go/internal/gcimporter: fix ureader.go handling of local defined types

In unified IR, local defined types are promoted to a global defined
type with a "vargen" suffix. These shouldn't actually be exposed to
go/types users, because they're only relevant within function bodies,
which go/types doesn't support importing.

Moreover, in the case of defined types that were declared within a
generic function, they can have ambient type parameters, which the
go/types importer doesn't know how to handle (because they shouldn't
be needed for that use case).

While here, prune the gcimporter_test.go skip list, because some of
the listed failures have actually been fixed and all of them are
specific to the Go 1.18 (nounified) frontend. They all work correctly
with GOEXPERIMENT=unified.

Fixes #55110.

Change-Id: I7dd8b86355d910dfed1d47edbad7695144c3f84d
Reviewed-on: https://go-review.googlesource.com/c/go/+/431495
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/internal/gcimporter/gcimporter_test.go

@@ -173,13 +173,14 @@ func TestImportTypeparamTests(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	skip := map[string]string{
-		"equal.go":      "inconsistent embedded sorting", // TODO(rfindley): investigate this.
-		"nested.go":     "fails to compile",              // TODO(rfindley): investigate this.
-		"issue50417.go": "inconsistent interface member sorting",
-		"issue53419.go": "fails to compile",
-		"issue53477.go": "fails to compile",
-		"issue55101.go": "fails to compile",
+	var skip map[string]string
+	if !goexperiment.Unified {
+		// The Go 1.18 frontend still fails several cases.
+		skip = map[string]string{
+			"equal.go":      "inconsistent embedded sorting", // TODO(rfindley): investigate this.
+			"nested.go":     "fails to compile",              // TODO(rfindley): investigate this.
+			"issue55101.go": "fails to compile",
+		}
 	}
 
 	for _, entry := range list {

src/go/internal/gcimporter/support.go

@@ -167,3 +167,17 @@ type typeInfo struct {
 	idx     pkgbits.Index
 	derived bool
 }
+
+// See cmd/compile/internal/types.SplitVargenSuffix.
+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, ""
+}

src/go/internal/gcimporter/ureader.go

@@ -469,6 +469,11 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
 		return objPkg, objName
 	}
 
+	// Ignore local types promoted to global scope (#55110).
+	if _, suffix := splitVargenSuffix(objName); suffix != "" {
+		return objPkg, objName
+	}
+
 	if objPkg.Scope().Lookup(objName) == nil {
 		dict := pr.objDictIdx(idx)