go/ssa: use correct type for variadic parameter in wrapper methods

For some reason, the type of the last Param in a wrapper function
for a variadic method (T) f(...U) had type [][]U instead of []U.
(Possibly a workaround for a long-since fixed bug in go/types?)

Added a sanity check to ensure that the common suffix of
fn.Params and fn.Signature.Params match in type.

Fixes golang/go#27453

Change-Id: I9506f4f67a7ff3a283e9ec0142f638aad00287a9
Reviewed-on: https://go-review.googlesource.com/134515
Reviewed-by: Robert Griesemer <gri@golang.org>

go/ssa/sanity.go

@@ -446,6 +446,17 @@ func (s *sanity) checkFunction(fn *Function) bool {
 		if p.Parent() != fn {
 			s.errorf("Param %s at index %d has wrong parent", p.Name(), i)
 		}
+		// Check common suffix of Signature and Params match type.
+		if sig := fn.Signature; sig != nil {
+			j := i - len(fn.Params) + sig.Params().Len() // index within sig.Params
+			if j < 0 {
+				continue
+			}
+			if !types.Identical(p.Type(), sig.Params().At(j).Type()) {
+				s.errorf("Param %s at index %d has wrong type (%s, versus %s in Signature)", p.Name(), i, p.Type(), sig.Params().At(j).Type())
+
+			}
+		}
 		s.checkReferrerList(p)
 	}
 	for i, fv := range fn.FreeVars {

go/ssa/wrappers.go

@@ -141,13 +141,9 @@ func makeWrapper(prog *Program, sel *types.Selection) *Function {
 // start is the index of the first regular parameter to use.
 //
 func createParams(fn *Function, start int) {
-	var last *Parameter
 	tparams := fn.Signature.Params()
 	for i, n := start, tparams.Len(); i < n; i++ {
-		last = fn.addParamObj(tparams.At(i))
+		fn.addParamObj(tparams.At(i))
-	}
-	if fn.Signature.Variadic() {
-		last.typ = types.NewSlice(last.typ)
 	}
 }