Disambiguation for zero- and one-element positional patterns

[gafter]

From today's LDM:

Pattern-Matching

Nople pattern syntax

Is () OK as a zero-element positional pattern? Is var () OK?

Resolution: Yes for both. To be scheduled.

Oneple pattern syntax

We previously said that a type is required in a positional pattern with one element. But that doesn't work as a disambiguation.

What about this

  bool M(SomeTypeThatImplementsItuple it) => it is WhatType(3) {P:4};

Here, what type could we possibly write if we are trying to match using ITuple?

I think a better disambiguation rule is to require a property subpattern (even if the empty {}) whenever a positional pattern has only one nested pattern and no type.

Approved (resolution): the presence of a property part disambiguates from an expression even without a type. To be scheduled.

[gafter]

Implemented in #40543