cmd/gofmt, go/printer: fix mis-alignment of comment on one-line function

Fixes #19544.

Change-Id: I5df67383e9471f030ddafabadf2bc19ce6816f0f
Reviewed-on: https://go-review.googlesource.com/46002
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/printer/nodes.go

@@ -1532,6 +1532,16 @@ func (p *printer) nodeSize(n ast.Node, maxSize int) (size int) {
 	return
 }
 
+// numLines returns the number of lines spanned by node n in the original source.
+func (p *printer) numLines(n ast.Node) int {
+	if from := n.Pos(); from.IsValid() {
+		if to := n.End(); to.IsValid() {
+			return p.lineFor(to) - p.lineFor(from) + 1
+		}
+	}
+	return infinity
+}
+
 // bodySize is like nodeSize but it is specialized for *ast.BlockStmt's.
 func (p *printer) bodySize(b *ast.BlockStmt, maxSize int) int {
 	pos1 := b.Pos()
@@ -1668,7 +1678,9 @@ func (p *printer) declList(list []ast.Decl) {
 			if prev != tok || getDoc(d) != nil {
 				min = 2
 			}
-			p.linebreak(p.lineFor(d.Pos()), min, ignore, false)
+			// start a new section if the next declaration is a function
+			// that spans multiple lines (see also issue #19544)
+			p.linebreak(p.lineFor(d.Pos()), min, ignore, tok == token.FUNC && p.numLines(d) > 1)
 		}
 		p.decl(d)
 	}

src/go/printer/testdata/declarations.golden

@@ -778,6 +778,12 @@ func _() {
 	/* multi-line func because block is on multiple lines */
 }
 
+// test case for issue #19544
+func _()	{}
+func _longer_name_() {	// this comment must not force the {} from above to alignment
+	// multiple lines
+}
+
 // ellipsis parameters
 func _(...int)
 func _(...*int)

src/go/printer/testdata/declarations.input

@@ -795,6 +795,11 @@ func _() { /* multi-line function because of "long-ish" comment - much more comm
 func _() {
 /* multi-line func because block is on multiple lines */ }
 
+// test case for issue #19544
+func _() {}
+func _longer_name_() { // this comment must not force the {} from above to alignment
+	// multiple lines
+}
 
 // ellipsis parameters
 func _(...int)