https://issues.dlang.org/show_bug.cgi?id=17330

          Issue ID: 17330
           Summary: [REG 2.072] BigInt's constructor used to be pure
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: regression
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: ag0aep6g@gmail.com

Found by Russel Winder who posted to D.learn: http://forum.dlang.org/post/mailman.1458.1491584811.31550.digitalmars-d-learn@puremagic.com

This used to work, and it should be made to work again:

----
void main()
{
    import std.bigint: BigInt;
    auto b = immutable BigInt("123");
}
----

Fails compilation since 2.072.0. Regression was introduced by <https://github.com/dlang/phobos/pull/3876>.

The issue is that purity cannot be inferred for the constructor. It used to be explicitly marked `pure` when it wasn't a template.

I've previously filed this as a compiler bug, thinking that a templated constructor should infer `pure`. That issue has been closed as WONTFIX. See issue 17309.

I'm not sure if this can be fixed satisfactorily. So this may end up as WONTFIX as well. But that would be quite disappointing.

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