cmd/compile: fixing 15.go for -G=3

Required two changes:

 - avoid creating a closure in the case where the actual receiver of an
   embedded method is not generic even though the base operand of the
   selector is generic. This is similar to the test suggested by wayne
   zuo - I thought it was clear in buildClosure, and easier to comment.

 - Propagate //go:nointerface to base generic methods and then to
   instantiations.

Change-Id: If30c834e4223c2639b7f7e74d44e6087aa9ccd76
Reviewed-on: https://go-review.googlesource.com/c/go/+/344251
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Wayne Zuo <wdvxdr1123@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>

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

@@ -104,6 +104,12 @@ func (g *irgen) funcDecl(out *ir.Nodes, decl *syntax.FuncDecl) {
 		// the Fields to represent the receiver's method set.
 		if recv := fn.Type().Recv(); recv != nil {
 			typ := types.ReceiverBaseType(recv.Type)
+			if typ.OrigSym != nil {
+				// For a generic method, we mark the methods on the
+				// base generic type, since those are the methods
+				// that will be stenciled.
+				typ = typ.OrigSym.Def.Type()
+			}
 			meth := typecheck.Lookdot1(fn, typecheck.Lookup(decl.Name.Value), typ, typ.Methods(), 0)
 			meth.SetNointerface(true)
 		}

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

@@ -304,6 +304,11 @@ func (g *irgen) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 		// of se.Selection, since that will be the type that actually has
 		// the method.
 		recv := deref(se.Selection.Type.Recv().Type)
+		if len(recv.RParams()) == 0 {
+			// The embedded type that actually has the method is not
+			// actually generic, so no need to build a closure.
+			return x
+		}
 		baseType := recv.OrigSym.Def.Type()
 		var gf *ir.Name
 		for _, m := range baseType.Methods().Slice() {
@@ -491,6 +496,9 @@ func (g *irgen) instantiateMethods() {
 		baseSym := typ.OrigSym
 		baseType := baseSym.Def.(*ir.Name).Type()
 		for j, _ := range typ.Methods().Slice() {
+			if baseType.Methods().Slice()[j].Nointerface() {
+				typ.Methods().Slice()[j].SetNointerface(true)
+			}
 			baseNname := baseType.Methods().Slice()[j].Nname.(*ir.Name)
 			// Eagerly generate the instantiations and dictionaries that implement these methods.
 			// We don't use the instantiations here, just generate them (and any

test/run.go

@@ -2185,8 +2185,7 @@ var g3Failures = setOf(
 
 	"typeparam/nested.go", // -G=3 doesn't support function-local types with generics
 
-	"typeparam/mdempsky/4.go",  // -G=3 can't export functions with labeled breaks in loops
-	"typeparam/mdempsky/15.go", // ICE in (*irgen).buildClosure
+	"typeparam/mdempsky/4.go", // -G=3 can't export functions with labeled breaks in loops
 )
 
 var unifiedFailures = setOf(