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Parsing support for if expressions. (#2883)

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We model `if a then b else` as a prefix operator for parsing precedence purposes. The rule that a statement starting with `if` is never an `if` expression is handled implicitly because the statement parser never invokes the expression parser for a statement starting with `if`.

This exposed a bug in our diagnosis of the whitespace rule for prefix operators, which was incorrectly being applied to non-symbolic operators in some cases, and was producing a bogus second diagnostic in some cases, which is also fixed here.
This commit is contained in:
Richard Smith
2023-06-09 16:41:10 -07:00
committed by GitHub
parent 4e1adcf4c7
commit 2c45cb3be8
18 changed files with 400 additions and 22 deletions
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toolchain/parser/testdata/if_expression/fail_else_expr_missing.carbon
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// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// AUTOUPDATE
// CHECK:STDOUT: [
// CHECK:STDOUT: {kind: 'FunctionIntroducer', text: 'fn'},
// CHECK:STDOUT: {kind: 'DeclaredName', text: 'F'},
// CHECK:STDOUT: {kind: 'ParameterListStart', text: '('},
// CHECK:STDOUT: {kind: 'ParameterList', text: ')', subtree_size: 2},
// CHECK:STDOUT: {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 5},
// CHECK:STDOUT: {kind: 'VariableIntroducer', text: 'var'},
// CHECK:STDOUT: {kind: 'DeclaredName', text: 'n'},
// CHECK:STDOUT: {kind: 'Literal', text: 'i32'},
// CHECK:STDOUT: {kind: 'PatternBinding', text: ':', subtree_size: 3},
// CHECK:STDOUT: {kind: 'VariableInitializer', text: '='},
// CHECK:STDOUT: {kind: 'NameReference', text: 'true'},
// CHECK:STDOUT: {kind: 'IfExpressionThen', text: 'then'},
// CHECK:STDOUT: {kind: 'Literal', text: '1'},
// CHECK:STDOUT: {kind: 'IfExpressionElse', text: 'else'},
// CHECK:STDOUT: {kind: 'InvalidParse', text: ';', has_error: yes},
// CHECK:STDOUT: {kind: 'IfExpression', text: 'if', has_error: yes, subtree_size: 6},
// CHECK:STDOUT: {kind: 'VariableDeclaration', text: ';', subtree_size: 12},
// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
// CHECK:STDOUT: ]
fn F() {
// CHECK:STDERR: fail_else_expr_missing.carbon:[[@LINE+1]]:35: Expected expression.
var n: i32 = if true then 1 else;
}