cmd/compile: fix reflect naming of local generic types

To disambiguate local types, we append a "·N" suffix to their name and
then trim it off again when producing their runtime type descriptors.

However, if a local type is generic, then we were further appending
the type arguments after this suffix, and the code in types/fmt.go
responsible for trimming didn't know to handle this.

We could extend the types/fmt.go code to look for the "·N" suffix
elsewhere in the type name, but this is risky because it could
legitimately (albeit unlikely) appear in struct field tags.

Instead, the most robust solution is to just change the mangling logic
to keep the "·N" suffix at the end, where types/fmt.go can easily and
reliably trim it.

Note: the "·N" suffix is still visible within the type arguments
list (e.g., the "·3" suffixes in nested.out), because we currently use
the link strings in the type arguments list.

Fixes #54456.

Change-Id: Ie9beaf7e5330982f539bff57b8d48868a3674a37
Reviewed-on: https://go-review.googlesource.com/c/go/+/424901
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/noder/reader.go

@@ -777,8 +777,13 @@ func (dict *readerDict) mangle(sym *types.Sym) *types.Sym {
 		return sym
 	}
 
+	// If sym is a locally defined generic type, we need the suffix to
+	// stay at the end after mangling so that types/fmt.go can strip it
+	// out again when writing the type's runtime descriptor (#54456).
+	base, suffix := types.SplitVargenSuffix(sym.Name)
+
 	var buf strings.Builder
-	buf.WriteString(sym.Name)
+	buf.WriteString(base)
 	buf.WriteByte('[')
 	for i, targ := range dict.targs {
 		if i > 0 {
@@ -791,6 +796,7 @@ func (dict *readerDict) mangle(sym *types.Sym) *types.Sym {
 		buf.WriteString(targ.LinkString())
 	}
 	buf.WriteByte(']')
+	buf.WriteString(suffix)
 	return sym.Pkg.Lookup(buf.String())
 }
 

src/cmd/compile/internal/noder/writer.go

@@ -933,8 +933,11 @@ func (w *writer) qualifiedIdent(obj types2.Object) {
 		decl, ok := w.p.typDecls[obj.(*types2.TypeName)]
 		assert(ok)
 		if decl.gen != 0 {
-			// TODO(mdempsky): Find a better solution than embedding middle
+			// For local defined types, we embed a scope-disambiguation
-			// dot in the symbol name; this is terrible.
+			// number directly into their name. types.SplitVargenSuffix then
+			// knows to look for this.
+			//
+			// TODO(mdempsky): Find a better solution; this is terrible.
 			name = fmt.Sprintf("%s·%v", name, decl.gen)
 		}
 	}

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

@@ -340,13 +340,9 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 		// non-fmtTypeID modes.
 		sym := t.Sym()
 		if mode != fmtTypeID {
-			i := len(sym.Name)
+			base, _ := SplitVargenSuffix(sym.Name)
-			for i > 0 && sym.Name[i-1] >= '0' && sym.Name[i-1] <= '9' {
+			if len(base) < len(sym.Name) {
-				i--
+				sym = &Sym{Pkg: sym.Pkg, Name: base}
-			}
-			const dot = "·"
-			if i >= len(dot) && sym.Name[i-len(dot):i] == dot {
-				sym = &Sym{Pkg: sym.Pkg, Name: sym.Name[:i-len(dot)]}
 			}
 		}
 		sconv2(b, sym, verb, mode)
@@ -704,6 +700,21 @@ func fldconv(b *bytes.Buffer, f *Field, verb rune, mode fmtMode, visited map[*Ty
 	}
 }
 
+// SplitVargenSuffix returns name split into a base string and a ·N
+// suffix, if any.
+func SplitVargenSuffix(name string) (base, suffix string) {
+	i := len(name)
+	for i > 0 && name[i-1] >= '0' && name[i-1] <= '9' {
+		i--
+	}
+	const dot = "·"
+	if i >= len(dot) && name[i-len(dot):i] == dot {
+		i -= len(dot)
+		return name[:i], name[i:]
+	}
+	return name, ""
+}
+
 // Val
 
 func FmtConst(v constant.Value, sharp bool) string {

test/run.go

@@ -1990,6 +1990,7 @@ var go118Failures = setOf(
 	"fixedbugs/issue54343.go",  // 1.18 compiler assigns receiver parameter to global variable
 	"typeparam/nested.go",      // 1.18 compiler doesn't support function-local types with generics
 	"typeparam/issue51521.go",  // 1.18 compiler produces bad panic message and link error
+	"typeparam/issue54456.go",  // 1.18 compiler fails to distinguish local generic types
 	"typeparam/issue54497.go",  // 1.18 compiler is more conservative about inlining due to repeated issues
 	"typeparam/mdempsky/16.go", // 1.18 compiler uses interface shape type in failed type assertions
 	"typeparam/mdempsky/17.go", // 1.18 compiler mishandles implicit conversions from range loops

test/typeparam/issue54456.go

@@ -0,0 +1,37 @@
+// run
+
+// Copyright 2022 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.
+
+// The Go 1.18 frontend failed to disambiguate instantiations of
+// different, locally defined generic types with the same name.
+//
+// The unified frontend also exposed the scope-disambiguation mangling
+// to end users in reflect data.
+
+package main
+
+import (
+	"reflect"
+)
+
+func one() any { type T[_ any] int; return T[int](0) }
+func two() any { type T[_ any] int; return T[int](0) }
+
+func main() {
+	p, q := one(), two()
+
+	// p and q have different dynamic types; this comparison should
+	// evaluate false.
+	if p == q {
+		panic("bad type identity")
+	}
+
+	for _, x := range []any{p, q} {
+		// The names here should not contain "·1" or "·2".
+		if name := reflect.TypeOf(x).String(); name != "main.T[int]" {
+			panic(name)
+		}
+	}
+}

test/typeparam/nested.out

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