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I was facing the same issue while running docker containers with flask.

I downgraded Flask to 1.1.4 and markupsafe to 2.0.1 which solved my issue.

Check this for reference.

Looks like a bug to me. Modifying case A to case nullptr gives the following error message (on the template definition):

You are correct, this is due to match ergonomics. The first case should hopefully be self explanatory, but the second and third cases can be a bit counter-intuitive.

In the second case:

• (x,) is a non-reference pattern (see the second example in the RFC). The t tuple reference is dereferenced, and x is bound as a ref as it also is a non-reference pattern. Note that t.0 was a reference to begin with, thus resulting in x being a double reference.

• (&y,) is also a non-reference pattern. The t tuple is dereferenced again to a (&i32,). However, &y is a reference pattern being matched to a &i32 reference. Hence y is bound with move mode and is an i32.

In the third case:

• Using the same reasoning as the second case, u is dereferenced via Deref coercion to an (i32,), and x, a non-reference pattern, is bound in ref mode. Hence x is an &i32.

• Again with the same reasoning as the second case, u is dereferenced to an (i32,). The &y reference pattern is then matched to an i32, a non-reference, which causes an error.

This problem has a fun O(n) solution.

If you draw a graph of cumulative sum vs index, then:

The average value in the subarray between any two indexes is the slope of the line between those points on the graph.

The first highest-average-prefix will end at the point that makes the highest angle from 0. The next highest-average-prefix must then have a smaller average, and it will end at the point that makes the highest angle from the first ending. Continuing to the end of the array, we find that...

These segments of highest average are exactly the segments in the upper convex hull of the cumulative sum graph.

Find these segments using the monotone chain algorithm. Since the points are already sorted, it takes O(n) time.

I fixed it by changing uses value to

• uses: google-github-actions/setup-gcloud@master

References can't bind to objects with different type directly. Given const int& s = u;, u is implicitly converted to int firstly, which is a temporary, a brand-new object and then s binds to the temporary int. (Lvalue-references to const (and rvalue-references) could bind to temporaries.) The lifetime of the temporary is prolonged to the lifetime of s, i.e. it'll be destroyed when get out of main.

It is correct that the constructor template is generally a better match for the constructor call with argument of type DerivedFoo& or Foo& than the copy constructors are, since it doesn't require a const conversion.

However, [over.match.funcs.general]/8 essentially (almost) says, in more general wording, that an inherited constructor that would have the form of a move or copy constructor is excluded from overload resolution, even if it is instantiated from a constructor template. Therefore the template constructor will not be considered.

Therefore the implicit copy constructor of DerivedFoo will be chosen by overload resolution for

The original code can be re-written in the following way:

This is Clang bug #49513; the situation and analysis is similar to this answer.

sizeof(T)>0 is an atomic constraint, so [temp.constr.atomic]/3 applies:

To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression, the constraint is not satisfied. [...]

sizeof(void)>0 is an invalid expression, so that constraint is not satisfied, and constraint evaluation proceeds to sizeof(U)>0.

As in the linked question, an alternative workaround is to use "requires requires requires"; demo: