLocation(4, 18),
+                // (4,18): error CS0121: The call is ambiguous between the following methods or properties: 'C.C()' and 'C.C()'
+                //     public C() : Garbage()
+                Diagnostic(ErrorCode.ERR_AmbigCall, "").WithArguments("C.C()", "C.C()").WithLocation(4, 18)
+                );
+        }
+
+        [Fact, WorkItem(55797, "https://github.com/dotnet/roslyn/issues/55797")]
+        public void TwoParameterlessConstructors_2()
+        {
+            string source = @"
+public class C
+{
+    public C() : this()
+    {
+    }
+    public C()
+    {
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (4,18): error CS0121: The call is ambiguous between the following methods or properties: 'C.C()' and 'C.C()'
+                //     public C() : this()
+                Diagnostic(ErrorCode.ERR_AmbigCall, "this").WithArguments("C.C()", "C.C()").WithLocation(4, 18),
+                // (7,12): error CS0111: Type 'C' already defines a member called 'C' with the same parameter types
+                //     public C()
+                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "C").WithArguments("C", "C").WithLocation(7, 12)
+                );
+        }
+
+        [Fact, WorkItem(55797, "https://github.com/dotnet/roslyn/issues/55797")]
+        public void TwoParameterlessConstructors_3()
+        {
+            string source = @"
+public class C
+{
+    public C() : this()
+    {
+    }
+    public C2()
+    {
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (4,18): error CS0121: The call is ambiguous between the following methods or properties: 'C.C()' and 'C.C()'
+                //     public C() : this()
+                Diagnostic(ErrorCode.ERR_AmbigCall, "this").WithArguments("C.C()", "C.C()").WithLocation(4, 18),
+                // (7,12): error CS1520: Method must have a return type
+                //     public C2()
+                Diagnostic(ErrorCode.ERR_MemberNeedsType, "C2").WithLocation(7, 12)
+                );
+        }
+
+        [Fact, WorkItem(55797, "https://github.com/dotnet/roslyn/issues/55797")]
+        public void TwoParameterlessConstructors_Struct()
+        {
+            string source = @"
+public struct C
+{
+    public C() : this()
+    {
+    }
+    public C2()
+    {
+    }
+}
+";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (4,18): error CS0121: The call is ambiguous between the following methods or properties: 'C.C()' and 'C.C()'
+                //     public C() : this()
+                Diagnostic(ErrorCode.ERR_AmbigCall, "this").WithArguments("C.C()", "C.C()").WithLocation(4, 18),
+                // (7,12): error CS1520: Method must have a return type
+                //     public C2()
+                Diagnostic(ErrorCode.ERR_MemberNeedsType, "C2").WithLocation(7, 12)
+                );
+        }
     }
 }