[diff.cpp20.expr]

# Annex C (informative) Compatibility [[diff]](./#diff)

## C.2 C++ and ISO C++ 2020 [[diff.cpp20]](diff.cpp20#expr)

### C.2.3 [[expr]](expr "7 Expressions"): expressions [diff.cpp20.expr]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L501)

**Affected subclause:** [[expr.prim.id.unqual]](expr.prim.id.unqual)  


**Change:** Change move-eligible [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]")*s* from lvalues to xvalues[.](#1.sentence-1)

  


**Rationale:** Simplify the rules for implicit move[.](#1.sentence-2)

  


**Effect on original feature:** Valid C++ 2020 code that relies on a returned [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]")'s
being an lvalue may change behavior or fail to compile[.](#1.sentence-3)

[*Example [1](#example-1)*: decltype(auto) f(int&& x) { return (x); } // returns int&&; previously returned int&int& g(int&& x) { return x; } // ill-formed; previously well-formed — *end example*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L516)

**Affected subclause:** [[expr.sub]](expr.sub)  


**Change:** Change the meaning of comma in subscript expressions[.](#2.sentence-1)

  


**Rationale:** Enable repurposing a deprecated syntax to support multidimensional indexing[.](#2.sentence-2)

  


**Effect on original feature:** Valid C++ 2020 code that uses a comma expression within a
subscript expression may fail to compile[.](#2.sentence-3)

[*Example [2](#example-2)*: arr[1, 2] // was equivalent to arr[(1, 2)],// now equivalent to arr.operator[](1, 2) or ill-formed — *end example*]