[Proposal]: Variable declarations under disjunctive patterns

[333fred]

Variable declarations under disjunctive patterns

• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/pattern-variables.md
• Discussion: Proposal: Permit variable declarations under disjunctive patterns #8621

Summary

• Allow variable declarations under or and not patterns and across case labels in a switch section to share code.

   if (e is C c or Wrapper { Prop: C c })
       return c;
  
   Expr Simplify(Expr e)
   {
     switch (e) {
       case Mult(Const(1), var x):
       case Mult(var x, Const(1)): 
       case Add(Const(0), var x):
       case Add(var x, Const(0)):
   	return Simplify(x);
       // ..
     }
   }

  Instead of:

   if (e is C c1) 
       return c1;
   if (e is Wrapper { Prop: C c2 }) 
       return c2;
  
   Expr Simplify(Expr e)
   {
     switch (e) {
       case Mult(Const(1), var x):
   	return Simplify(x);
       case Mult(var x, Const(1)): 
   	return Simplify(x);
       case Add(Const(0), var x):
   	return Simplify(x);
       case Add(var x, Const(0)):
   	return Simplify(x);
       // ..
     }
   }

• Also relax single-declaration rules within expression boundaries as long as each variable is assigned once.

   if (e is C c || e is Wrapper { Prop: C c }) ;
   if (b ? e is C c : e is Wrapper { Prop: C c }) ;

• Also relax single-declaration rules within conditions of an if/else:

   if (e is C c) { }
       else if (e is Wrapper { Prop: C c }) { }

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