From 8b01b13b2fb53f2d9485268a6e6c2137cbb7e710 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Mart=C3=AD?= <mvdan@mvdan.cc>
Date: Sat, 8 Apr 2023 21:32:20 +0100
Subject: [PATCH] relax rule to insert newlines in func signatures

In particular, in cases where a field list fits all in one line,
adding a newline at the end of the field list is clearly wrong.

Fixes #235.
---
 format/format.go                 |  9 ++++++++-
 testdata/script/typeparams.txtar | 31 +++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/format/format.go b/format/format.go
index 36da9fb..b51f75e 100644
--- a/format/format.go
+++ b/format/format.go
@@ -546,12 +546,19 @@ func (f *fumpter) applyPre(c *astutil.Cursor) {
 
 			if f.Line(sign.Pos()) != endLine {
 				handleMultiLine := func(fl *ast.FieldList) {
+					// Refuse to insert a newline before the closing token
+					// if the list is empty or all in one line.
 					if fl == nil || len(fl.List) == 0 {
 						return
 					}
+					fieldOpeningLine := f.Line(fl.Opening)
+					fieldClosingLine := f.Line(fl.Closing)
+					if fieldOpeningLine == fieldClosingLine {
+						return
+					}
+
 					lastFieldEnd := fl.List[len(fl.List)-1].End()
 					lastFieldLine := f.Line(lastFieldEnd)
-					fieldClosingLine := f.Line(fl.Closing)
 					isLastFieldOnFieldClosingLine := lastFieldLine == fieldClosingLine
 					isLastFieldOnSigClosingLine := lastFieldLine == endLine
 
diff --git a/testdata/script/typeparams.txtar b/testdata/script/typeparams.txtar
index 26c2f6d..2552004 100644
--- a/testdata/script/typeparams.txtar
+++ b/testdata/script/typeparams.txtar
@@ -36,6 +36,22 @@ type CombineEmbeds interface {
 func Caller() {
 	Foo[int,int](1,2)
 }
+
+func Issue235[K interface {
+	comparable
+	constraints.Ordered
+}, V any](m map[K]V) []K {
+	keys := maps.Keys(m)
+	slices.Sort(keys)
+	return keys
+}
+
+func multilineParams[V any](p1 V,
+	p2 V) {
+
+	println("body")
+
+}
 -- foo.go.golden --
 package p
 
@@ -65,3 +81,18 @@ type CombineEmbeds interface {
 func Caller() {
 	Foo[int, int](1, 2)
 }
+
+func Issue235[K interface {
+	comparable
+	constraints.Ordered
+}, V any](m map[K]V) []K {
+	keys := maps.Keys(m)
+	slices.Sort(keys)
+	return keys
+}
+
+func multilineParams[V any](p1 V,
+	p2 V,
+) {
+	println("body")
+}