diff --git a/compiler/src/dmd/dtoh.d b/compiler/src/dmd/dtoh.d
index 6badbada3b32..f8b8ec822ffa 100644
--- a/compiler/src/dmd/dtoh.d
+++ b/compiler/src/dmd/dtoh.d
@@ -536,6 +536,13 @@ public:
     }
 
     /// Checks whether `t` is a type that can be exported to C++
+    private static bool containsSizeT(AST.Type t)
+    {
+        for (auto tp = t.isTypePointer(); tp; tp = t.isTypePointer())
+            t = tp.next;
+        return t == AST.Type.tsize_t || t == AST.Type.tptrdiff_t;
+    }
+
     private bool isSupportedType(AST.Type t)
     {
         if (!t)
@@ -951,7 +958,7 @@ public:
             return;
         }
 
-        if (vd.originalType && vd.type == AST.Type.tsize_t)
+        if (vd.originalType && containsSizeT(vd.type))
             origType = vd.originalType;
         scope(exit) origType = null;
 
@@ -1131,9 +1138,10 @@ public:
                 return;
             }
 
-            // for function pointers we need to original type
-            if (ad.originalType && ad.type.ty == AST.Tpointer &&
-                (cast(AST.TypePointer)t).nextOf.ty == AST.Tfunction)
+            // for function pointers and size_t/ptrdiff_t we need the original type
+            if (ad.originalType &&
+                ((ad.type.ty == AST.Tpointer && (cast(AST.TypePointer)t).nextOf.ty == AST.Tfunction) ||
+                 containsSizeT(ad.type)))
             {
                 origType = ad.originalType;
             }
@@ -1414,7 +1422,10 @@ public:
                         buf.writestring(", ");
                     assert(vd.type);
                     assert(vd.ident);
+                    if (vd.originalType && containsSizeT(vd.type))
+                        origType = vd.originalType;
                     typeToBuffer(vd.type, vd, true);
+                    origType = null;
                     // Don't print default value for first parameter to not clash
                     // with the default ctor defined above
                     if (!first)
@@ -3012,6 +3023,11 @@ public:
      */
     private void ensureDeclared(AST.Dsymbol sym)
     {
+        // size_t and ptrdiff_t are provided by #include <stddef.h>
+        if (auto ad = sym.isAliasDeclaration())
+            if (ad.ident == Id._size_t || ad.ident == Id._ptrdiff_t)
+                return;
+
         auto par = sym.toParent2();
         auto ed = sym.isEnumDeclaration();
 
diff --git a/compiler/src/dmd/frontend.h b/compiler/src/dmd/frontend.h
index a5b710cfd503..ee13aea91141 100644
--- a/compiler/src/dmd/frontend.h
+++ b/compiler/src/dmd/frontend.h
@@ -676,7 +676,7 @@ class Dsymbol : public ASTNode
 
 struct BitArray final
 {
-    typedef uint64_t Chunk_t;
+    typedef size_t Chunk_t;
     enum : uint64_t { ChunkSize = 8LLU };
 
     enum : uint64_t { BitsPerChunk = 64LLU };
@@ -684,14 +684,14 @@ struct BitArray final
 private:
     ~BitArray();
     size_t len;
-    uint64_t* ptr;
+    size_t* ptr;
 public:
     BitArray() :
         len(),
         ptr()
     {
     }
-    BitArray(uint64_t len, uint64_t* ptr = nullptr) :
+    BitArray(size_t len, size_t* ptr = nullptr) :
         len(len),
         ptr(ptr)
         {}
diff --git a/compiler/test/compilable/dtoh_18015.d b/compiler/test/compilable/dtoh_18015.d
new file mode 100644
index 000000000000..f86a9d3120b8
--- /dev/null
+++ b/compiler/test/compilable/dtoh_18015.d
@@ -0,0 +1,48 @@
+/*
+REQUIRED_ARGS: -HC -c -o-
+PERMUTE_ARGS:
+TEST_OUTPUT:
+---
+// Automatically generated by Digital Mars D Compiler
+
+#pragma once
+
+#include <assert.h>
+#include <math.h>
+#include <stddef.h>
+#include <stdint.h>
+
+struct BitArray final
+{
+    typedef size_t Chunk_t;
+    size_t len;
+    size_t* ptr;
+    BitArray() :
+        len(),
+        ptr()
+    {
+    }
+    BitArray(size_t len, size_t* ptr = nullptr) :
+        len(len),
+        ptr(ptr)
+        {}
+};
+
+typedef ptrdiff_t MyDiff;
+
+---
+*/
+
+// https://github.com/dlang/dmd/issues/18015
+// dtoh: Insufficient size_t/ptrdiff_t detection
+
+extern(C++):
+
+struct BitArray
+{
+    alias Chunk_t = size_t;
+    size_t len;
+    size_t* ptr;
+}
+
+alias MyDiff = ptrdiff_t;