Paul Berry
637dd76c7f
This change fixes parsing of case clauses such as:
case foo when !flag:
Constructions like this require some lookahead in order to parse
correctly, because the token `when` is valid both as an identifier and
as a part of the grammar for a case clause. Therefore, at the time
`foo` is encountered, the parser must decide whether it is looking at
a variable pattern (`foo when`, where `when` is the name of the
variable) or an identifier pattern (`foo`, where `when` begins the
case's guard clause). Previous to this fix, the algorithm for
disambiguating these two choices was as follows:
- If the token sequence starting at `foo` looked like a type, and the
token that follows was an identifier, the parser assumed it was
looking at a variable pattern with a type; otherwise it assumed it
was looking at an identifier pattern.
- EXCEPT that if the token that followed the supposed type was `when`
or `as` (both of which are valid identifiers), then it probed
further:
- If the token that followed `when` or `as` was a token that could
legitimately follow a pattern, then it assumed that it was looking
at a variable pattern with a type. (The tokens that could
legitimately follow a pattern are `,`, `:`, `||`, `&&`, `)`, `}`,
`]`, `as`, `when`, `?`, `!`).
- Otherwise it assumed that it was looking at an identifier pattern.
This didn't fully disambiguate, because the third bullet didn't
account for the fact that the tokens `as`, `when`, and `!` could
_either_ legitimately follow a pattern _or_ legitimately begin an
expression (or, in the case of `when`, a type), therefore constructs
like the following were incorrectly parsed:
- `case foo when as:` (where `as` is a local boolean variable)
- `case foo when when:` (where `when` is a local boolean variable)
- `case foo when !flag:` (where `flag` is a local boolean variable)
- `case foo as when:` (where `when` is the name of a type)
The solution is to simplify the disambiguation logic so that if if the
token that follows the supposed type is `when` or `as`, then the
parser assumes that it's looking at an identifier pattern, _not_ a
typed variable pattern.
The consequence of this is that the above four constructions are
parsed correctly; however it is no longer possible for a typed
variable pattern to name a variable `when` or `as`.
For consistency we would like to prohibit _any_ variable pattern from
naming a variable `when` or `as`, however to keep this change as small
as possible (and reduce the risk involved in a possible cherry-pick)
that will be postponed until a later CL.
Fixes #52199.
Bug: https://github.com/dart-lang/sdk/issues/52199
Change-Id: Ibab9b92f01e3e4020d7d64f1ff000a9b964a4564
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/299400
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
51 lines
2.0 KiB
Plaintext
51 lines
2.0 KiB
Plaintext
beginCompilationUnit(void)
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beginMetadataStar(void)
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endMetadataStar(0)
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beginTopLevelMember(void)
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beginTopLevelMethod(, null, null)
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handleVoidKeyword(void)
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handleIdentifier(f, topLevelFunctionDeclaration)
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handleNoTypeVariables(()
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beginFormalParameters((, MemberKind.TopLevelMethod)
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beginMetadataStar(x)
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endMetadataStar(0)
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beginFormalParameter(x, MemberKind.TopLevelMethod, null, null, null)
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handleNoType(()
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handleIdentifier(x, formalParameterDeclaration)
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handleFormalParameterWithoutValue())
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endFormalParameter(null, null, null, x, null, null, FormalParameterKind.requiredPositional, MemberKind.TopLevelMethod)
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endFormalParameters(1, (, ), MemberKind.TopLevelMethod)
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handleAsyncModifier(null, null)
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beginBlockFunctionBody({)
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beginSwitchStatement(switch)
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handleIdentifier(x, expression)
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handleNoTypeArguments())
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handleNoArguments())
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handleSend(x, ))
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handleParenthesizedCondition((, null, null)
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beginSwitchBlock({)
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beginCaseExpression(case)
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beginPattern(case)
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beginConstantPattern(null)
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handleIdentifier(foo, expression)
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handleNoTypeArguments(when)
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handleNoArguments(when)
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handleSend(foo, when)
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endConstantPattern(null)
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endPattern(foo)
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beginSwitchCaseWhenClause(when)
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handleIdentifier(when, expression)
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handleNoTypeArguments(:)
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handleNoArguments(:)
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handleSend(when, :)
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endSwitchCaseWhenClause(when)
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endCaseExpression(case, when, :)
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beginSwitchCase(0, 1, case)
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endSwitchCase(0, 1, null, null, 0, case, })
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endSwitchBlock(1, {, })
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endSwitchStatement(switch, })
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endBlockFunctionBody(1, {, })
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endTopLevelMethod(void, null, })
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endTopLevelDeclaration()
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endCompilationUnit(1, )
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