Collection literals: output type inferences #7293
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| @@ -317,6 +317,13 @@ A *collection literal inference* is made *from* an expression `E` *to* a type ` | |||
| - Otherwise, if `Eᵢ` has type `Uᵢ` then a *lower-bound inference* is made *from* `Uᵢ` *to* `Tₑ`. | |||
| - Otherwise, no inferences are made for `Eᵢ`. | |||
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| > 11.6.3.7 Output type inferences | |||
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It looks like the spec has it as 12.6.3.7 now. Don't know whether there's a need for us to keep in sync.
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The links here are to standard-v6 currently. We could update all to v7 in the future and update the section numbers at the same time.
proposals/collection-literals.md
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| > An *output type inference* is made *from* an expression `E` *to* a type `T` in the following way: | ||
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| > - **If `E` is a *collection literal* with elements `Eᵢ` and `T` is a [*collection type*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/statements.md#1295-the-foreach-statement) with an [*iteration type*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/statements.md#1295-the-foreach-statement) `Tₑ`, then an *output type inference* is made *from* each `Eᵢ` *to* `Tₑ`.** |
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I didn't understand why this part is necessary. I thought the language earlier in this section (L314-L318) covers the same thing.
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Maybe for stuff like M(([1], ["a"]))?
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The output type inference change is to walk into elements to infer from the output types of elements, such as the lambda return type in:
F([null, () => 1]);
static void F<T>(Func<T>[] arg) { }
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| > 11.6.3.2 The first phase | ||
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| > For each of the method arguments `Eᵢ`: | ||
| > - An *input type inference* is made *from* `Eᵢ` *to* the corresponding *parameter type* `Tᵢ`. |
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Extracted existing rules to an input type inference section based on suggestion from @MadsTorgersen.
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| > An *input type inference* is made *from* an expression `E` *to* a type `T` in the following way: | ||
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| > - If `E` is a *collection literal* with elements `Eᵢ` and `T` is a [*collection type*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/statements.md#1295-the-foreach-statement) with an [*iteration type*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/statements.md#1295-the-foreach-statement) `Tₑ`, then an *input type inference* is made *from* each `Eᵢ` *to* `Tₑ`. |
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I'm a little concerned about the interaction of this rule with tuples. I think what the LDM wants is multi-level walks here, where a tuple of collection literals would walk through multiple levels if T is such a type, but this wouldn't do that.
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The rule for collection literals and the rule for tuple literals both use this input type inference rule for their parts.
| The [*type inference*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#1163-type-inference) rules are **updated** to include inferences from collection literal expressions. | ||
| The [*type inference*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#1163-type-inference) rules are updated as follows. | ||
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| The existing rules for the [*first phase*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#11632-the-first-phase) are extracted to a new *input type inference* section, and a rule is added to *input type inference* and *output type inference* for collection literal expressions. |
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| The existing rules for the [*first phase*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#11632-the-first-phase) are extracted to a new *input type inference* section, and a rule is added to *input type inference* and *output type inference* for collection literal expressions. | |
| The existing rules for the [*first phase*](https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/expressions.md#11632-the-first-phase) are extracted to a new *input type inference* section, and a rule is added to *input type inference* and *output type inference* for collection literal expressions. |
Add rules for collection literal expressions to input type inference and output type inference sections.