as expressions (#845)

This proposal provides an `as` expression for casting. This supports implicit conversions plus some safe and unsurprising conversions that we do not support implicitly:

* lossy but fully defined conversions to floating-point types
* conversion from `bool` to integer types
* conversion between adaptors and their adapted type, and more generally between compatible types

This facility can be extended by implementing the `As(TargetType)` interface for a type.

Co-authored-by: Geoff Romer <[email protected]>
Co-authored-by: josh11b <[email protected]>

docs/design/expressions/README.md

@@ -11,7 +11,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 ## Table of contents
 
 -   [Overview](#overview)
--   [Implicit conversions](#implicit-conversions)
+-   [Conversions and casts](#conversions-and-casts)
 
 <!-- tocstop -->
 
@@ -29,12 +29,15 @@ fn Foo(a: i32*) -> i32 {
 Here, the parameter type `i32*`, the return type `i32`, and the operand `*a` of
 the `return` statement are all expressions.
 
-## Implicit conversions
+## Conversions and casts
 
 When an expression appears in a context in which an expression of a specific
 type is expected, [implicit conversions](implicit_conversions.md) are applied to
 convert the expression to the target type.
 
+Expressions can also be converted to a specific type using an
+[`as` expression](as_expressions.md).
+
 ```
 fn Bar(n: i32);
 fn Baz(n: i64) {

docs/design/expressions/as_expressions.md

@@ -0,0 +1,178 @@
+# `as` expressions
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Overview](#overview)
+-   [Precedence and associativity](#precedence-and-associativity)
+-   [Built-in types](#built-in-types)
+    -   [Data types](#data-types)
+    -   [Compatible types](#compatible-types)
+-   [Extensibility](#extensibility)
+-   [Alternatives considered](#alternatives-considered)
+-   [References](#references)
+
+<!-- tocstop -->
+
+## Overview
+
+An expression of one type can be explicitly cast to another type by using an
+`as` expression:
+
+```
+var n: i32 = Get();
+var f: f32 = n as f32;
+```
+
+An `as` expression can be used to perform any implicit conversion, either when
+the context does not imply a destination type or when it is valuable to a reader
+of the code to make the conversion explicit. In addition, `as` expressions can
+perform safe conversions that nonetheless should not be performed implicitly,
+such as lossy conversions or conversions that lose capabilities or change the
+way a type would be interpreted.
+
+As guidelines, an `as` conversion should be permitted when:
+
+-   The conversion is _complete_: it produces a well-defined output value for
+    each input value.
+-   The conversion is _unsurprising_: the resulting value is the expected value
+    in the destination type.
+
+For example:
+
+-   A conversion from `fM` to `iN` is not complete, because it is not defined
+    for input values that are out of the range of the destination type, such as
+    infinities or, if `N` is too small, large finite values.
+-   A conversion from `iM` to `iN`, where `N` < `M`, is either not complete or
+    not unsurprising, because there is more than one possible expected behavior
+    for an input value that is not within the destination type, and those
+    behaviors are not substantially the same -- we could perform two's
+    complement wrapping, saturate, or produce undefined behavior analogous to
+    arithmetic overflow.
+-   A conversion from `iM` to `fN` can be unsurprising, because even though
+    there may be a choice of which way to round, the possible values are
+    substantially the same.
+
+It is possible for user-defined types to [extend](#extensibility) the set of
+valid explicit casts that can be performed by `as`. Such extensions are expected
+to follow these guidelines.
+
+## Precedence and associativity
+
+`as` expressions are non-associative.
+
+```
+var b: bool = true;
+// OK
+var n: i32 = (b as i1) as i32;
+var m: auto = b as (bool as Hashable);
+// Error, ambiguous
+var m: auto = b as T as U;
+```
+
+The `as` operator has lower precedence than operators that visually bind
+tightly:
+
+-   prefix symbolic operators
+    -   dereference (`*a`)
+    -   negation (`-a`)
+    -   complement (`~a`)
+-   postfix symbolic operators
+    -   pointer type formation (`T*`),
+    -   function call (`a(...)`),
+    -   array indexing (`a[...]`), and
+    -   member access (`a.m`).
+
+The `as` operator has higher precedence than assignment and comparison. It is
+unordered with respect to binary arithmetic, bitwise operators, and unary `not`.
+
+```
+// OK
+var x: i32* as Comparable;
+// OK, `x as (U*)` not `(x as U)*`.
+var y: auto = x as U*;
+
+var a: i32;
+var b: i32;
+// OK, `(a as i64) < ((*x) as i64)`.
+if (a as i64 < *x as i64) {}
+// Ambiguous: `(a + b) as i64` or `a + (b as i64)`?
+var c: i32 = a + b as i64;
+// Ambiguous: `(a as i64) + b` or `a as (i64 + b)`?
+var d: i32 = a as i64 + b;
+
+// OK, `(-a) as f64`, not `-(a as f64)`.
+// Unfortunately, the former is undefined if `a` is `i32.MinValue()`;
+// the latter is not.
+var u: f64 = -a as f64;
+
+// OK, `i32 as (GetType())`, not `(i32 as GetType)()`.
+var e: i32 as GetType();
+```
+
+## Built-in types
+
+### Data types
+
+In addition to the [implicit conversions](implicit_conversions.md#data-types),
+the following numeric conversions are supported by `as`:
+
+-   `iN`, `uN`, or `fN` -> `fM`, for any `N` and `M`. Values that cannot be
+    exactly represented are suitably rounded to one of the two nearest
+    representable values. Very large finite values may be rounded to an
+    infinity. NaN values are converted to NaN values.
+
+-   `bool` -> `iN` or `uN`. `false` converts to `0` and `true` converts to `1`
+    (or to `-1` for `i1`).
+
+Conversions from numeric types to `bool` are not supported with `as`; instead of
+using `as bool`, such conversions can be performed with `!= 0`.
+
+Lossy conversions between `iN` or `uN` and `iM` or `uM` are not supported with
+`as`, and similarly conversions from `fN` to `iM` are not supported.
+
+**Future work:** Add mechanisms to perform these conversions.
+
+### Compatible types
+
+The following conversion is supported by `as`:
+
+-   `T` -> `U` if `T` is
+    [compatible](../generics/terminology.md#compatible-types) with `U`.
+
+**Future work:** We may need a mechanism to restrict which conversions between
+adapters are permitted and which code can perform them. Some of the conversions
+permitted by this rule may only be allowed in certain contexts.
+
+## Extensibility
+
+Explicit casts can be defined for user-defined types such as
+[classes](../classes.md) by implementing the `As` interface:
+
+```
+interface As(Dest:! Type) {
+  fn Convert[me: Self]() -> Dest;
+}
+```
+
+The expression `x as U` is rewritten to `x.(As(U).Convert)()`.
+
+## Alternatives considered
+
+-   [Do not distinguish between safe and unsafe casts](/docs/proposals/p0845.md#merge-as-and-assume_as)
+-   [Do not distinguish between `as` and implicit conversions](/docs/proposals/p0845.md#as-only-performs-implicit-conversions)
+-   [Allow `iN as bool`](/docs/proposals/p0845.md#integer-to-bool-conversions)
+-   [Disallow `bool as iN`](/docs/proposals/p0845.md#bool-to-integer-conversions)
+
+## References
+
+-   [Implicit conversions in C++](https://en.cppreference.com/w/cpp/language/implicit_conversion)
+-   Proposal
+    [#845: `as` expressions](https://github.com/carbon-language/carbon-lang/pull/845).

docs/design/expressions/implicit_conversions.md

@@ -21,7 +21,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
     -   [Pointer conversions](#pointer-conversions)
         -   [Pointer conversion examples](#pointer-conversion-examples)
     -   [Type-of-types](#type-of-types)
--   [Semantics](#semantics)
+-   [Consistency with `as`](#consistency-with-as)
 -   [Extensibility](#extensibility)
 -   [Alternatives considered](#alternatives-considered)
 -   [References](#references)
@@ -245,37 +245,33 @@ can be implicitly converted to the type-of-type `TT2` if `T`
 [satisfies the requirements](../generics/details.md#subtyping-between-type-of-types)
 of `TT2`.
 
-## Semantics
+## Consistency with `as`
 
 An implicit conversion of an expression `E` of type `T` to type `U`, when
-permitted, always has the same meaning as the explicit cast expression `E as U`.
-Moreover, such an implicit conversion is expected to exactly preserve the value.
-For example, `(E as U) as T`, if valid, should be expected to result in the same
-value as produced by `E`.
-
-**Note:** The explicit cast expression syntax has not yet been decided. The use
-of `E as T` in this document is provisional.
+permitted, always has the same meaning as the
+[explicit cast expression `E as U`](as_expressions.md). Moreover, because such
+an implicit conversion is expected to exactly preserve the value,
+`(E as U) as T`, if valid, should be expected to result in the same value as
+produced by `E` even if the `as T` cast cannot be performed as an implicit
+conversion.
 
 ## Extensibility
 
 Implicit conversions can be defined for user-defined types such as
-[classes](../classes.md) by implementing the `ImplicitAs` interface:
+[classes](../classes.md) by implementing the `ImplicitAs` interface, which
+extends
+[the `As` interface used to implement `as` expressions](as_expressions.md#extensibility):
 
 ```
-interface As(Dest:! Type) {
-  fn Convert[me: Self]() -> Dest;
+interface ImplicitAs(Dest:! Type) extends As(Dest) {
+  // Inherited from As(Dest):
+  // fn Convert[me: Self]() -> Dest;
 }
-interface ImplicitAs(Dest:! Type) extends As(Dest) {}
 ```
 
 When attempting to implicitly convert an expression `x` to type `U`, the
 expression is rewritten to `x.(ImplicitAs(U).Convert)()`.
 
-**Note:** The `As` interface is intended to be used as the implementation
-vehicle for explicit casts: `x as U` would be rewritten as
-`x.(As(U).Convert)()`. However, the explicit cast expression syntax has not yet
-been decided, so this rewrite is provisional.
-
 Note that implicit conversions are not transitive. Even if an
 `impl A as ImplicitAs(B)` and an `impl B as ImplicitAs(C)` are both provided, an
 expression of type `A` cannot be implicitly converted to type `C`. Allowing

docs/design/generics/details.md

@@ -1631,7 +1631,7 @@ addition to using the same data representation, they both implement one
 interface, `Hashable`, and use the same implementation for that interface. The
 one difference between them is that `Song as Hashable` may be implicitly
 converted to `Song`, which implements interface `Printable`, and
-`PlayableSong as Hashable` may be implicilty converted to `PlayableSong`, which
+`PlayableSong as Hashable` may be implicitly converted to `PlayableSong`, which
 implements interface `Media`. This means that it is safe to convert between
 `HashMap(Song, i32)` and `HashMap(PlayableSong, i32)` (though maybe only with an
 explicit cast), since the implementation of all the methods will use the same

docs/design/lexical_conventions/words.md

@@ -37,6 +37,7 @@ The following words are interpreted as keywords:
 -   `alias`
 -   `and`
 -   `api`
+-   `as`
 -   `auto`
 -   `base`
 -   `break`