Fix #21098 (#21132)

By reverting #10718 - it seems superfluous nowadays?

compiler/test/compilable/imports/test21098_phobos.d

@@ -0,0 +1,77 @@
+struct Nullable(T)
+{
+    static struct DontCallDestructorT
+    {
+        T payload;
+    }
+
+    DontCallDestructorT _value;
+
+    string toString() const
+    {
+        Appender!string app;
+        formatValueImpl(app, _value);
+        return null;
+    }
+}
+
+
+
+struct Appender(A)
+{
+    InPlaceAppender!A impl;
+}
+
+struct InPlaceAppender(T)
+{
+    static void toStringImpl(const T[] data)
+    {
+        string app;
+        formatValue(app, data);
+    }
+}
+
+
+
+void formatValueImpl(Writer, T)(Writer, const(T)) {}
+
+void formatValueImpl(Writer, T)(Writer w, T obj)
+if (is(T == U[], U))
+{
+    formatValue(w, obj[0]);
+}
+
+enum HasToStringResult
+{
+    none,
+    bla
+}
+
+template hasToString(T)
+{
+    static if (is(typeof(
+        (T val) {
+            val.toString(s);
+        })))
+        enum hasToString = HasToStringResult.bla;
+    else
+        enum hasToString = HasToStringResult.none;
+}
+
+void formatValueImpl(Writer, T)(ref Writer w, T val)
+if (is(T == struct) || is(T == union))
+{
+    static if (hasToString!T)
+        int dummy;
+    formatElement(w, val.tupleof);
+}
+
+void formatElement(Writer, T)(Writer w, T val)
+{
+    formatValueImpl(w, val);
+}
+
+void formatValue(Writer, T)(Writer w, T val)
+{
+    formatValueImpl(w, val);
+}

compiler/test/compilable/imports/test21098b.d

@@ -0,0 +1,12 @@
+import imports.test21098_phobos : Appender, Nullable;
+
+struct Type {
+    Nullable!(Type[]) templateArgs;
+}
+
+Type[] parseDeclarations() {
+    Appender!(Type[]) members;
+    return null;
+}
+
+enum ast = parseDeclarations();

compiler/test/compilable/test21098.d

@@ -0,0 +1,4 @@
+// https://github.com/dlang/dmd/issues/21098
+
+// EXTRA_FILES: imports/test21098b.d imports/test21098_phobos.d
+import imports.test21098b;