Support structs that are merely a forward reference. See mini/forwdecl1.d

gen/structs.cpp

@@ -131,11 +131,17 @@ void DtoResolveStruct(StructDeclaration* sd)
     Logger::println("DtoResolveStruct(%s): %s", sd->toChars(), sd->loc.toChars());
     LOG_SCOPE;
 
-    if (sd->prot() == PROTprivate && sd->getModule() != gIR->dmodule)
-        Logger::println("using a private struct from outside its module");
-
     TypeStruct* ts = (TypeStruct*)sd->type->toBasetype();
 
+    // this struct is a forward declaration
+    // didn't even know had those ...
+    if (sd->sizeok != 1)
+    {
+        sd->ir.irStruct = new IrStruct(ts);
+        ts->ir.type = new llvm::PATypeHolder(llvm::OpaqueType::get());
+        return;
+    }
+
     bool ispacked = (ts->alignsize() == 1);
 
     IrStruct* irstruct = new IrStruct(ts);

gen/toir.cpp

@@ -129,10 +129,10 @@ DValue* VarExp::toElem(IRState* p)
             if (vd->isDataseg() || (vd->storage_class & STCextern)) {
                 vd->toObjFile(0); // TODO: multiobj
             }
-            if (!vd->ir.isSet() || !vd->ir.getIrValue() || DtoType(vd->type)->isAbstract()) {
-                error("global variable %s not resolved", vd->toChars());
+            if (!vd->ir.isSet() || !vd->ir.getIrValue()) {
+                error("variable %s not resolved", vd->toChars());
                 if (Logger::enabled())
-                    Logger::cout() << "unresolved global had type: " << *DtoType(vd->type) << '\n';
+                    Logger::cout() << "unresolved variable had type: " << *DtoType(vd->type) << '\n';
                 fatal();
             }
             if (vd->isDataseg() || (vd->storage_class & STCextern)) {

tests/mini/forwdecl1.d

@@ -0,0 +1,12 @@
+struct Foo;
+
+Foo* foo()
+{
+    return null;
+}
+
+void main()
+{
+    Foo* f = foo();
+    assert(f is null);
+}