fix: don't include generic types in variants

Author: alecthomas

bin/hermit.hcl

@@ -0,0 +1,3 @@
+env = {
+  "PATH": "${HERMIT_ENV}/scripts:${PATH}",
+}

cmd/go-check-sumtype/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"flag"
 	"log"
 	"os"
 	"strings"
@@ -11,7 +12,8 @@ import (
 
 func main() {
 	log.SetFlags(0)
-	if len(os.Args) < 2 {
+	flag.Parse()
+	if len(flag.Args()) < 1 {
 		log.Fatalf("Usage: sumtype <packages>\n")
 	}
 	args := os.Args[1:]

decl.go

@@ -57,6 +57,7 @@ func findSumTypeDecls(pkgs []*packages.Package) ([]sumTypeDecl, error) {
 					}
 					pos = pkg.Fset.Position(tspec.Pos())
 					decl := sumTypeDecl{Package: pkg, TypeName: tspec.Name.Name, Pos: pos}
+					debugf("found sum type decl: %s.%s", decl.Package.PkgPath, decl.TypeName)
 					decls = append(decls, decl)
 					break
 				}

def.go

@@ -1,11 +1,21 @@
 package gochecksumtype
 
 import (
+	"flag"
 	"fmt"
 	"go/token"
 	"go/types"
+	"log"
 )
 
+var debug = flag.Bool("debug", false, "enable debug logging")
+
+func debugf(format string, args ...interface{}) {
+	if *debug {
+		log.Printf(format, args...)
+	}
+}
+
 // Error as returned by Run()
 type Error interface {
 	error
@@ -107,6 +117,7 @@ func newSumTypeDef(pkg *types.Package, decl sumTypeDecl) (*sumTypeDef, error) {
 		Decl: decl,
 		Ty:   iface,
 	}
+	debugf("searching for variants of %s.%s\n", pkg.Path(), decl.TypeName)
 	for _, name := range pkg.Scope().Names() {
 		obj, ok := pkg.Scope().Lookup(name).(*types.TypeName)
 		if !ok {
@@ -116,7 +127,12 @@ func newSumTypeDef(pkg *types.Package, decl sumTypeDecl) (*sumTypeDef, error) {
 		if types.Identical(ty.Underlying(), iface) {
 			continue
 		}
+		// Skip generic types.
+		if named, ok := ty.(*types.Named); ok && named.TypeParams() != nil {
+			continue
+		}
 		if types.Implements(ty, iface) || types.Implements(types.NewPointer(ty), iface) {
+			debugf("  found variant: %s.%s\n", pkg.Path(), obj.Name())
 			def.Variants = append(def.Variants, obj)
 		}
 	}

scripts/go-check-sumtype

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -euo pipefail
+basedir="$(dirname "$0")/.."
+name="$(basename "$0")"
+dest="${basedir}/build/devel"
+mkdir -p "$dest"
+(cd "${basedir}" && ./bin/go build -ldflags="-s -w -buildid=" -o "$dest/${name}" "./cmd/${name}") && exec "$dest/${name}" "$@"

testdata/sum.go

@@ -0,0 +1,23 @@
+package testdata
+
+//sumtype:decl
+type Sum interface{ sum() }
+
+type A struct{}
+
+func (A) sum() {}
+
+type B struct{}
+
+func (B) sum() {}
+
+type C[T any] struct{}
+
+func (C[T]) sum() {}
+
+func SumSwitch(x Sum) {
+	switch x.(type) {
+	case A:
+	case B:
+	}
+}