@@ -6274,7 +6274,7 @@ class Parser {
62746274 Token next = token.next! ;
62756275 if (allowPatterns && optional ('case' , next)) {
62766276 Token case_ = token = next;
6277- token = parsePattern (token);
6277+ token = parsePattern (token, isRefutableContext : true );
62786278 next = token.next! ;
62796279 Token ? when ;
62806280 if (optional ('when' , next)) {
@@ -8295,7 +8295,7 @@ class Parser {
82958295 }
82968296 listener.beginCaseExpression (caseKeyword);
82978297 if (allowPatterns) {
8298- token = parsePattern (caseKeyword);
8298+ token = parsePattern (caseKeyword, isRefutableContext : true );
82998299 } else {
83008300 token = parseExpression (caseKeyword);
83018301 }
@@ -9157,10 +9157,21 @@ class Parser {
91579157 /// castPattern ::= primaryPattern 'as' type
91589158 /// nullAssertPattern ::= primaryPattern '!'
91599159 /// nullCheckPattern ::= primaryPattern '?'
9160- Token parsePattern (Token token, {int precedence = 1 }) {
9160+ ///
9161+ /// [isRefutableContext] should be `true` if the pattern occurs in a
9162+ /// `guardedPattern` or any of its sub-patterns (i.e. in a
9163+ /// `switchStatementCase` , `switchExpressionCase` , or `ifCondition` ); these
9164+ /// are contexts where a pattern match might be expected to fail, and bare
9165+ /// identifiers are treated as constant patterns. It should be `false` if the
9166+ /// pattern occurs in a `localVariableDeclaration` , `forLoopParts` , or
9167+ /// `patternAssignment` ; these are contexts where a pattern match failure is
9168+ /// either prohibited statically or causes a runtime exception, and bare
9169+ /// identifiers are treated as variable patterns.
9170+ Token parsePattern (Token token,
9171+ {int precedence = 1 , required bool isRefutableContext}) {
91619172 assert (precedence >= 1 );
91629173 assert (precedence <= SELECTOR_PRECEDENCE );
9163- token = parsePrimaryPattern (token);
9174+ token = parsePrimaryPattern (token, isRefutableContext : isRefutableContext );
91649175 while (true ) {
91659176 Token next = token.next! ;
91669177 int tokenLevel = _computePrecedence (next, forPattern: true );
@@ -9191,7 +9202,9 @@ class Parser {
91919202 case '|' :
91929203 listener.beginBinaryPattern (next);
91939204 // Left associative so we parse the RHS one precedence level higher
9194- token = parsePattern (next, precedence: tokenLevel + 1 );
9205+ token = parsePattern (next,
9206+ precedence: tokenLevel + 1 ,
9207+ isRefutableContext: isRefutableContext);
91959208 listener.endBinaryPattern (next);
91969209 break ;
91979210 default :
@@ -9229,7 +9242,7 @@ class Parser {
92299242 /// extractorPattern ::= extractorName typeArguments?
92309243 /// '(' patternFields? ')'
92319244 /// extractorName ::= typeIdentifier | qualifiedName
9232- Token parsePrimaryPattern (Token token) {
9245+ Token parsePrimaryPattern (Token token, { required bool isRefutableContext} ) {
92339246 TypeParamOrArgInfo typeArg =
92349247 computeTypeParamOrArg (token, /* inDeclaration = */ true );
92359248 Token next = typeArg.skip (token).next! ;
@@ -9238,13 +9251,15 @@ class Parser {
92389251 case '[' :
92399252 // listPattern ::= typeArguments? '[' patterns? ']'
92409253 token = typeArg.parseArguments (token, this );
9241- return parseListPatternSuffix (token);
9254+ return parseListPatternSuffix (token,
9255+ isRefutableContext: isRefutableContext);
92429256 case '{' :
92439257 // mapPattern ::= typeArguments? '{' mapPatternEntries? '}'
92449258 // mapPatternEntries ::= mapPatternEntry ( ',' mapPatternEntry )* ','?
92459259 // mapPatternEntry ::= expression ':' pattern
92469260 token = typeArg.parseArguments (token, this );
9247- return parseMapPatternSuffix (token);
9261+ return parseMapPatternSuffix (token,
9262+ isRefutableContext: isRefutableContext);
92489263 }
92499264 // Whatever was after the optional type arguments didn't parse as a pattern
92509265 // that can start with type arguments, so back up and reparse assuming that
@@ -9265,7 +9280,8 @@ class Parser {
92659280 listener.handleRecordPattern (next, /* count = */ 0 );
92669281 return nextNext;
92679282 } else {
9268- return parseParenthesizedPatternOrRecordPattern (token);
9283+ return parseParenthesizedPatternOrRecordPattern (token,
9284+ isRefutableContext: isRefutableContext);
92699285 }
92709286 case 'const' :
92719287 // constantPattern ::= booleanLiteral
@@ -9335,7 +9351,8 @@ class Parser {
93359351 if (optional ('(' , afterToken) && ! potentialTypeArg.recovered) {
93369352 TypeParamOrArgInfo typeArg = potentialTypeArg;
93379353 token = typeArg.parseArguments (token, this );
9338- token = parseExtractorPatternRest (token);
9354+ token = parseExtractorPatternRest (token,
9355+ isRefutableContext: isRefutableContext);
93399356 listener.handleExtractorPattern (firstIdentifier, dot, secondIdentifier);
93409357 return token;
93419358 } else if (firstIdentifier.lexeme == '_' && dot == null ) {
@@ -9393,7 +9410,8 @@ class Parser {
93939410 /// bracket.
93949411 ///
93959412 /// listPattern ::= typeArguments? '[' patterns? '] '
9396- Token parseListPatternSuffix (Token token) {
9413+ Token parseListPatternSuffix (Token token,
9414+ {required bool isRefutableContext}) {
93979415 Token beforeToken = token;
93989416 Token beginToken = token = token.next! ;
93999417 assert (optional ('[' , token) || optional ('[]' , token));
@@ -9413,7 +9431,7 @@ class Parser {
94139431 token = next;
94149432 break ;
94159433 }
9416- token = parsePattern (token);
9434+ token = parsePattern (token, isRefutableContext : isRefutableContext );
94179435 next = token.next! ;
94189436 ++ count;
94199437 if (! optional (',' , next)) {
@@ -9455,7 +9473,7 @@ class Parser {
94559473 /// mapPattern ::= typeArguments? '{' mapPatternEntries? '}'
94569474 /// mapPatternEntries ::= mapPatternEntry ( ',' mapPatternEntry )* ','?
94579475 /// mapPatternEntry ::= expression ':' pattern
9458- Token parseMapPatternSuffix (Token token) {
9476+ Token parseMapPatternSuffix (Token token, { required bool isRefutableContext} ) {
94599477 Token leftBrace = token = token.next! ;
94609478 assert (optional ('{' , leftBrace));
94619479 Token next = token.next! ;
@@ -9476,7 +9494,7 @@ class Parser {
94769494 codes.templateExpectedButGot.withArguments (':' ),
94779495 new SyntheticToken (TokenType .PERIOD , next.charOffset));
94789496 }
9479- token = parsePattern (colon);
9497+ token = parsePattern (colon, isRefutableContext : isRefutableContext );
94809498 listener.handleMapPatternEntry (colon, token.next! );
94819499 ++ count;
94829500 next = token.next! ;
@@ -9520,7 +9538,8 @@ class Parser {
95209538 /// recordPattern ::= '(' patternFields? ')'
95219539 /// patternFields ::= patternField ( ',' patternField )* ','?
95229540 /// patternField ::= ( identifier? ':' )? pattern
9523- Token parseParenthesizedPatternOrRecordPattern (Token token) {
9541+ Token parseParenthesizedPatternOrRecordPattern (Token token,
9542+ {required bool isRefutableContext}) {
95249543 Token begin = token.next! ;
95259544 assert (optional ('(' , begin));
95269545
@@ -9551,7 +9570,7 @@ class Parser {
95519570 colon = token;
95529571 wasValidRecord = true ;
95539572 }
9554- token = parsePattern (token);
9573+ token = parsePattern (token, isRefutableContext : isRefutableContext );
95559574 next = token.next! ;
95569575 if (wasRecord || colon != null ) {
95579576 listener.handlePatternField (colon);
@@ -9594,7 +9613,8 @@ class Parser {
95949613 ///
95959614 /// extractorPattern ::= extractorName typeArguments?
95969615 /// '(' patternFields? ')'
9597- Token parseExtractorPatternRest (Token token) {
9616+ Token parseExtractorPatternRest (Token token,
9617+ {required bool isRefutableContext}) {
95989618 Token begin = token = token.next! ;
95999619 assert (optional ('(' , begin));
96009620 int argumentCount = 0 ;
@@ -9616,7 +9636,7 @@ class Parser {
96169636 .next! ;
96179637 colon = token;
96189638 }
9619- token = parsePattern (token);
9639+ token = parsePattern (token, isRefutableContext : isRefutableContext );
96209640 next = token.next! ;
96219641 listener.handlePatternField (colon);
96229642 ++ argumentCount;
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