Collection expressions: inferred element type

meetings/working-groups/collection-literals/collection-expressions-inferred-type.md

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+# Collection expressions: inferred type
+
+## Summary
+
+Inferring an element type for collection expressions based on the elements in the collection, and choosing a containing collection type, would allow using collection expressions in locations that are implicitly-typed.
+
+## Motivation
+
+Inferring an element type would allow collection expressions to be used in `foreach` and in spread elements.
+In these cases, the inferred *element type* is observable, but the containing *collection type* is not observable.
+
+```csharp
+foreach (var i in [x, y, z]) { }
+
+int[] items = [x, y, .. b ? [z] : []];
+```
+
+If the compiler chooses a specific containing *collection type*, collection expressions could be used in other implicitly-typed locations where the collection type is observable.
+
+```csharp
+var a = [x, y];                       // var
+var b = [x, y].Where(e => e != null); // extension methods
+var c = Identity([x, y]);             // type inference: T Identity<T>(T)
+```
+
+## Inferred element type
+
+The *element type* `E` inferred for a collection expressions is the [*best common type*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#116315-finding-the-best-common-type-of-a-set-of-expressions) of the elements, where for each element `Eᵢ`:
+* If `Eᵢ` is an *expression element*, the contribution is the *type* of `Eᵢ`. If `Eᵢ` does not have a type, there is no contribution.
+* If `Eᵢ` is a *spread element* `..Sᵢ`, the contribution is the [*iteration type*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/statements.md#1295-the-foreach-statement) of `Sᵢ`. If `Sᵢ` does not have a type, there is no contribution.
+
+If there is no *best common type*, the collection expression has no type.
+
+Inferring the element type is sufficient for scenarios such as `foreach` and spreads where the *collection type* is *not observable*. For those cases, the compiler may use any conforming representation for the collection, including eliding the collection instance altogether.
+
+```csharp
+foreach (var i in [1, .. b ? [2, 3] : []]) { } // ok: collection of int
+foreach (var i in []) { }                      // error: cannot determine element type
+foreach (var i in [1, null]) { }               // error: no common type for int, <null>
+```
+
+## Natural collection type
+
+For implicitly-typed scenarios where the *collection type* is observable, the compiler needs to choose a specific collection type in addition to inferring the element type.
+
+The choice of collection type has a few implications:
+- **Mutability**: Can the collection instance or the elements be modified?
+- **Allocations**: How many allocations are required to create the instance?
+- **`IEnumerable<T>`**: Is the collection instance implicitly convertible to `IEnumerable<T>`, and perhaps other collection interfaces?
+- **Non-type arguments**: Does the collection type support elements that are not valid as type arguments, such as pointers or `ref struct`?
+- **Async code**: Can the collection be used in `async` code or an iterator?
+
+The table below includes some possible collection types, and implications for each type.
+
+|Collection type|Mutable|Allocs|`IEnumerable<T>`|Non-type args|Async|Details|
+|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
+|`T[]`|elements only|1|Yes|pointers|Yes| |
+|`List<T>`|Yes|2|Yes|No|Yes| |
+|`ReadOnlySpan<T>`|No|0/1|No|No|No|stack/heap allocated buffer|
+|`ReadOnlyMemory<T>`|No|1|Yes|No|Yes|heap allocated buffer|
+|`IEnumerable<T>`|No|1+|Yes|No|Yes|context-dependent implementation|
+|*Anonymous type*|?|1+|Yes|Yes|Yes|compiler-generated type|
+
+## Breaking changes
+
+Previously, collection expressions [*conversions*](https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/collection-expressions.md#conversions) relied on *conversion from expression* to a target type.
+And previously, collection expression [*type inference*](https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/collection-expressions.md#type-inference) relied on input and output type inference for the elements.
+For scenarios where the natural type of the collection could be used instead, there is a potential breaking change.
+
+If the natural type is `IEnumerable<T>`, the following is a breaking change due to the **conversion** from `List<int>` to `IEnumerable<int>`
+
+```csharp
+bool b = true;
+List<int> x = [1, 2, 3];
+var y = b ? x : [4]; // y: previously List<int>, now IEnumerable<int>
+```
+
+The following is a breaking change in overload resolution due to **type inference** from the natural type to `T`:
+
+```csharp
+Log([1, 2, 3]); // previously Log<T>(IEnumerable<T>), now ambiguous
+
+void Log<T>(T item) { ... }
+void Log<T>(IEnumerable<T> items) { ... }
+```
+
+## Open questions
+
+### Support `foreach` and spread over typeless collection expressions?
+
+Should we support collection expressions in `foreach` expressions and in spread elements, even if we decide not to support a natural *collection type* in general?
+
+What are the set of locations where the collection type is not observable that should be supported?
+
+### Collection type?
+
+If we do support a natural *collection type*, which collection type should we use? See the table above for some considerations.
+
+### Conversions from type?
+
+Is the natural type considered for *conversion from type*?
+
+For instance, can a collection expression with natural type be assigned to `object`?
+
+```csharp
+object obj = [1, 2, 3]; // convert col<int> to object?
+
+[Value([1, 2, 3])]      // convert col<int> to object?
+static void F() { }
+
+class ValueAttribute : Attribute
+{
+    public ValueAttribute(object value) { }
+}
+```
+
+### Target-type `foreach` collection?
+
+Should the collection expression be target-typed when used in `foreach` with an *explicitly typed iteration variable*?
+
+In short, if the iteration variable type is explicitly typed as `E`, should we use `col<E>` as the target type of the `foreach` expression?
+
+```csharp
+foreach (bool? b in [false, true, null]) { } // target type: col<bool?>?
+foreach (byte b in [1, 2, 3]) { }            // target type: col<byte>?
+```
+
+### Target-type spread collection?
+
+If a spread element is contained in a *target-typed* collection expression, should the spread element expression be target-typed?
+
+In short, if the containing collection expression has a target type with *element type* `E`, should we use `col<E>` as the target type for any spread element expressions?
+
+```csharp
+int[] x = [1, ..[]];           // spread target type: col<int>?
+object[] y = [2, ..[default]]; // spread target type: col<object>?
+```