Certain IdentifierPatterns in the old parser now parsed as IdentifierExpr

[allevato]

Issue Kind

Parse of Valid Source Produced Invalid Syntax Tree

Source Code

switch x { case let .y(z): break }

Description

This code snippet produces the following parse tree:

SourceFileSyntax children=2
  0: CodeBlockItemListSyntax children=1
    0: CodeBlockItemSyntax children=1
      0: SwitchStmtSyntax children=5
        0: switchKeyword
        1: IdentifierExprSyntax children=1
          0: identifier("x")
        2: leftBrace
        3: SwitchCaseListSyntax children=1
          0: SwitchCaseSyntax children=2
            0: SwitchCaseLabelSyntax children=3
              0: caseKeyword
              1: CaseItemListSyntax children=1
                0: CaseItemSyntax children=1
                  0: ValueBindingPatternSyntax children=2
                    0: letKeyword
                    1: ExpressionPatternSyntax children=1
                      0: FunctionCallExprSyntax children=4
                        0: MemberAccessExprSyntax children=2
                          0: prefixPeriod
                          1: identifier("y")
                        1: leftParen
                        2: TupleExprElementListSyntax children=1
                          0: TupleExprElementSyntax children=1
                            0: IdentifierExprSyntax children=1
                              0: identifier("z")
                        3: rightParen
              2: colon
            1: CodeBlockItemListSyntax children=1
              0: CodeBlockItemSyntax children=1
                0: BreakStmtSyntax children=1
                  0: breakKeyword
        4: rightBrace
  1: eof

For z, the C++ parser instead produced an UnresolvedPatternExprSyntax with an IdentifierPatternSyntax as its child.

The C++ tree distinguishes identifiers that are bound in the pattern from those which aren't or can't be. For example,

• if the pattern was let .y(z, someFunction(5)), z would be an IdentifierPattern while someFunction would be an IdentifierExprs.
• if the pattern was let .y(z, someFunction(w)), z and w would be IdentifierPatterns while someFunction would be an IdentifierExpr (and the compiler would later emit an error that pattern variable binding cannot appear in an expression).

[ahoppen]

rdar://100301666