Function return type inference (#826)

Allow `auto` as a return type for functions. Only support functions with one `return` statement for now (open question). This explicitly suggests removing the executable semantics `fn name(args) => expression` syntax, and is motivated by reconciling executable semantics with approved Carbon state. [example](https://github.com/carbon-language/carbon-lang/blob/3d1716f6c692a840d8b4b513ddfb8119432a5150/executable_semantics/testdata/fun_named_params.carbon) This aspect is a decision that may be affected by lambda syntax, but we might also choose to keep lambda syntax and function syntax separate -- I don't think there's enough benefit to providing the alternate function syntax right now. Co-authored-by: Geoff Romer <[email protected]> Co-authored-by: Chandler Carruth <[email protected]>
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+# Function return type inference
+
+<!--
+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
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/826)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Problem](#problem)
+-   [Background](#background)
+    -   [Lambdas](#lambdas)
+    -   [`auto` keyword](#auto-keyword)
+    -   [`const` qualification](#const-qualification)
+-   [Proposal](#proposal)
+-   [Details](#details)
+    -   [Explicit return required](#explicit-return-required)
+    -   [Executable semantics changes](#executable-semantics-changes)
+    -   [Disallow direct recursion](#disallow-direct-recursion)
+        -   [Indirect recursion](#indirect-recursion)
+-   [Rationale based on Carbon's goals](#rationale-based-on-carbons-goals)
+-   [Open questions](#open-questions)
+    -   [Multiple returns](#multiple-returns)
+    -   [`const` qualification](#const-qualification-1)
+-   [Alternatives considered](#alternatives-considered)
+    -   [Only allow `auto` return types if parameters are generic](#only-allow-auto-return-types-if-parameters-are-generic)
+    -   [Provide alternate function syntax for concise return type inference](#provide-alternate-function-syntax-for-concise-return-type-inference)
+    -   [Allow separate declaration and definition](#allow-separate-declaration-and-definition)
+
+<!-- tocstop -->
+
+## Problem
+
+Should there be a shorter way of declaring a function? This embodies two
+questions:
+
+-   Should there be a way for a declarer to ask that the return type of a
+    function be inferred from returned values?
+-   Should there be an alternate function syntax that provides briefer function
+    declarations for simple cases of return type inference?
+
+## Background
+
+Under
+[Proposal #438: Add statement syntax for function declarations](https://github.com/carbon-language/carbon-lang/pull/438),
+the syntax approved was:
+
+> `fn` _identifier_ `(` _args_ `)` _[_ `->` _expression ]_ `{` _statements_ `}`
+
+Executable semantics currently supports the syntax:
+
+> `fn` _identifier_ `(` _args_ `) =>` _expression_
+
+In C++, there is similar automatic type inference for return types, although the
+use of `=>` reflects syntax tentatively discussed for matching use.
+
+### Lambdas
+
+Lambdas are also under discussion for Carbon: however, a different syntax is
+likely to arise. This proposal does not try to address lambda syntax, although
+it is possible that a decision on lambda syntax may lead to an alternate syntax
+for declaring functions in order to maintain syntax parity.
+
+### `auto` keyword
+
+This is the first proposal to formally include a use of `auto`; although
+[`var` statement #339](p0339.md) mentions it in an alternative, the proposal did
+not explicitly propose the keyword. However, the `var x: auto` use-case is
+expected in Carbon, and so uses of `auto` here can be considered in that
+context.
+
+Note the `auto` keyword name and behavior can generally be considered consistent
+with C++.
+
+### `const` qualification
+
+C++ initially used the matching return type for lambdas. However, it switched to
+its `auto` variable rules, which are defined in terms of template argument
+deduction and discard `const` qualifiers. This yields subtle behavioral
+differences.
+
+Note that `const` is not yet defined in Carbon.
+
+## Proposal
+
+Support automatic type inference in Carbon with a new `auto` keyword, as in:
+
+> `fn` _identifier_ `(` _args_ `) -> auto {` _statements_ `}`
+
+For now, only one return statement is allowed: we will avoid defining rules for
+handling differing return types.
+
+Separate declarations and definitions are not supported with `auto` return types
+because the declaration must have a known return type.
+
+## Details
+
+### Explicit return required
+
+Functions using `-> auto` must return an expression; using the implicit return
+or `return;` is invalid. This is consistent with the
+[design for returning empty tuples](/docs/design/control_flow/return.md#returning-empty-tuples).
+
+### Executable semantics changes
+
+Executable semantics will remove the `=>` function syntax.
+
+In practice, this means the current executable semantics syntax:
+
+```
+fn Add(x: i32, y: i32) => x + y;
+```
+
+Becomes:
+
+```
+fn Add(x: i32, y: i32) -> auto { return x + y; }
+```
+
+### Disallow direct recursion
+
+Direct recursive calls can create complexities in inferring the return type. For
+example:
+
+```
+// In the return statement.
+fn Factorial(x: i32) -> auto {
+    return (if x == 1 then x else x * Factorial(x - 1));
+}
+
+// Before the return statement, but affecting the return type.
+fn Factorial(x: i32) -> auto {
+    var x: auto = (if x == 1 then x else x * Factorial(x - 1));
+    return x;
+}
+```
+
+As a consequence, direct recursion is rejected: that is, a function with an
+`auto` return type may not call itself.
+
+#### Indirect recursion
+
+Indirect recursion will typically look like:
+
+```
+fn ExplicitReturn() -> i32;
+fn AutoReturn() -> auto { return ExplicitReturn(); }
+fn ExplicitReturn() -> i32 { return AutoReturn(); }
+```
+
+This is valid because the return type of `AutoReturn()` can be calculated to be
+`i32` from the forward declaration of `ExplicitReturn`.
+
+Due to how name lookup works, there is no risk of indirect recursion causing
+problems for typing:
+
+-   A similar separate declaration of an `auto` return is
+    [disallowed](#proposal).
+-   Without a separate declaration, name lookup would fail. That is, the below
+    has a name lookup error at `return B();`:
+
+    ```
+    fn A() -> auto { return B(); }
+    fn B() -> auto { return A(); }
+    ```
+
+As a consequence, no specific rules about indirect recursion are needed, unlike
+direct recursion.
+
+## Rationale based on Carbon's goals
+
+-   [Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write)
+
+    -   We are avoiding providing an alternative function syntax in order to
+        provide users less syntax they'll need to understand.
+    -   As a practical measure, there are likely to be cases in generic code
+        that are more feasible to write.
+
+-   [Interoperability with and migration from existing C++ code](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code)
+
+    -   The intent is to have `auto` as a return type work similarly to C++'s
+        type inference, in order to ease migration.
+
+## Open questions
+
+### Multiple returns
+
+It's likely that we should support `auto` on functions with multiple returns.
+This proposal declines to address that use-case because of the complexities
+which arise when return types may differ. Instead, the use-case is left for
+future proposals to handle.
+
+### `const` qualification
+
+As noted in background, [`const` qualification](#const-qualification) has seen
+changes in C++. While we should work to choose a story that can remain
+long-term, this will likely be revisited when rules around deduction for
+templates are addressed.
+
+## Alternatives considered
+
+### Only allow `auto` return types if parameters are generic
+
+`auto` return types are likely to be a negative impact to readability, because a
+reader must read the function body in order to determine the return type. We
+could detect whether parameters used in determining an `auto` return type are
+generic, and only allow `auto` to be used when they are.
+
+Advantages:
+
+-   Limits the readability impact of `auto`.
+    -   `auto` could not be used where the return type is clearly deterministic,
+        and thus easy to write. In these cases, readers would never need to look
+        at the function body to determine the return.
+    -   `auto` could be used where the return type may be difficult to easily
+        write as a consequence of potential generic parameters.
+
+Disadvantages:
+
+-   Increases the rule set around use of `auto` in return types.
+-   Breaks C++ compatibility.
+
+The decision to allow `auto` in more cases is primarily for C++ compatibility.
+
+### Provide alternate function syntax for concise return type inference
+
+Executable semantics currently demonstrates an alternate function syntax with:
+
+```
+fn Add(x: i32, y: i32) => x + y;
+```
+
+This is a more concise syntax that builds upon return type inference. We could
+keep that, or come up with some other syntax.
+
+Advantages:
+
+-   For short functions, provides a more succinct manner of defining the
+    function.
+-   `=>` use echoes tentative matching syntax.
+
+Disadvantages:
+
+-   Creates an additional syntax which can be used for equivalent behavior.
+-   Overlaps with lambda use-cases, but lambdas will likely end up looking
+    different; that is, we should not assume the syntax will converge.
+    -   We may in particular take a more sharply divergent syntax for lambdas,
+        in order to allow for significant brevity in typing, which may not make
+        sense to support for function declarations.
+-   Limits use of `=>` for other purposes.
+    -   To the extent that `=>` might primarily be replacing `-> auto`, assuming
+        Carbon adopted Rust-style
+        [block expression returns](https://doc.rust-lang.org/reference/expressions/block-expr.html),
+        this offers limited value for the additional token use.
+
+This proposal suggests not adding an inference-focused alternate function syntax
+at this time; while consistent with tentative matching syntax, the split from
+function syntax is significant. The
+[one way principle](/docs/project/principles/one_way.md) applies here.
+
+If an alternate function syntax is added, it should be done in tandem with a
+lambda proposal in order to ensure consistency in syntax.
+
+### Allow separate declaration and definition
+
+The return type must be possible to determine from the declaration. As a
+consequence, the declaration and definition of a function returning `auto`
+cannot be significantly separated: a caller must be able to see the definition.
+Thus, although a separate declaration would be insufficient for a call, we could
+allow using `auto` with a separate declaration and definition when the caller
+can see _both_.
+
+This example would be valid because `CallAdd` can see the `Add` definition:
+
+```
+fn Add(x: i32, y: i32) -> auto;
+
+fn Add(x: i32, y: i32) -> auto { return x + y; }
+
+fn CallAdd() -> i32 { return Add(1, 2); }
+```
+
+However, the following example would be invalid because `CallAdd` can only see
+the `Add` declaration (even though the definition may be in the same file), and
+it would need the definition to determine the return type:
+
+```
+fn Add(x: i32, y: i32) -> auto;
+
+fn CallAdd() -> i32 { return Add(1, 2); }
+
+fn Add(x: i32, y: i32) -> auto { return x + y; }
+```
+
+Advantages:
+
+-   Separating the declaration and definition could be used in files to cluster
+    brief declarations, then put definitions below.
+    -   A particularly common use-case of this might be in class declarations,
+        where a user might want to offer a declaration and define it out of line
+        where it doesn't interrupt the class's API.
+    -   It may be preferred to only use `auto` return types with short
+        functions, which limits this advantage as a short function could easily
+        be inlined.
+
+Disadvantages:
+
+-   Cannot be used to break call cycles, which is the usual use of separate
+    declarations and definitions.
+    -   A separate declaration still cannot be called until after the definition
+        is provided.
+    -   This may be particularly confusing to humans.
+
+The decision is to not support separate declarations and definitions with `auto`
+return types due to the limited utility.