Merge v6 standard updates into the v7 branch

.gitattributes

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+###############################################################################
+# Set default behavior to automatically normalize line endings.
+###############################################################################
+* text=auto
+
+###############################################################################
+# Set default behavior for command prompt diff.
+#
+# This is need for earlier builds of msysgit that does not have it on by
+# default for csharp files.
+# Note: This is only used by command line
+###############################################################################
+#*.cs     diff=csharp
+
+###############################################################################
+# Set the merge driver for project and solution files
+#
+# Merging from the command prompt will add diff markers to the files if there
+# are conflicts (Merging from VS is not affected by the settings below, in VS
+# the diff markers are never inserted). Diff markers may cause the following 
+# file extensions to fail to load in VS. An alternative would be to treat
+# these files as binary and thus will always conflict and require user
+# intervention with every merge. To do so, just uncomment the entries below
+###############################################################################
+#*.sln       merge=binary
+#*.csproj    merge=binary
+#*.vbproj    merge=binary
+#*.vcxproj   merge=binary
+#*.vcproj    merge=binary
+#*.dbproj    merge=binary
+#*.fsproj    merge=binary
+#*.lsproj    merge=binary
+#*.wixproj   merge=binary
+#*.modelproj merge=binary
+#*.sqlproj   merge=binary
+#*.wwaproj   merge=binary
+
+###############################################################################
+# behavior for image files
+#
+# image files are treated as binary by default.
+###############################################################################
+#*.jpg   binary
+#*.png   binary
+#*.gif   binary
+
+###############################################################################
+# diff behavior for common document formats
+# 
+# Convert binary document formats to text before diffing them. This feature
+# is only available from the command line. Turn it on by uncommenting the 
+# entries below.
+###############################################################################
+#*.doc   diff=astextplain
+#*.DOC   diff=astextplain
+#*.docx  diff=astextplain
+#*.DOCX  diff=astextplain
+#*.dot   diff=astextplain
+#*.DOT   diff=astextplain
+#*.pdf   diff=astextplain
+#*.PDF   diff=astextplain
+#*.rtf   diff=astextplain
+#*.RTF   diff=astextplain

.github/workflows/dependencies/ReplaceAndAdd.md

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+# Replacements & Additions
+
+A “replacement & additions” file can be passed to `BuildGrammar`’s `--modification-file`/`-m`
+option and contains replacement and additional rules to be added to: verify/test a grammar,
+explore/test new language features, etc. It is a markdown (`.md`) file which follows the same
+conventions as those used for the Standard.
+
+The file is processed by `BuildGrammar` similarly to other markdown files; clauses are
+noted and text in ```` ```ANTLR ... ``` ```` code blocks is “parsed” (being generous here);
+as ANTLR rules, mode commands, and comments (and nothing else, don’t go trying to set options
+or other stuff); and added to the corresponding section in the produced grammar (`.g4`) file.
+
+The rules add to, or replace, existing rules in the corresponding clause; new rules are added
+to the end of the clause’s rule list, replacements occur in-place. Further in the replacement
+case the previous rule is kept as a comment, and this happens regardless of the previous rules
+clause i.e. a replacement can move a rule to a different clause.
+
+A rule is only a replacement if it changes the rule’s definition or clause.
+Rules with are neither additions or replacements are ignored.
+
+> *Note:* This means that a
+replacement & additions file can actually be a complete Standard section markdown file
+and only the changed rules will be extracted to update the grammar. So while working, say,
+on a PR if the original Standard’s section files are passed to `BuildGrammar` as input files,
+the section files changed by the PR are passed as modification files, then the resultant grammar will
+reflect the PR’s changes with the old rules as comments.
+
+> **Important:** section numbers in replacement & additions files are not maintained by
+the automatic section numbering tooling, they **must** be maintained manually.
+
+---
+
+# Verification-Only Replacements & Additions
+
+This set of replacements and additions is the bare minimum required to allow the grammar to verify and run, though
+it may not produce the desired parse (that requires at least the use of modes and/or
+lexical predicates).
+
+This set can be used as a basic check that the grammar is a correct ANTLR grammar.
+
+---
+
+## Top Level Rule
+
+The Standard’s *compilation_unit* as is will allow garbage at the end of a file, this
+rule has an EOF requirement to ensure the whole of the input must be a correct program.
+
+> *Note: The section number makes this the first rule in the grammar, not required but it
+has to go somewhere…*
+
+### 0.0.0 Top Level Rule
+
+```ANTLR
+// [ADDED] Rule added as the start point
+prog: compilation_unit EOF;
+```
+---
+
+## Discarding Whitespace
+
+The following changes in §7.3.2, §7.3.3 and §7.3.4, add `-> skip` to the “whitespace”
+token rules so that are not passed to the parser. This behaviour is implicit in the
+Standard.
+
+### 7.3.2 Line terminators
+
+```ANTLR
+// [SKIP]
+New_Line
+    : ( New_Line_Character
+      | '\u000D\u000A'    // carriage return, line feed 
+      ) -> skip
+    ;
+```
+
+### 7.3.3 Comments
+
+```ANTLR
+// [SKIP]
+Comment
+    : ( Single_Line_Comment
+      | Delimited_Comment
+      ) -> skip
+    ;
+```
+
+### 7.3.4 White space
+
+```ANTLR
+// [SKIP]
+Whitespace
+    : ( [\p{Zs}]  // any character with Unicode class Zs
+      | '\u0009'  // horizontal tab
+      | '\u000B'  // vertical tab
+      | '\u000C'  // form feed
+      ) -> skip
+    ;
+
+```
+
+---
+
+## Pre-processing directives
+
+This change causes all pre-processor directives to be discarded, they don’t need to be
+processed to validate the grammar (processing them would exercise the *implementation*
+of the pre-processor, which is not part of the Standard).
+
+### 7.5.1 General
+
+```ANTLR
+// [CHANGE] Discard pre-processor directives
+PP_Directive
+    : (PP_Start PP_Kind PP_New_Line) -> skip
+    ;
+```
+
+---
+
+## Mutual Left Recursion Removal
+
+All but one mutual left recursive (MLR) group has been removed from the grammar (and we should
+strive not to introduce any new ones).
+
+This change resolves the one remaining MLR group by inlining some of the non-terminal
+alternatives in *primary_no_array_creation_expression*.
+
+Non-terminals that are inlined are commented out and the inlined body is indented.
+
+This change has not been made to the Standard itself as it makes *primary_no_array_creation_expression*
+“uglier” and would obfuscate somewhat the description in the Standard.
+
+As MLR is not supported by ANTLR without this change the grammar would be rejected.
+
+### 12.7.1 General
+
+```ANTLR
+// [CHANGE] This removes a mutual left-recursion group which we have (currently?)
+// [CHANGE] decided to leave in the Standard. Without this change the grammar will fail.
+primary_no_array_creation_expression
+    : literal
+    | simple_name
+    | parenthesized_expression
+    // | member_access
+        | primary_no_array_creation_expression '.' identifier type_argument_list?
+        | array_creation_expression '.' identifier type_argument_list?
+        | predefined_type '.' identifier type_argument_list?
+        | qualified_alias_member '.' identifier type_argument_list?
+    // | invocation_expression
+        | primary_no_array_creation_expression '(' argument_list? ')'
+        | array_creation_expression '(' argument_list? ')'
+    // | element_access and pointer_element_access (unsafe code support)
+        | primary_no_array_creation_expression '[' argument_list ']'
+    | this_access
+    | base_access
+    // | post_increment_expression
+        | primary_no_array_creation_expression '++'
+        | array_creation_expression '++'
+    // | post_decrement_expression
+        | primary_no_array_creation_expression '--'
+        | array_creation_expression '--'
+    | object_creation_expression
+    | delegate_creation_expression
+    | anonymous_object_creation_expression
+    | typeof_expression
+    | sizeof_expression
+    | checked_expression
+    | unchecked_expression
+    | default_value_expression
+    | nameof_expression
+    | anonymous_method_expression
+    // | pointer_member_access     // unsafe code support
+        | primary_no_array_creation_expression '->' identifier type_argument_list?
+        | array_creation_expression '->' identifier type_argument_list?
+    // | pointer_element_access    // unsafe code support
+        // covered by element_access replacement above
+    ;
+```

.github/workflows/grammar-validator.yaml

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+name: ANTLR Grammar validator
+
+# Triggers the workflow on pull request events that update the branch
+on: 
+  pull_request:
+    types: [opened, synchronize, reopened]
+  workflow_dispatch:
+    inputs:
+      reason:
+        description: 'The reason for running the workflow'
+        required: true
+        default: 'Manual run'
+
+jobs:
+  grammar-validator:
+    runs-on: ubuntu-18.04
+    env:
+      DOTNET_NOLOGO: true
+
+    steps:
+    - name: Check out our repo
+      uses: actions/checkout@v2
+
+    - name: Setup .NET 6.0
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 6.0.x
+
+    - name: Set up JDK 15
+      uses: actions/setup-java@v1
+      with:
+        java-version: 15.0
+
+      # Install build grammar global tool
+    - name: Install BuildGrammar tool
+      run: |
+        dotnet tool install --version 1.0.0-alpha.1 --global --add-source ./.github/workflows/dependencies/ EcmaTC49.BuildGrammar
+
+
+    - name: run validate
+      run: |
+        cd tools
+        ./validate-grammar.sh

.github/workflows/markdown-links-verifier.yaml

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+name: Markdown links verifier
+on: pull_request
+
+jobs:
+  validate_links:
+    name: Markdown links verifier
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout the repository
+      uses: actions/checkout@v1
+
+    - name: Validate links
+      uses: Youssef1313/markdown-links-verifier@v0.1.3

.github/workflows/markdownlint-problem-matcher.json

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+{
+    "problemMatcher": [
+        {
+            "owner": "markdownlint",
+            "pattern": [
+                {
+                    "regexp": "^([^:]*):(\\d+):?(\\d+)?\\s([\\w-\\/]*)\\s(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "code": 4,
+                    "message": 5
+                }
+            ]
+        }
+    ]
+}

.github/workflows/markdownlint.yml

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+name: Markdownlint
+
+on:
+  push:
+    branches:
+      - draft-v6
+    paths:
+      - "standard/*.md"
+      - ".markdownlint.json"
+  pull_request:
+    paths:
+      - "standard/*.md"
+      - ".markdownlint.json"
+  workflow_dispatch:
+    inputs:
+      reason:
+        description: 'The reason for running the workflow'
+        required: true
+        default: 'Manual run'
+
+jobs:
+  lint:
+
+    runs-on: ubuntu-latest
+    permissions:
+        statuses: write
+
+    steps:
+    - uses: actions/checkout@a81bbbf8298c0fa03ea29cdc473d45769f953675 #@v2
+    - name: Use Node.js
+      uses: actions/setup-node@56899e050abffc08c2b3b61f3ec6a79a9dc3223d #@v1
+      with:
+        node-version: 12.x
+    - name: Run Markdownlint
+      run: |
+        echo "::add-matcher::.github/workflows/markdownlint-problem-matcher.json"
+        npm i -g markdownlint-cli
+        markdownlint "standard/*.md"

.github/workflows/renumber-sections.yaml

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+name: Renumber standard TOC
+
+# Triggers the workflow on pull request events that update the branch
+on: 
+  pull_request:
+    types: [opened, synchronize, reopened]
+  workflow_dispatch:
+    inputs:
+      reason:
+        description: 'The reason for running the workflow'
+        required: true
+        default: 'Manual run'
+
+jobs:
+  renumber-sections:
+    runs-on: ubuntu-18.04
+    env:
+      DOTNET_NOLOGO: true
+
+    steps:
+    - name: Check out our repo
+      uses: actions/checkout@v2
+
+    - name: Setup .NET 6.0
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 6.0.x
+
+    - name: Run section renumbering dry run
+      run: |
+        cd tools
+        ./run-section-renumber.sh --dryrun