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spec: document numeric operations behavior for generic types
Includes a few minor cosmetic changes. Change-Id: I6c307d958b47d83671142688630ea7835168439f Reviewed-on: https://go-review.googlesource.com/c/go/+/384622 Trust: Robert Griesemer <gri@golang.org> Run-TryBot: Robert Griesemer <gri@golang.org> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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@ -1,6 +1,6 @@
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<!--{
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"Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
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"Subtitle": "Version of Feb 9, 2022",
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"Subtitle": "Version of Feb 10, 2022",
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"Path": "/ref/spec"
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}-->
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@ -4752,7 +4752,7 @@ Arithmetic operators apply to numeric values and yield a result of the same
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type as the first operand. The four standard arithmetic operators (<code>+</code>,
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<code>-</code>, <code>*</code>, <code>/</code>) apply to
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<a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
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<a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</.
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<a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
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The bitwise logical and shift operators apply to integers only.
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</p>
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@ -4772,6 +4772,37 @@ The bitwise logical and shift operators apply to integers only.
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>> right shift integer >> integer >= 0
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</pre>
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<p>
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Excluding shifts, if the operand type is a <a href="#Type_parameters">type parameter</a>,
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it must have <a href="#Structure_of_interfaces">specific types</a>, and the operator must
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apply to each specific type.
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The operands are represented as values of the type argument that the type parameter
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is <a href="#Instantiations">instantiated</a> with, and the operation is computed
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with the precision of that type argument. For example, given the function:
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</p>
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<pre>
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func dotProduct[F ~float32|~float64](v1, v2 []F) F {
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var s F
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for i, x := range v1 {
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y := v2[i]
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s += x * y
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}
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return s
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}
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</pre>
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<p>
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the the product <code>x * y</code> and the addition <code>s += x * y</code>
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are computed with <code>float32</code> or <code>float64</code> precision,
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respectively, depending on the type argument for <code>F</code>.
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</p>
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<p>
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For shifts, the <a href="#Core_types">core type</a> of both operands must be
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an integer.
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</p>
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<h4 id="Integer_operators">Integer operators</h4>
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<p>
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@ -4857,10 +4888,10 @@ follows:
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<h4 id="Integer_overflow">Integer overflow</h4>
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<p>
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For unsigned integer values, the operations <code>+</code>,
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For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
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<code>-</code>, <code>*</code>, and <code><<</code> are
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computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
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the <a href="#Numeric_types">unsigned integer</a>'s type.
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the unsigned integer's type.
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Loosely speaking, these unsigned integer operations
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discard high bits upon overflow, and programs may rely on "wrap around".
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</p>
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@ -4875,7 +4906,6 @@ A compiler may not optimize code under the assumption that overflow does
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not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
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</p>
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<h4 id="Floating_point_operators">Floating-point operators</h4>
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<p>
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@ -4931,7 +4961,6 @@ s += " and good bye"
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String addition creates a new string by concatenating the operands.
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</p>
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<h3 id="Comparison_operators">Comparison operators</h3>
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<p>
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@ -5220,7 +5249,7 @@ string(65.0) // illegal: 65.0 is not an integer constant
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<p>
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Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
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with the value represented as a value of the type argument that the type parameter
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is instantiated with.
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is <a href="#Instantiations">instantiated</a> with.
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For example, given the function:
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</p>
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