Bug 20237 – with gagged errors from is(typeof()), compiler forgets that it failed to compile a method

Consider the following code: https://run.dlang.io/is/pXwdyp: struct Struct() { void method() { static assert(false); } } static Struct!() foo()() { return Struct!().init; } enum b = is(typeof(foo().method())); void main() { foo().method(); } Clearly, this code should not compile. At the latest, `main` should error out because `foo` is clearly not a valid function. However, this code hits a *linker error* instead - the frontend lets it through it with zero complaints. It seems plausible that since `foo` is only evaluated once per parameters, and `foo` is first instantiated while in the gagged `is(typeof())` context, the struct is instantiated successfully despite missing a method.

This is a dupe of 15459. *** This issue has been marked as a duplicate of issue 15459 ***