cmd/compile: ensure we replace package placeholder in type names

We want package names exposed by reflect to be things like
main.F[main.foo], not main.F["".foo].

Fixes #49547

Change-Id: I182411a75d56ce1f64fde847e5b9ee74ce44e00b
Reviewed-on: https://go-review.googlesource.com/c/go/+/363656
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/types/fmt.go

@@ -140,11 +140,17 @@ func sconv2(b *bytes.Buffer, s *Sym, verb rune, mode fmtMode) {
 }
 
 func symfmt(b *bytes.Buffer, s *Sym, verb rune, mode fmtMode) {
+	name := s.Name
 	if q := pkgqual(s.Pkg, verb, mode); q != "" {
 		b.WriteString(q)
 		b.WriteByte('.')
+		if mode == fmtTypeIDName {
+			// If name is a generic instantiation, it might have local package placeholders
+			// in it. Replace those placeholders with the package name. See issue 49547.
+			name = strings.Replace(name, LocalPkg.Prefix, q, -1)
+		}
 	}
-	b.WriteString(s.Name)
+	b.WriteString(name)
 }
 
 // pkgqual returns the qualifier that should be used for printing

test/typeparam/issue49547.go

@@ -0,0 +1,22 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "fmt"
+
+type foo int
+
+func main() {
+	want := "main.F[main.foo]"
+	got := fmt.Sprintf("%T", F[foo]{})
+	if got != want {
+		fmt.Printf("want: %s, got: %s\n", want, got)
+	}
+}
+
+type F[T any] struct {
+}

test/typeparam/nested.out

@@ -1,4 +1,4 @@
 0,3: main.T·2[int;int]
-4,7: main.T·2[int;"".U·3[int;int]]
-22,23: main.T·2["".Int;"".Int]
-26,27: main.T·2["".Int;"".U·3["".Int;"".Int]]
+4,7: main.T·2[int;main.U·3[int;int]]
+22,23: main.T·2[main.Int;main.Int]
+26,27: main.T·2[main.Int;main.U·3[main.Int;main.Int]]