diff --git a/compiler/src/dmd/aggregate.d b/compiler/src/dmd/aggregate.d
index 982e914f9dd3..becc864b4de4 100644
--- a/compiler/src/dmd/aggregate.d
+++ b/compiler/src/dmd/aggregate.d
@@ -25,7 +25,7 @@ import dmd.declaration;
 import dmd.dscope;
 import dmd.dstruct;
 import dmd.dsymbol;
-import dmd.dsymbolsem : dsymbolSemantic, determineFields, search;
+import dmd.dsymbolsem : dsymbolSemantic, determineFields, search, determineSize;
 import dmd.dtemplate;
 import dmd.errors;
 import dmd.expression;
@@ -187,70 +187,12 @@ extern (C++) abstract class AggregateDeclaration : ScopeDsymbol
         return fields.length - isNested() - (vthis2 !is null);
     }
 
-    /***************************************
-     * Collect all instance fields, then determine instance size.
-     * Returns:
-     *      false if failed to determine the size.
-     */
-    extern (D) final bool determineSize(const ref Loc loc)
-    {
-        //printf("AggregateDeclaration::determineSize() %s, sizeok = %d\n", toChars(), sizeok);
-
-        // The previous instance size finalizing had:
-        if (type.ty == Terror || errors)
-            return false;   // failed already
-        if (sizeok == Sizeok.done)
-            return true;    // succeeded
-
-        if (!members)
-        {
-            .error(loc, "%s `%s` unknown size", kind, toPrettyChars);
-            return false;
-        }
-
-        if (_scope)
-            dsymbolSemantic(this, null);
-
-        // Determine the instance size of base class first.
-        if (auto cd = isClassDeclaration())
-        {
-            cd = cd.baseClass;
-            if (cd && !cd.determineSize(loc))
-                goto Lfail;
-        }
-
-        // Determine instance fields when sizeok == Sizeok.none
-        if (!this.determineFields())
-            goto Lfail;
-        if (sizeok != Sizeok.done)
-            finalizeSize();
-
-        // this aggregate type has:
-        if (type.ty == Terror)
-            return false;   // marked as invalid during the finalizing.
-        if (sizeok == Sizeok.done)
-            return true;    // succeeded to calculate instance size.
-
-    Lfail:
-        // There's unresolvable forward reference.
-        if (type != Type.terror)
-            error(loc, "%s `%s` no size because of forward reference", kind, toPrettyChars);
-        // Don't cache errors from speculative semantic, might be resolvable later.
-        // https://issues.dlang.org/show_bug.cgi?id=16574
-        if (!global.gag)
-        {
-            type = Type.terror;
-            errors = true;
-        }
-        return false;
-    }
-
     abstract void finalizeSize();
 
     override final uinteger_t size(const ref Loc loc)
     {
         //printf("+AggregateDeclaration::size() %s, scope = %p, sizeok = %d\n", toChars(), _scope, sizeok);
-        bool ok = determineSize(loc);
+        bool ok = determineSize(this, loc);
         //printf("-AggregateDeclaration::size() %s, scope = %p, sizeok = %d\n", toChars(), _scope, sizeok);
         return ok ? structsize : SIZE_INVALID;
     }
diff --git a/compiler/src/dmd/dsymbolsem.d b/compiler/src/dmd/dsymbolsem.d
index e7e054930cc1..c2f8bdfc036f 100644
--- a/compiler/src/dmd/dsymbolsem.d
+++ b/compiler/src/dmd/dsymbolsem.d
@@ -7647,3 +7647,61 @@ private Expression callScopeDtor(VarDeclaration vd, Scope* sc)
     }
     return e;
 }
+
+/***************************************
+ * Collect all instance fields, then determine instance size.
+ * Returns:
+ *      false if failed to determine the size.
+ */
+bool determineSize(AggregateDeclaration ad, const ref Loc loc)
+{
+    //printf("AggregateDeclaration::determineSize() %s, sizeok = %d\n", toChars(), sizeok);
+
+    // The previous instance size finalizing had:
+    if (ad.type.ty == Terror || ad.errors)
+        return false;   // failed already
+    if (ad.sizeok == Sizeok.done)
+        return true;    // succeeded
+
+    if (!ad.members)
+    {
+        .error(loc, "%s `%s` unknown size", ad.kind, ad.toPrettyChars);
+        return false;
+    }
+
+    if (ad._scope)
+        dsymbolSemantic(ad, null);
+
+    // Determine the instance size of base class first.
+    if (auto cd = ad.isClassDeclaration())
+    {
+        cd = cd.baseClass;
+        if (cd && !cd.determineSize(loc))
+            goto Lfail;
+    }
+
+    // Determine instance fields when sizeok == Sizeok.none
+    if (!ad.determineFields())
+        goto Lfail;
+    if (ad.sizeok != Sizeok.done)
+        ad.finalizeSize();
+
+    // this aggregate type has:
+    if (ad.type.ty == Terror)
+        return false;   // marked as invalid during the finalizing.
+    if (ad.sizeok == Sizeok.done)
+        return true;    // succeeded to calculate instance size.
+
+Lfail:
+    // There's unresolvable forward reference.
+    if (ad.type != Type.terror)
+        error(loc, "%s `%s` no size because of forward reference", ad.kind, ad.toPrettyChars);
+    // Don't cache errors from speculative semantic, might be resolvable later.
+    // https://issues.dlang.org/show_bug.cgi?id=16574
+    if (!global.gag)
+    {
+        ad.type = Type.terror;
+        ad.errors = true;
+    }
+    return false;
+}
diff --git a/compiler/src/dmd/mtype.d b/compiler/src/dmd/mtype.d
index 7243a9998cca..d40b92b17a1a 100644
--- a/compiler/src/dmd/mtype.d
+++ b/compiler/src/dmd/mtype.d
@@ -30,6 +30,7 @@ import dmd.dtemplate;
 import dmd.enumsem;
 import dmd.errors;
 import dmd.expression;
+import dmd.dsymbolsem : determineSize;
 import dmd.globals;
 import dmd.hdrgen;
 import dmd.id;
diff --git a/compiler/src/dmd/safe.d b/compiler/src/dmd/safe.d
index bb4e3f49873b..24d874585516 100644
--- a/compiler/src/dmd/safe.d
+++ b/compiler/src/dmd/safe.d
@@ -21,6 +21,7 @@ import dmd.dcast : implicitConvTo;
 import dmd.dclass;
 import dmd.declaration;
 import dmd.dscope;
+import dmd.dsymbolsem : determineSize;
 import dmd.errors;
 import dmd.expression;
 import dmd.func;