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go_spec: don't allow parens around the literal type of composite literals
Background: The current spec is imprecise with respect to the parsing ambiguity for composite literals: It says that the ambiguity arises when the TypeName form of the LiteralType is used. The following code: if (B) {} ... is not using the TypeName form (but the parenthesized TypeName form) and thus could be interpreted as: if ((B){}) ... instead of if B {} ... Both compilers and gofmt choose the latter interpretation. One could fix the spec by making the clause regarding the parsing ambiguity more precise ("...using the _possibly parenthesized_ TypeName form of the LiteralType..."). The alternative (chosen here) is to simply disallow parenthesized literal types. Except for a single test case (test/parentype.go) there appears to be no Go code under $GOROOT containing parenthesized literal types. Furthermore, parentheses are never needed around a literal type for correct parsing. R=golang-dev CC=golang-dev https://golang.org/cl/1913041
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@ -1,5 +1,5 @@
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<!-- title The Go Programming Language Specification -->
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<!-- subtitle Version of July 14, 2010 -->
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<!-- subtitle Version of July 29, 2010 -->
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<!--
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TODO
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@ -1974,7 +1974,7 @@ a single expression or a key-value pair.
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<pre class="ebnf">
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CompositeLit = LiteralType "{" [ ElementList [ "," ] ] "}" .
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LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
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SliceType | MapType | TypeName | "(" LiteralType ")" .
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SliceType | MapType | TypeName .
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ElementList = Element { "," Element } .
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Element = [ Key ":" ] Value .
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Key = FieldName | ElementIndex .
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@ -2096,10 +2096,11 @@ and is a shortcut for a slice operation applied to an array literal:
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<p>
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A parsing ambiguity arises when a composite literal using the
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TypeName form of the LiteralType appears in the condition of an
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TypeName form of the LiteralType appears between the
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<a href="#Keywords">keyword</a> and the opening brace of the block of an
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"if", "for", or "switch" statement, because the braces surrounding
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the expressions in the literal are confused with those introducing
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a block of statements. To resolve the ambiguity in this rare case,
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the block of statements. To resolve the ambiguity in this rare case,
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the composite literal must appear within
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parentheses.
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</p>
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@ -11,9 +11,7 @@ func g() {}
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func main() {
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f(map[string]string{"a":"b","c":"d"});
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f([...]int{1,2,3});
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f(([...]int){1,2,3});
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f((map[string]string){"a":"b","c":"d"});
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f((map[string]func()){"a":g,"c":g});
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f(map[string]func(){"a":g,"c":g});
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f(make(chan(<-chan int)));
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f(make(chan<-(chan int)));
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}
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