Address #1285 & #1428

1285:
- text incorrectly placed in §6.2.5 removed
- text added to §12.14.1 to cover disambiguation is-type/is-pattern
- the comment regarding “precedence at least as high" is covered
  under #1428, see HTML comment bullet below

1428:
- The suggested grammar changes reduce to changing one rule,
  *constant_expression*
- Inserted HTML comment [TODO] note in §12.14.1 regarding reviewing
  decision on “is pattern”

New sample “Pattern precedence” to check relational_expression,
and lambdas in conjunction with patterns, have correct precedence

Author: Nigel-Ecma

standard/expressions.md

@@ -4392,11 +4392,14 @@ equality_expression
     ;
 ```
 
-> *Note*: Lookup for the right operand of the `is` operator must first test as a *type*, then as an *expression* which may span multiple tokens. In the case where the operand is an *expression*, the pattern expression must have precedence at least as high as *shift_expression*. *end note*  
-<!-- markdownlint-disable MD028 -->
-
-<!-- markdownlint-enable MD028 -->
-> *Note*: There is a grammar ambiguity between *type* and *constant_pattern* in a `relational_expression` on the right-hand-side of `is`; either might be a valid parse of a qualified identifier. In such a case, only if it fails to bind as a type (for compatibility with previous versions of the language), is it resolved to be the first thing found (which must be either a constant or a type). This ambiguity is only present on the right-hand side of such an expression. *end note*
+<!--
+[TODO] PR#1428 recommended changing *pattern* on the RHS of 'is' above to restrict constant_pattern to relational_pattern to
+avoid precedence issues, this was not done. This might be required for a certain compiler but ANTLR grammar semantics does the
+right thing, maybe not obviously for some, and so no change was made – making the change in the grammar is effectively harmless
+as it does not effect the resulting parse at all, but the prose gets messier. For C#9 this decision should be checked to be
+still valid after the new pattern changes.
+-->
+When recognising a *relational_expression* ([§12.14.1](expressions.md#12141-general)) if both the “*relational_expression* `is` *type*” and “*relational_expression* `is` *constant_pattern*” alternatives are applicable, and *type* resolves to an accessible type, then the “*relational_expression* `is` *type*” alternative shall be chosen.
 
 The `is` operator is described in [§12.14.12](expressions.md#121412-the-is-operator) and the `as` operator is described in [§12.14.13](expressions.md#121413-the-as-operator).
 
@@ -7151,7 +7154,7 @@ A constant expression is an expression that shall be fully evaluated at compile-
 
 ```ANTLR
 constant_expression
-    : expression
+    : conditional_expression
     ;
 ```
 

standard/lexical-structure.md

@@ -134,8 +134,6 @@ then the *type_argument_list* shall be retained as part of the disambiguated pro
 >
 > *end example*
 
-When recognising a *relational_expression* ([§12.14.1](expressions.md#12141-general)) if both the “*relational_expression* `is` *type*” and “*relational_expression* `is` *pattern*” alternatives are applicable, and *type* resolves to an accessible type, then the “*relational_expression* `is` *type*” alternative shall be chosen.
-
 ## 6.3 Lexical analysis
 
 ### 6.3.1 General

tools/GrammarTesting/Tests/Parsing/Samples/v8/Pattern precedence/ReadMe.md

@@ -0,0 +1,3 @@
+# Sample: Pattern precedence
+
+Samples showing pattern precedence in relational_expression & lambda_expression

tools/GrammarTesting/Tests/Parsing/Samples/v8/Pattern precedence/Reference/sample.gruntree.red.txt

@@ -0,0 +1,303 @@
+[@0,0:4='class',<'class'>,1:0]
+[@1,6:14='IsPattern',<Simple_Identifier>,1:6]
+[@2,16:16='{',<'{'>,2:0]
+[@3,21:24='void',<'void'>,3:3]
+[@4,26:34='IsPattern',<Simple_Identifier>,3:8]
+[@5,35:35='(',<'('>,3:17]
+[@6,36:36=')',<')'>,3:18]
+[@7,41:41='{',<'{'>,4:3]
+[@8,362:365='bool',<'bool'>,11:6]
+[@9,367:367='A',<Simple_Identifier>,11:11]
+[@10,369:369='=',<'='>,11:13]
+[@11,371:375='false',<'false'>,11:15]
+[@12,376:376=';',<';'>,11:20]
+[@13,384:388='const',<'const'>,12:6]
+[@14,390:393='bool',<'bool'>,12:12]
+[@15,395:395='B',<Simple_Identifier>,12:17]
+[@16,397:397='=',<'='>,12:19]
+[@17,399:402='true',<'true'>,12:21]
+[@18,403:403=';',<';'>,12:25]
+[@19,411:415='const',<'const'>,13:6]
+[@20,417:420='bool',<'bool'>,13:12]
+[@21,422:422='C',<Simple_Identifier>,13:17]
+[@22,424:424='=',<'='>,13:19]
+[@23,426:429='true',<'true'>,13:21]
+[@24,430:430=';',<';'>,13:25]
+[@25,438:440='int',<'int'>,14:6]
+[@26,442:442='X',<Simple_Identifier>,14:10]
+[@27,444:444='=',<'='>,14:12]
+[@28,446:446='0',<Integer_Literal>,14:14]
+[@29,447:447=';',<';'>,14:15]
+[@30,455:459='const',<'const'>,15:6]
+[@31,461:463='int',<'int'>,15:12]
+[@32,465:465='Y',<Simple_Identifier>,15:16]
+[@33,467:467='=',<'='>,15:18]
+[@34,469:470='42',<Integer_Literal>,15:20]
+[@35,471:471=';',<';'>,15:22]
+[@36,479:483='const',<'const'>,16:6]
+[@37,485:487='int',<'int'>,16:12]
+[@38,489:489='Z',<Simple_Identifier>,16:16]
+[@39,491:491='=',<'='>,16:18]
+[@40,493:494='24',<Integer_Literal>,16:20]
+[@41,495:495=';',<';'>,16:22]
+[@42,510:514='const',<'const'>,18:6]
+[@43,516:519='bool',<'bool'>,18:12]
+[@44,521:521='D',<Simple_Identifier>,18:17]
+[@45,523:523='=',<'='>,18:19]
+[@46,525:525='Y',<Simple_Identifier>,18:21]
+[@47,527:528='is',<'is'>,18:23]
+[@48,530:532='int',<'int'>,18:26]
+[@49,533:533=';',<';'>,18:29]
+[@50,542:545='bool',<'bool'>,20:6]
+[@51,547:547='N',<Simple_Identifier>,20:11]
+[@52,549:549='=',<'='>,20:13]
+[@53,551:551='A',<Simple_Identifier>,20:15]
+[@54,553:554='is',<'is'>,20:17]
+[@55,556:556='B',<Simple_Identifier>,20:20]
+[@56,558:559='is',<'is'>,20:22]
+[@57,561:561='C',<Simple_Identifier>,20:25]
+[@58,562:562=';',<';'>,20:26]
+[@59,598:601='bool',<'bool'>,21:6]
+[@60,603:603='O',<Simple_Identifier>,21:11]
+[@61,605:605='=',<'='>,21:13]
+[@62,607:607='A',<Simple_Identifier>,21:15]
+[@63,609:610='is',<'is'>,21:17]
+[@64,612:612='(',<'('>,21:20]
+[@65,613:613='Y',<Simple_Identifier>,21:21]
+[@66,615:616='is',<'is'>,21:23]
+[@67,618:620='int',<'int'>,21:26]
+[@68,621:621=')',<')'>,21:29]
+[@69,622:622=';',<';'>,21:30]
+[@70,637:640='bool',<'bool'>,23:6]
+[@71,642:642='P',<Simple_Identifier>,23:11]
+[@72,644:644='=',<'='>,23:13]
+[@73,646:646='X',<Simple_Identifier>,23:15]
+[@74,648:649='is',<'is'>,23:17]
+[@75,651:651='Y',<Simple_Identifier>,23:20]
+[@76,653:653='>',<'>'>,23:22]
+[@77,654:654='>',<'>'>,23:23]
+[@78,656:656='4',<Integer_Literal>,23:25]
+[@79,657:657=';',<';'>,23:26]
+[@80,693:696='bool',<'bool'>,24:6]
+[@81,698:698='Q',<Simple_Identifier>,24:11]
+[@82,700:700='=',<'='>,24:13]
+[@83,702:702='(',<'('>,24:15]
+[@84,703:703='X',<Simple_Identifier>,24:16]
+[@85,705:706='is',<'is'>,24:18]
+[@86,708:708='Y',<Simple_Identifier>,24:21]
+[@87,709:709=')',<')'>,24:22]
+[@88,711:711='>',<'>'>,24:24]
+[@89,712:712='>',<'>'>,24:25]
+[@90,714:714='4',<Integer_Literal>,24:27]
+[@91,715:715=';',<';'>,24:28]
+[@92,730:733='bool',<'bool'>,26:6]
+[@93,735:735='R',<Simple_Identifier>,26:11]
+[@94,737:737='=',<'='>,26:13]
+[@95,739:739='A',<Simple_Identifier>,26:15]
+[@96,741:742='is',<'is'>,26:17]
+[@97,744:744='B',<Simple_Identifier>,26:20]
+[@98,746:747='<=',<'<='>,26:22]
+[@99,749:749='C',<Simple_Identifier>,26:25]
+[@100,750:750=';',<';'>,26:26]
+[@101,786:789='bool',<'bool'>,27:6]
+[@102,791:791='S',<Simple_Identifier>,27:11]
+[@103,793:793='=',<'='>,27:13]
+[@104,795:795='A',<Simple_Identifier>,27:15]
+[@105,797:798='is',<'is'>,27:17]
+[@106,800:800='(',<'('>,27:20]
+[@107,801:801='Y',<Simple_Identifier>,27:21]
+[@108,803:804='<=',<'<='>,27:23]
+[@109,806:806='4',<Integer_Literal>,27:26]
+[@110,807:807=')',<')'>,27:27]
+[@111,808:808=';',<';'>,27:28]
+[@112,823:826='bool',<'bool'>,29:6]
+[@113,828:828='T',<Simple_Identifier>,29:11]
+[@114,830:830='=',<'='>,29:13]
+[@115,832:832='A',<Simple_Identifier>,29:15]
+[@116,834:835='is',<'is'>,29:17]
+[@117,837:837='B',<Simple_Identifier>,29:20]
+[@118,839:840='==',<'=='>,29:22]
+[@119,842:842='C',<Simple_Identifier>,29:25]
+[@120,843:843=';',<';'>,29:26]
+[@121,879:882='bool',<'bool'>,30:6]
+[@122,884:884='U',<Simple_Identifier>,30:11]
+[@123,886:886='=',<'='>,30:13]
+[@124,888:888='A',<Simple_Identifier>,30:15]
+[@125,890:891='is',<'is'>,30:17]
+[@126,893:893='(',<'('>,30:20]
+[@127,894:894='Y',<Simple_Identifier>,30:21]
+[@128,896:897='==',<'=='>,30:23]
+[@129,899:899='Z',<Simple_Identifier>,30:26]
+[@130,900:900=')',<')'>,30:27]
+[@131,901:901=';',<';'>,30:28]
+[@132,916:919='bool',<'bool'>,32:6]
+[@133,921:921='V',<Simple_Identifier>,32:11]
+[@134,923:923='=',<'='>,32:13]
+[@135,925:925='A',<Simple_Identifier>,32:15]
+[@136,927:928='is',<'is'>,32:17]
+[@137,930:930='B',<Simple_Identifier>,32:20]
+[@138,932:933='||',<'||'>,32:22]
+[@139,935:935='C',<Simple_Identifier>,32:25]
+[@140,936:936=';',<';'>,32:26]
+[@141,973:976='bool',<'bool'>,33:6]
+[@142,978:978='W',<Simple_Identifier>,33:11]
+[@143,980:980='=',<'='>,33:13]
+[@144,982:982='A',<Simple_Identifier>,33:15]
+[@145,984:985='is',<'is'>,33:17]
+[@146,987:987='(',<'('>,33:20]
+[@147,988:988='B',<Simple_Identifier>,33:21]
+[@148,990:991='||',<'||'>,33:23]
+[@149,993:993='C',<Simple_Identifier>,33:26]
+[@150,994:994=')',<')'>,33:27]
+[@151,995:995=';',<';'>,33:28]
+[@152,1043:1045='int',<'int'>,36:6]
+[@153,1046:1046='?',<'?'>,36:9]
+[@154,1048:1049='Yn',<Simple_Identifier>,36:11]
+[@155,1051:1051='=',<'='>,36:14]
+[@156,1053:1053='Y',<Simple_Identifier>,36:16]
+[@157,1054:1054=';',<';'>,36:17]
+[@158,1062:1064='int',<'int'>,37:6]
+[@159,1065:1065='?',<'?'>,37:9]
+[@160,1067:1068='Zn',<Simple_Identifier>,37:11]
+[@161,1070:1070='=',<'='>,37:14]
+[@162,1072:1075='null',<'null'>,37:16]
+[@163,1076:1076=';',<';'>,37:20]
+[@164,1084:1087='bool',<'bool'>,38:6]
+[@165,1089:1089='J',<Simple_Identifier>,38:11]
+[@166,1091:1091='=',<'='>,38:13]
+[@167,1093:1094='Yn',<Simple_Identifier>,38:15]
+[@168,1096:1097='is',<'is'>,38:18]
+[@169,1099:1101='int',<'int'>,38:21]
+[@170,1102:1102=';',<';'>,38:24]
+[@171,1133:1136='bool',<'bool'>,39:6]
+[@172,1138:1138='K',<Simple_Identifier>,39:11]
+[@173,1140:1140='=',<'='>,39:13]
+[@174,1142:1143='Zn',<Simple_Identifier>,39:15]
+[@175,1145:1146='is',<'is'>,39:18]
+[@176,1148:1150='int',<'int'>,39:21]
+[@177,1151:1151=';',<';'>,39:24]
+[@178,1183:1186='bool',<'bool'>,40:6]
+[@179,1188:1189='Jn',<Simple_Identifier>,40:11]
+[@180,1191:1191='=',<'='>,40:14]
+[@181,1193:1194='Yn',<Simple_Identifier>,40:16]
+[@182,1196:1197='is',<'is'>,40:19]
+[@183,1199:1201='int',<'int'>,40:22]
+[@184,1202:1202='?',<'?'>,40:25]
+[@185,1203:1203=';',<';'>,40:26]
+[@186,1232:1235='bool',<'bool'>,41:6]
+[@187,1237:1238='Kn',<Simple_Identifier>,41:11]
+[@188,1240:1240='=',<'='>,41:14]
+[@189,1242:1243='Zn',<Simple_Identifier>,41:16]
+[@190,1245:1246='is',<'is'>,41:19]
+[@191,1248:1250='int',<'int'>,41:22]
+[@192,1251:1251='?',<'?'>,41:25]
+[@193,1252:1252=';',<';'>,41:26]
+[@194,1282:1285='bool',<'bool'>,42:6]
+[@195,1287:1288='Ly',<Simple_Identifier>,42:11]
+[@196,1290:1290='=',<'='>,42:14]
+[@197,1292:1293='Yn',<Simple_Identifier>,42:16]
+[@198,1295:1296='is',<'is'>,42:19]
+[@199,1298:1298='7',<Integer_Literal>,42:22]
+[@200,1300:1300='*',<'*'>,42:24]
+[@201,1302:1302='6',<Integer_Literal>,42:26]
+[@202,1303:1303=';',<';'>,42:27]
+[@203,1331:1334='bool',<'bool'>,43:6]
+[@204,1336:1337='Lz',<Simple_Identifier>,43:11]
+[@205,1339:1339='=',<'='>,43:14]
+[@206,1341:1342='Zn',<Simple_Identifier>,43:16]
+[@207,1344:1345='is',<'is'>,43:19]
+[@208,1347:1347='7',<Integer_Literal>,43:22]
+[@209,1349:1349='*',<'*'>,43:24]
+[@210,1351:1351='6',<Integer_Literal>,43:26]
+[@211,1352:1352=';',<';'>,43:27]
+[@212,1381:1384='bool',<'bool'>,44:6]
+[@213,1386:1386='M',<Simple_Identifier>,44:11]
+[@214,1388:1388='=',<'='>,44:13]
+[@215,1390:1390='X',<Simple_Identifier>,44:15]
+[@216,1392:1393='is',<'is'>,44:17]
+[@217,1395:1397='int',<'int'>,44:20]
+[@218,1398:1398='?',<'?'>,44:23]
+[@219,1399:1399=';',<';'>,44:24]
+[@220,1430:1433='bool',<'bool'>,45:6]
+[@221,1435:1436='Mn',<Simple_Identifier>,45:11]
+[@222,1438:1438='=',<'='>,45:14]
+[@223,1440:1441='Zn',<Simple_Identifier>,45:16]
+[@224,1443:1444='is',<'is'>,45:19]
+[@225,1446:1449='null',<'null'>,45:22]
+[@226,1450:1450=';',<';'>,45:26]
+[@227,1486:1486='_',<'_'>,47:6]
+[@228,1488:1488='=',<'='>,47:8]
+[@229,1490:1490='A',<Simple_Identifier>,47:10]
+[@230,1492:1497='switch',<'switch'>,47:12]
+[@231,1505:1505='{',<'{'>,48:6]
+[@232,1516:1516='(',<'('>,49:9]
+[@233,1517:1517='a',<Simple_Identifier>,49:10]
+[@234,1519:1520='=>',<'=>'>,49:12]
+[@235,1522:1522='b',<Simple_Identifier>,49:15]
+[@236,1523:1523=')',<')'>,49:16]
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+[@239,1529:1529=',',<','>,49:22]
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+[@243,1754:1755='=>',<'=>'>,52:16]
+[@244,1757:1757='c',<Simple_Identifier>,52:19]
+[@245,1758:1758=',',<','>,52:20]
+[@246,1825:1825='a',<Simple_Identifier>,53:9]
+[@247,1827:1828='=>',<'=>'>,53:11]
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+[@249,1831:1831='b',<Simple_Identifier>,53:15]
+[@250,1833:1834='=>',<'=>'>,53:17]
+[@251,1836:1836='c',<Simple_Identifier>,53:20]
+[@252,1837:1837=')',<')'>,53:21]
+[@253,1870:1870='}',<'}'>,54:6]
+[@254,1871:1871=';',<';'>,54:7]
+[@255,1886:1891='static',<'static'>,56:6]
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+[@259,1900:1903='Func',<Simple_Identifier>,56:20]
+[@260,1904:1904='<',<'<'>,56:24]
+[@261,1905:1907='int',<'int'>,56:25]
+[@262,1908:1908=',',<','>,56:28]
+[@263,1910:1912='int',<'int'>,56:30]
+[@264,1913:1913='>',<'>'>,56:33]
+[@265,1915:1915='g',<Simple_Identifier>,56:35]
+[@266,1916:1916=')',<')'>,56:36]
+[@267,1924:1924='{',<'{'>,57:6]
+[@268,1936:1938='var',<'var'>,58:10]
+[@269,1940:1940='I',<Simple_Identifier>,58:14]
+[@270,1942:1942='=',<'='>,58:16]
+[@271,1944:1944='g',<Simple_Identifier>,58:18]
+[@272,1946:1951='switch',<'switch'>,58:20]
+[@273,1963:1963='{',<'{'>,59:10]
+[@274,1979:1982='null',<'null'>,60:14]
+[@275,1984:1985='=>',<'=>'>,60:19]
+[@276,1987:1987='0',<Integer_Literal>,60:22]
+[@277,1988:1988=',',<','>,60:23]
+[@278,2052:2054='var',<'var'>,61:14]
+[@279,2056:2056='k',<Simple_Identifier>,61:18]
+[@280,2058:2059='=>',<'=>'>,61:20]
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+[@283,2063:2064='42',<Integer_Literal>,61:25]
+[@284,2065:2065=')',<')'>,61:27]
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+[@294,2145:2146='42',<Integer_Literal>,62:28]
+[@295,2147:2147=')',<')'>,62:30]
+[@296,2217:2217='}',<'}'>,63:10]
+[@297,2218:2218=';',<';'>,63:11]
+[@298,2226:2226='}',<'}'>,64:6]
+[@299,2227:2227=';',<';'>,64:7]
+[@300,2232:2232='}',<'}'>,65:3]
+[@301,2234:2234='}',<'}'>,66:0]
+[@302,2235:2234='<EOF>',<EOF>,66:1]