fix Issue 19734 - isDataseg returns true for non-static declarations

src/dmd/declaration.d

@@ -1300,6 +1300,7 @@ extern (C++) class VarDeclaration : Declaration
             else if (storage_class & (STC.static_ | STC.extern_ | STC.tls | STC.gshared) ||
                 parent.isModule() || parent.isTemplateInstance() || parent.isNspace())
             {
+                assert(!isParameter() && !isResult());
                 isdataseg = 1; // It is in the DataSegment
             }
         }

test/runnable/test19734.d

@@ -0,0 +1,36 @@
+// https://issues.dlang.org/show_bug.cgi?id=19734
+// REQUIRED_ARGS: -main
+
+class C19734
+{
+    import core.stdc.stdarg;
+
+    extern
+    {
+        // Invalid 'this' parameter because of applied 'extern' storage class.
+        void testin(typeof(this) p)
+            in(this is p)
+        {
+        }
+
+        // Undefined reference to __result.
+        int testout()
+            out(; __result == 2)
+        {
+            return 2;
+        }
+
+        // Undefined reference to var.
+        int testlocal()
+        {
+            int var;
+            return var + 2;
+        }
+
+        // Variable _argptr cannot have initializer.
+        int testvarargs(...)
+        {
+            return 0;
+        }
+    }
+}