syntax: don't parse $foo#bar as a comment

And the same goes for:

    ${foo}#bar
    'foo'#bar
    "foo"#bar

Note that this isn't a complete fix, per the added TODOs,
since we don't have a fix for command substitutions.
However, this still fixes four cases where we got it wrong.

Fixes #1003.

syntax/filetests_test.go

@@ -1743,6 +1743,13 @@ var fileTests = []testCase{
 		Strs:   []string{"foo#bar"},
 		common: litWord("foo#bar"),
 	},
+	{
+		Strs: []string{"$foo#bar foo#$bar"},
+		common: call(
+			word(litParamExp("foo"), lit("#bar")),
+			word(lit("foo#"), litParamExp("bar")),
+		),
+	},
 	{
 		Strs:   []string{"{ echo } }; }"},
 		common: block(litStmt("echo", "}", "}")),

syntax/lexer.go

@@ -270,6 +270,18 @@ skipSpace:
 		case ';', '"', '\'', '(', ')', '$', '|', '&', '>', '<', '`':
 			p.tok = p.regToken(r)
 		case '#':
+			// If we're parsing $foo#bar, ${foo}#bar, 'foo'#bar, or "foo"#bar,
+			// #bar is a continuation of the same word, not a comment.
+			// TODO: support $(foo)#bar and `foo`#bar as well, which is slightly tricky,
+			// as we can't easily tell them apart from (foo)#bar and `#bar`,
+			// where #bar should remain a comment.
+			if !p.spaced {
+				switch p.tok {
+				case _LitWord, rightBrace, sglQuote, dblQuote:
+					p.advanceLitNone(r)
+					return
+				}
+			}
 			r = p.rune()
 			p.newLit(r)
 		runeLoop:

syntax/printer_test.go

@@ -649,6 +649,13 @@ var printTests = []printCase{
 	},
 	samePrint("(\n\t((foo++))\n)"),
 	samePrint("(foo && bar)"),
+	samePrint(`$foo#bar ${foo}#bar 'foo'#bar "foo"#bar`),
+	// TODO: support cases with command substitutions as well
+	// {
+	// 	"`foo`#bar",
+	// 	"$(foo)#bar",
+	// },
+	// samePrint(`$("foo"#bar)#bar`),
 }
 
 func TestPrintWeirdFormat(t *testing.T) {