[dev.typeparams] cmd/compile: keep export format unchanged if no type params are exported

Added new export tags 'G' and 'U' to export parameterized
functions/methods and parameterized types respectively. This has the
advantage that the Go 1.18 format remains backward-compatible with the
Go 1.17 format if no type parameters are exported.

Change-Id: I9dba8faaa65609eb3f9c693bd0c79daee98bd865
Reviewed-on: https://go-review.googlesource.com/c/go/+/340989
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 6 insertions, 13 deletions

diff --git a/src/go/internal/gcimporter/iimport.go b/src/go/internal/gcimporter/iimport.go
index b300860e94..d4778d3a74 100644
--- a/src/go/internal/gcimporter/iimport.go
+++ b/src/go/internal/gcimporter/iimport.go
@@ -290,24 +290,14 @@ func (r *importReader) obj(name string) {
 		r.declare(types.NewConst(pos, r.currPkg, name, typ, val))
 
 	case 'F':
-		if r.p.exportVersion >= iexportVersionGenerics {
-			numTparams := r.uint64()
-			if numTparams > 0 {
-				errorf("unexpected tparam")
-			}
-		}
 		sig := r.signature(nil)
 
 		r.declare(types.NewFunc(pos, r.currPkg, name, sig))
 
-	case 'T':
-		if r.p.exportVersion >= iexportVersionGenerics {
-			numTparams := r.uint64()
-			if numTparams > 0 {
-				errorf("unexpected tparam")
-			}
-		}
+	case 'G':
+		errorf("unexpected parameterized function/method")
 
+	case 'T':
 		// Types can be recursive. We need to setup a stub
 		// declaration before recursing.
 		obj := types.NewTypeName(pos, r.currPkg, name, nil)
@@ -328,6 +318,9 @@ func (r *importReader) obj(name string) {
 			}
 		}
 
+	case 'U':
+		errorf("unexpected parameterized type")
+
 	case 'V':
 		typ := r.typ()