Allow to initialize a struct with other struct

parse.c

@@ -715,6 +715,18 @@ static void initializer2(Token **rest, Token *tok, Initializer *init) {
   }
 
   if (init->ty->kind == TY_STRUCT) {
+    // A struct can be initialized with another struct. E.g.
+    // `struct T x = y;` where y is a variable of type `struct T`.
+    // Handle that case first.
+    if (!equal(tok, "{")) {
+      Node *expr = assign(rest, tok);
+      add_type(expr);
+      if (expr->ty->kind == TY_STRUCT) {
+        init->expr = expr;
+        return;
+      }
+    }
+
     struct_initializer(rest, tok, init);
     return;
   }
@@ -755,7 +767,7 @@ static Node *create_lvar_init(Initializer *init, Type *ty, InitDesg *desg, Token
     return node;
   }
 
-  if (ty->kind == TY_STRUCT) {
+  if (ty->kind == TY_STRUCT && !init->expr) {
     Node *node = new_node(ND_NULL_EXPR, tok);
 
     for (Member *mem = ty->members; mem; mem = mem->next) {

test/initializer.c

@@ -55,6 +55,8 @@ int main() {
   ASSERT(5, ({ typedef struct {int a,b,c,d,e,f;} T; T x={1,2,3,4,5,6}; T y; y=x; y.e; }));
   ASSERT(2, ({ typedef struct {int a,b;} T; T x={1,2}; T y, z; z=y=x; z.b; }));
 
+  ASSERT(1, ({ typedef struct {int a,b;} T; T x={1,2}; T y=x; y.a; }));
+
   printf("OK\n");
   return 0;
 }