cmd/gofmt: call("arg", newline /* comment */) is changed to syntax error

[rillig]

What version of Go are you using (go version)?

go version go1.10.3 windows/amd64

Does this issue reproduce with the latest release?

Yes, see https://play.golang.org/p/Gv-AmNeMSdT

What did you do?

Open https://play.golang.org/p/Gv-AmNeMSdT, click Format.

What did you expect to see?

After formatting, the program still compiles.

What did you see instead?

Gofmt removes the comma, leading to a syntax error:

prog.go:8:33: syntax error: unexpected newline, expecting comma or )

[dominikh]

/cc @griesemer

[agnivade]

Seems like a valid bug to me. Any reason this needs to be 'NeedsDecision' ?

[dominikh]

@agnivade No reason other than me misunderstanding the labels, it seems.

[rillig]

The related code is in package go/printer:

• printer.exprList checks whether the arguments are on a single line (line == endLine).
• If the arguments span multiple lines, the comma is inserted correctly.
• If the code for the single-line case is deleted, the comma is inserted correctly, but the comment is not indented properly.

Test cases:

package main

func issue27574a() {
	print("",
		/**/)
}

func issue27574b() {
	print(`
	`,
		/**/)
}

func issue27574c() {
	print(
		"",
		/**/)
}

func issue27574d() {
	print(
		"",
		/**/)
}

func issue27574e() {
	print("", "",
		/**/)
}

[griesemer]

Too late for 1.12

[linsite]

Now gofmt rejects the input. Is this fixed or maybe a new one?

8:33: missing ',' before newline in argument list

[rillig]

Maybe gofmt got more careful not to destroy valid code by validating that the reformatted code still compiles, before actually writing the reformatted code to a file.