Fix a bug when default value is struct type

Author: hadashiA

src/MemoryPack.Generator/MemoryPackGenerator.Emitter.cs

@@ -1381,7 +1381,7 @@ string EmitConstantValue(object? constantValue)
                 _ => constantValue.ToString()
             };
         }
-        return "null";
+        return "default!";
     }
 
     string EmitExpression(ExpressionSyntax expression)
@@ -1431,9 +1431,9 @@ static string GetTypeFullName(ITypeSymbol typeSymbol)
         if (typeSymbol.ContainingType is { } containingType)
         {
             // nested type
-            var containgTypeFullName = containingType.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat);
+            var containingTypeFullName = containingType.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat);
             var typeName = typeSymbol.ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat);
-            return $"{containgTypeFullName}.{typeName}";
+            return $"{containingTypeFullName}.{typeName}";
         }
         return typeSymbol.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat);
     }

tests/MemoryPack.Tests/Models/DefaultValues.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 
 namespace MemoryPack.Tests.Models;
@@ -26,14 +27,21 @@ partial class FieldDefaultValue
 [MemoryPackable]
 partial class PropertyDefaultValue
 {
+    internal enum NestedEnum
+    {
+        A, B
+    }
+
     public int X { get; set; }
     public int Y { get; set; } = 12345;
     public float Z { get; set; } = 678.9f;
     public string S { get; set; } = "aaaaaaaaa";
     public bool B { get; set; } = true;
     public List<string> Alpha { get; set; } = new List<string>(new HashSet<string>());
-    public TestEnum TestEnum { get; set; } = TestEnum.A;
+    public TestEnum E { get; set; } = TestEnum.A;
+    public NestedEnum E2 { get; set; } = NestedEnum.A;
     public (TestEnum, List<string>) Tuple { get; set; } = (TestEnum.A, new List<string>(new HashSet<string>()));
+    public DateTime Struct { get; set; } = default!;
 }
 
 [MemoryPackable]
@@ -44,14 +52,18 @@ partial class CtorParamDefaultValue
     public float Z;
     public string S;
     public bool B;
+    public decimal D;
+    public DateTime StructValue;
 
     [MemoryPackConstructor]
-    public CtorParamDefaultValue(int x, int y = 12345, float z = 678.9f, string s = "aaaaaa", bool b = true, decimal d = 99M)
+    public CtorParamDefaultValue(int x, int y = 12345, float z = 678.9f, string s = "aaaaaa", bool b = true, decimal d = 99M, DateTime structValue = default)
     {
         X = x;
         Y = y;
         Z = z;
         S = s;
         B = b;
+        D = d;
+        StructValue = structValue;
     }
 }