C# 7.x: out Variables #44

RexJaeschke · 2020-11-24T18:20:55Z

The handling of discards in tuple deconstruction and pattern matching is deferred to the PRs that add support for tuples and pattern matching, respectively. This PR only handles the out parameter and assignment scenarios.

A couple of > TODO placeholders will need attention once those other PRs are created.

standard/lexical-structure.md

jcouv · 2022-08-05T19:03:19Z

standard/expressions.md

@@ -541,7 +541,7 @@ argument_name
 argument_value
    : expression
    | 'ref' variable_reference
-    | 'out' variable_reference
+    | 'out' local_variable_type? identifier


The speclets for out vars and deconstructions relied on declaration_expression.
But in this doc, I see 'out' local_variable_type? identifier and in the deconstruction draft I see tuple_deconstruction_expression.
I'm not sure what is the benefit of this structure.

I think having a declaration_expression that appears in multiple contexts is a good factorization. Then we don't need new syntax for deconstructions (it's declaration_expression and tuple_expression).

Tagging @cston since I'll be away.

Some more context on syntax shape (was discussed with LDM): #285

Updated to use declaration_expression.

Tagging @Nigel-Ecma for grammar checks.

declaration_expression is different and cannot simply be substituted in this location. Here local_variable_type is optional, it is not in declaration_expression. I'll find the change and add a suggestion there…

standard/expressions.md

Part of adding V7.3 support for out variables in extra places.

This text has been moved to PR 596.

Some of this text has been moved to PR 596

- Overhauled argument list text - Added better conversion text - Removed discards-in-assignment text

BillWagner · 2023-04-14T17:19:52Z

@jskeet This one is now ready for the upcoming meeting.

Latest edits:

rebase
update all links
remove any discard related text (that has been done in a separate PR.)

Nigel-Ecma

I haven't done a full review just looked at the grammar change as requested – unfortunately the change was wrong so needs reverting, suggestion in comment.

standard/expressions.md

jskeet · 2023-04-19T14:00:07Z

standard/expressions.md

@@ -1033,6 +1081,8 @@ Given an implicit conversion `C₁` that converts from an expression `E` to a ty
 - `E` exactly matches both or neither of `T₁` and `T₂`, and `T₁` is a better conversion target than `T₂` ([§12.6.4.6](expressions.md#12646-better-conversion-target))
 - `E` is a method group ([§12.2](expressions.md#122-expression-classifications)), `T₁` is compatible ([§20.4](delegates.md#204-delegate-compatibility)) with the single best method from the method group for conversion `C₁`, and `T₂` is not compatible with the single best method from the method group for conversion `C₂`

+The  conversion from an implicitly-typed out variable declaration is not considered better than any other conversion from expression.


While I know what this means, it doesn't feel terribly clear - maybe we just need a reference to the "conversion from expression" section?

I'm also not sure it adds anything. Should this paragraph be removed? For discussion.

standard/lexical-structure.md

jskeet · 2023-04-19T14:11:07Z

standard/expressions.md

@@ -6051,7 +6101,7 @@ assignment_operator

 The left operand of an assignment shall be an expression classified as a variable, a property access, an indexer access, an event access or a tuple. A declaration expression is not directly permitted as a left operand, but may occur as a step in the evaluation of a deconstructing assignment.

-The `=` operator is called the ***simple assignment operator***. It assigns the value or values of the right operand to the variable, property, indexer element or tuple elements given by the left operand. The left operand of the simple assignment operator shall not be an event access (except as described in [§15.8.2](classes.md#1582-field-like-events)). The simple assignment operator is described in [§12.21.2](expressions.md#12212-simple-assignment).
+The `=` operator is called the ***simple assignment operator***. When the left operand is not a discard (§9.2.8.1), this operator assigns the value of the right operand to the variable, property, or indexer element given by the left operand. Otherwise, the right operand is evaluated with its value going unused. The left operand of the simple assignment operator shall not be an event access (except as described in [§15.8.2](classes.md#1582-field-like-events)). The simple assignment operator is described in [§12.21.2](expressions.md#12212-simple-assignment).


Is this change and the next one intended to be in this PR? It's not really about out parameters... was it just something else that should have been in a discards PR?

That's correct. This PR in its original form included both discards and out parameters. I left this change in place because it wasn't in the updated discards and tuples PR.

Let's discuss at the meeting if it should be removed.

Action from 2023-04-19: @BillWagner to remove from this PR, but add it to the issue that @MadsTorgersen has already created for polishing on discards.

standard/lexical-structure.md

jskeet

Some simple final changes, then let's merge.

standard/expressions.md

jskeet · 2023-04-19T20:54:06Z

standard/expressions.md

@@ -6051,7 +6101,7 @@ assignment_operator

 The left operand of an assignment shall be an expression classified as a variable, a property access, an indexer access, an event access or a tuple. A declaration expression is not directly permitted as a left operand, but may occur as a step in the evaluation of a deconstructing assignment.

-The `=` operator is called the ***simple assignment operator***. It assigns the value or values of the right operand to the variable, property, indexer element or tuple elements given by the left operand. The left operand of the simple assignment operator shall not be an event access (except as described in [§15.8.2](classes.md#1582-field-like-events)). The simple assignment operator is described in [§12.21.2](expressions.md#12212-simple-assignment).
+The `=` operator is called the ***simple assignment operator***. When the left operand is not a discard (§9.2.8.1), this operator assigns the value of the right operand to the variable, property, or indexer element given by the left operand. Otherwise, the right operand is evaluated with its value going unused. The left operand of the simple assignment operator shall not be an event access (except as described in [§15.8.2](classes.md#1582-field-like-events)). The simple assignment operator is described in [§12.21.2](expressions.md#12212-simple-assignment).


Action from 2023-04-19: @BillWagner to remove from this PR, but add it to the issue that @MadsTorgersen has already created for polishing on discards.

standard/lexical-structure.md

Co-authored-by: Jon Skeet <[email protected]> Co-authored-by: Nigel-Ecma <[email protected]>

MadsTorgersen · 2023-05-17T18:24:32Z

To the best of my analysis, this PR is fully replaced by #664.

jskeet · 2023-05-17T20:13:29Z

Closing as obsolete.

RexJaeschke added this to the C# 7.x milestone Dec 8, 2020

RexJaeschke mentioned this pull request Jan 13, 2021

8.2.5 Grammar ambiguities #180

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RexJaeschke added the Review: pending Proposal is available for review label Sep 16, 2021

RexJaeschke marked this pull request as draft December 4, 2021 17:52

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Test RexJaeschke/hello-world#3

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C# 7.x: Unifying treatment of discards and local variables in patterns, deconstruction and out variables #561

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C# 7.x: Patterns #61

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Champion "out var" (C# 7.0) dotnet/csharplang#60

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Champion "discards" (C# 7.0) dotnet/csharplang#422

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RexJaeschke mentioned this pull request Jun 19, 2022

Add discard feature #596

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RexJaeschke changed the title ~~C# 7.x: Discards and out Variables~~ C# 7.x: out Variables Jun 20, 2022

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BillWagner marked this pull request as ready for review October 5, 2022 22:18

MadsTorgersen mentioned this pull request Nov 2, 2022

Discards, tuples and out variables #664

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BillWagner self-assigned this Dec 1, 2022

BillWagner force-pushed the RexJaeschke-v7-discards branch from 1f55d4c to 02f89c6 Compare February 14, 2023 16:04

BillWagner mentioned this pull request Mar 27, 2023

Add rules for ref safety #742

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RexJaeschke and others added 7 commits April 14, 2023 10:48

Update lexical-structure.md

5ff4256

Update variables.md

bb1a288

Update expressions.md

26f2727

Update classes.md

82913af

Fixup link for local variable scope to new patterns subclause

be8c954

Part of adding V7.3 support for out variables in extra places.

Fix markdown lint errors, which included a missed conflict

e63bd17

undo earlier edits

500de88

This text has been moved to PR 596.

RexJaeschke and others added 8 commits April 14, 2023 11:02

undo previous edits

2a6b252

Some of this text has been moved to PR 596

add new conversion

e0e9440

undo earlier edits

30d2371

- Overhauled argument list text - Added better conversion text - Removed discards-in-assignment text

undo earlier edits

3adc144

var-ref becomes identifier

7cd6bcc

fix spacing

f39b927

fix spacing

faa8a55

fix build warning

91d0247

BillWagner force-pushed the RexJaeschke-v7-discards branch from 93a8bee to 91d0247 Compare April 14, 2023 15:09

fix build warnings

fb0efd8

BillWagner force-pushed the RexJaeschke-v7-discards branch from 5066217 to fb0efd8 Compare April 14, 2023 15:20

Final edits.

9aace93

BillWagner force-pushed the RexJaeschke-v7-discards branch from 2ee4c22 to 9aace93 Compare April 14, 2023 17:10

BillWagner closed this Apr 14, 2023

BillWagner reopened this Apr 14, 2023

Nigel-Ecma requested changes Apr 14, 2023

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Respond to pre-meeting feedback.

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BillWagner added the meeting: discuss This issue should be discussed at the next TC49-TG2 meeting label Apr 19, 2023

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Co-authored-by: Jon Skeet <[email protected]> Co-authored-by: Nigel-Ecma <[email protected]>

feedback from April meeting

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jskeet closed this May 17, 2023

jskeet removed meeting: discuss This issue should be discussed at the next TC49-TG2 meeting Review: pending Proposal is available for review labels May 17, 2023

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C# 7.x: out Variables #44

C# 7.x: out Variables #44

RexJaeschke commented Nov 24, 2020

jcouv Aug 5, 2022

jcouv Oct 3, 2022

BillWagner Apr 14, 2023

Nigel-Ecma Apr 14, 2023

BillWagner commented Apr 14, 2023 •

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jskeet commented May 17, 2023

C# 7.x: out Variables #44

C# 7.x: out Variables #44

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RexJaeschke commented Nov 24, 2020

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MadsTorgersen commented May 17, 2023

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