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Throughout the spec we use the notion of a constant x being representable by a value of type T. While intuitively clear, at least for floating-point and complex constants types, the concept was not well-defined. In the section on Conversions there was an extra rule for floating-point types only and it missed the case of floating-point values overflowing to an infinity after rounding. Since the concept is important to Go, and a compiler most certainly will have a function to test "representability", it seems warranted to define the term explicitly in the spec. This change introduces a new entry "Representability" under the section on "Properties of types and values", and defines the term explicitly, together with examples. The phrase used is "representable by" rather than "representable as" because the former use is prevalent in the spec. Additionally, it clarifies that a floating-point constant that overflows to an infinity after rounding is never representable by a value of a floating-point type, even though infinities are valid values of IEEE floating point types. This is required because there are not infinite value constants in the language (like there is also no -0.0) and representability also matters for constant conversions. This is not a language change, and type-checkers have been following this rule before. The change also introduces links throughout the spec to the new section as appropriate and removes duplicate text and examples elsewhere (Constants and Conversions sections), leading to simplifications in the relevant paragraphs. Fixes #15389. Change-Id: I8be0e071552df0f18998ef4c5ef521f64ffe8c44 Reviewed-on: https://go-review.googlesource.com/57530 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com> |
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