imports: fix lost line between package and import statement

Fixes golang/go#26290

Change-Id: Ide797a46bf1d0d6070940fb5b9db3e76502bc528
Reviewed-on: https://go-review.googlesource.com/122736
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go/ast/astutil/imports.go

@@ -101,8 +101,8 @@ func AddNamedImport(fset *token.FileSet, f *ast.File, name, ipath string) (added
 			impDecl.TokPos = f.Decls[lastImport].End()
 		} else {
 			// There are no existing imports.
-			// Our new import goes after the package declaration and after
-			// the comment, if any, that starts on the same line as the
+			// Our new import, preceded by a blank line,  goes after the package declaration
+			// and after the comment, if any, that starts on the same line as the
 			// package declaration.
 			impDecl.TokPos = f.Package
 
@@ -112,7 +112,8 @@ func AddNamedImport(fset *token.FileSet, f *ast.File, name, ipath string) (added
 				if file.Line(c.Pos()) > pkgLine {
 					break
 				}
-				impDecl.TokPos = c.End()
+				// +2 for a blank line
+				impDecl.TokPos = c.End() + 2
 			}
 		}
 		f.Decls = append(f.Decls, nil)

go/ast/astutil/imports_test.go

@@ -367,6 +367,7 @@ type T time.Time
 type T time.Time
 `,
 		out: `package main // comment
+
 import "time"
 
 type T time.Time
@@ -399,6 +400,7 @@ type T time.Time
 `,
 		out: `// comment before
 package main // comment on
+
 import "time"
 
 type T time.Time

imports/fix_test.go

@@ -1019,6 +1019,38 @@ func main() {
 		_ errors.Frame
 	)
 }
+`,
+	},
+
+	{
+		name: "issue #26290 1",
+		in: `package p // comment
+
+import "math"
+
+var _ = fmt.Printf
+`,
+		out: `package p // comment
+
+import "fmt"
+
+var _ = fmt.Printf
+`,
+	},
+
+	{
+		name: "issue #26290 2",
+		in: `package p
+
+import "math"
+
+var _ = fmt.Printf
+`,
+		out: `package p
+
+import "fmt"
+
+var _ = fmt.Printf
 `,
 	},
 }