spec: document new Go2 number literals

This CL documents the new binary and octal integer literals,
hexadecimal floats, generalized imaginary literals and digit
separators for all number literals in the spec.

Added empty lines between abutting paragraphs in some places
(a more thorough cleanup can be done in a separate CL).

A minor detail: A single 0 was considered an octal zero per the
syntax (decimal integer literals always started with a non-zero
digit). The new octal literal syntax allows 0o and 0O prefixes
and when keeping the respective octal_lit syntax symmetric with
all the others (binary_lit, hex_lit), a single 0 is not automatically
part of it anymore. Rather than complicating the new octal_lit syntax
to include 0 as before, it is simpler (and more natural) to accept
a single 0 as part of a decimal_lit. This is purely a notational
change.

R=Go1.13

Updates #12711.
Updates #19308.
Updates #28493.
Updates #29008.

Change-Id: Ib9fdc6e781f6031cceeed37aaed9d05c7141adec
Reviewed-on: https://go-review.googlesource.com/c/go/+/161098Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 16, 2019",
+	"Subtitle": "Version of March 12, 2019",
 	"Path": "/ref/spec"
 }-->
 
@@ -118,6 +118,7 @@ The underscore character <code>_</code> (U+005F) is considered a letter.
 <pre class="ebnf">
 letter        = unicode_letter | "_" .
 decimal_digit = "0" … "9" .
+binary_digit  = "0" | "1" .
 octal_digit   = "0" … "7" .
 hex_digit     = "0" … "9" | "A" … "F" | "a" … "f" .
 </pre>
@@ -273,41 +274,96 @@ The following character sequences represent <a href="#Operators">operators</a>
 <p>
 An integer literal is a sequence of digits representing an
 <a href="#Constants">integer constant</a>.
-An optional prefix sets a non-decimal base: <code>0</code> for octal, <code>0x</code> or
-<code>0X</code> for hexadecimal.  In hexadecimal literals, letters
-<code>a-f</code> and <code>A-F</code> represent values 10 through 15.
+An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
+for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
+and <code>0x</code> or <code>0X</code> for hexadecimal.
+A single <code>0</code> is considered a decimal zero.
+In hexadecimal literals, letters <code>a</code> through <code>f</code>
+and <code>A</code> through <code>F</code> represent values 10 through 15.
+</p>
+
+<p>
+For readability, an underscore character <code>_</code> may appear after
+a base prefix or between successive digits; such underscores do not change
+the literal's value.
 </p>
 <pre class="ebnf">
-int_lit     = decimal_lit | octal_lit | hex_lit .
-decimal_lit = ( "1" … "9" ) { decimal_digit } .
-octal_lit   = "0" { octal_digit } .
-hex_lit     = "0" ( "x" | "X" ) hex_digit { hex_digit } .
+int_lit        = decimal_lit | binary_lit | octal_lit | hex_lit .
+decimal_lit    = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
+binary_lit     = "0" ( "b" | "B" ) [ "_" ] binary_digits .
+octal_lit      = "0" [ "o" | "O" ] [ "_" ] octal_digits .
+hex_lit        = "0" ( "x" | "X" ) [ "_" ] hex_digits .
+
+decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
+binary_digits  = binary_digit { [ "_" ] binary_digit } .
+octal_digits   = octal_digit { [ "_" ] octal_digit } .
+hex_digits     = hex_digit { [ "_" ] hex_digit } .
 </pre>
 
 <pre>
 42
+4_2
 0600
+0_600
+0o600
+0O600       // second character is capital letter 'O'
 0xBadFace
+0xBad_Face
+0x_67_7a_2f_cc_40_c6
 170141183460469231731687303715884105727
+170_141183_460469_231731_687303_715884_105727
+
+_42         // an identifier, not an integer literal
+42_         // invalid: _ must separate successive digits
+4__2        // invalid: only one _ at a time
+0_xBadFace  // invalid: _ must separate successive digits
 </pre>
 
+
 <h3 id="Floating-point_literals">Floating-point literals</h3>
+
 <p>
-A floating-point literal is a decimal representation of a
+A floating-point literal is a decimal or hexadecimal representation of a
 <a href="#Constants">floating-point constant</a>.
-It has an integer part, a decimal point, a fractional part,
-and an exponent part.  The integer and fractional part comprise
-decimal digits; the exponent part is an <code>e</code> or <code>E</code>
-followed by an optionally signed decimal exponent.  One of the
-integer part or the fractional part may be elided; one of the decimal
-point or the exponent may be elided.
 </p>
+
+<p>
+A decimal floating-point literal consists of an integer part (decimal digits),
+a decimal point, a fractional part (decimal digits), and an exponent part
+(<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
+One of the integer part or the fractional part may be elided; one of the decimal point
+or the exponent part may be elided.
+An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
+</p>
+
+<p>
+A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
+prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
+and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
+One of the integer part or the fractional part may be elided; the radix point may be elided as well,
+but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
+An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
+</p>
+
+<p>
+For readability, an underscore character <code>_</code> may appear after
+a base prefix or between successive digits; such underscores do not change
+the literal value.
+</p>
+
 <pre class="ebnf">
-float_lit = decimals "." [ decimals ] [ exponent ] |
-            decimals exponent |
-            "." decimals [ exponent ] .
-decimals  = decimal_digit { decimal_digit } .
-exponent  = ( "e" | "E" ) [ "+" | "-" ] decimals .
+float_lit         = decimal_float_lit | hex_float_lit .
+
+decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
+                    decimal_digits decimal_exponent |
+                    "." decimal_digits [ decimal_exponent ] .
+decimal_exponent  = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
+
+hex_float_lit     = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
+hex_mantissa      = [ "_" ] hex_digits "." [ hex_digits ] |
+                    [ "_" ] hex_digits |
+                    "." hex_digits .
+hex_exponent      = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
 </pre>
 
 <pre>
@@ -320,24 +376,54 @@ exponent  = ( "e" | "E" ) [ "+" | "-" ] decimals .
 1E6
 .25
 .12345E+5
+1_5.         // == 15.0
+0.15e+0_2    // == 15.0
+
+0x1p-2       // == 0.25
+0x2.p10      // == 2048.0
+0x1.Fp+0     // == 1.9375
+0X.8p-0      // == 0.5
+0X_1FFFP-16  // == 0.1249847412109375
+0x15e-2      // == 0x15e - 2 (integer subtraction)
+
+0x.p1        // invalid: mantissa has no digits
+1p-2         // invalid: p exponent requires hexadecimal mantissa
+0x1.5e-2     // invalid: hexadecimal mantissa requires p exponent
+1_.5         // invalid: _ must separate successive digits
+1._5         // invalid: _ must separate successive digits
+1.5_e1       // invalid: _ must separate successive digits
+1.5e_1       // invalid: _ must separate successive digits
+1.5e1_       // invalid: _ must separate successive digits
 </pre>
 
+
 <h3 id="Imaginary_literals">Imaginary literals</h3>
+
 <p>
-An imaginary literal is a decimal representation of the imaginary part of a
+An imaginary literal represents the imaginary part of a
 <a href="#Constants">complex constant</a>.
-It consists of a
-<a href="#Floating-point_literals">floating-point literal</a>
-or decimal integer followed
-by the lower-case letter <code>i</code>.
+It consists of an <a href="#Integer_literals">integer</a> or
+<a href="#Floating-point_literals">floating-point</a> literal
+followed by the lower-case letter <code>i</code>.
+The value of an imaginary literal is the value of the respective
+integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
 </p>
+
 <pre class="ebnf">
-imaginary_lit = (decimals | float_lit) "i" .
+imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
 </pre>
 
+<p>
+For backward compatibility, an imaginary literal's integer part consisting
+entirely of decimal digits (and possibly underscores) is considered a decimal
+integer, even if it starts with a leading <code>0</code>.
+</p>
+
 <pre>
 0i
-011i  // == 11i
+0123i         // == 123i for backward-compatibility
+0o123i        // == 0o123 * 1i == 83i
+0xabci        // == 0xabc * 1i == 2748i
 0.i
 2.71828i
 1.e+0i
@@ -345,6 +431,7 @@ imaginary_lit = (decimals | float_lit) "i" .
 1E6i
 .25i
 .12345E+5i
+0x1p-2i       // == 0x1p-2 * 1i == 0.25i
 </pre>
 
 
@@ -361,6 +448,7 @@ of the character itself,
 while multi-character sequences beginning with a backslash encode
 values in various formats.
 </p>
+
 <p>
 The simplest form represents the single character within the quotes;
 since Go source text is Unicode characters encoded in UTF-8, multiple
@@ -370,6 +458,7 @@ a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
 <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
 a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
 </p>
+
 <p>
 Several backslash escapes allow arbitrary values to be encoded as
 ASCII text.  There are four ways to represent the integer value
@@ -380,6 +469,7 @@ plain backslash <code>\</code> followed by exactly three octal digits.
 In each case the value of the literal is the value represented by
 the digits in the corresponding base.
 </p>
+
 <p>
 Although these representations all result in an integer, they have
 different valid ranges.  Octal escapes must represent a value between
@@ -388,9 +478,11 @@ by construction. The escapes <code>\u</code> and <code>\U</code>
 represent Unicode code points so within them some values are illegal,
 in particular those above <code>0x10FFFF</code> and surrogate halves.
 </p>
+
 <p>
 After a backslash, certain single-character escapes represent special values:
 </p>
+
 <pre class="grammar">
 \a   U+0007 alert or bell
 \b   U+0008 backspace
@@ -403,6 +495,7 @@ After a backslash, certain single-character escapes represent special values:
 \'   U+0027 single quote  (valid escape only within rune literals)
 \"   U+0022 double quote  (valid escape only within string literals)
 </pre>
+
 <p>
 All other sequences starting with a backslash are illegal inside rune literals.
 </p>
@@ -446,6 +539,7 @@ A string literal represents a <a href="#Constants">string constant</a>
 obtained from concatenating a sequence of characters. There are two forms:
 raw string literals and interpreted string literals.
 </p>
+
 <p>
 Raw string literals are character sequences between back quotes, as in
 <code>`foo`</code>.  Within the quotes, any character may appear except
@@ -457,6 +551,7 @@ contain newlines.
 Carriage return characters ('\r') inside raw string literals
 are discarded from the raw string value.
 </p>
+
 <p>
 Interpreted string literals are character sequences between double
 quotes, as in <code>&quot;bar&quot;</code>.
@@ -596,6 +691,7 @@ precision in the language, a compiler may implement them using an
 internal representation with limited precision.  That said, every
 implementation must:
 </p>
+
 <ul>
 	<li>Represent integer constants with at least 256 bits.</li>
 
@@ -613,12 +709,14 @@ implementation must:
 	    represent a floating-point or complex constant due to limits
 	    on precision.</li>
 </ul>
+
 <p>
 These requirements apply both to literal constants and to the result
 of evaluating <a href="#Constant_expressions">constant
 expressions</a>.
 </p>
 
+
 <h2 id="Variables">Variables</h2>
 
 <p>