go.tools/ssa: fix bug in makeBridgeMethod for promoted interfaces.

The method index was hard-coded to zero, which works some of
the time.  Apparently I just forgot to implement the
method-table lookup...

Added regression test.

R=gri
CC=golang-dev
https://golang.org/cl/9916043

importer/pkginfo.go

@@ -20,7 +20,7 @@ type PackageInfo struct {
 	// Type-checker deductions.
 	types     map[ast.Expr]types.Type        // inferred types of expressions
 	constants map[ast.Expr]exact.Value       // values of constant expressions
-	idents    map[*ast.Ident]types.Object    // resoved objects for named entities
+	idents    map[*ast.Ident]types.Object    // resolved objects for named entities
 	typecases map[*ast.CaseClause]*types.Var // implicit vars for single-type typecases
 }
 
@@ -47,6 +47,7 @@ func (info *PackageInfo) TypeOf(e ast.Expr) types.Type {
 
 // ValueOf returns the value of expression e if it is a constant, nil
 // otherwise.
+// Precondition: e belongs to the package's ASTs.
 //
 func (info *PackageInfo) ValueOf(e ast.Expr) exact.Value {
 	return info.constants[e]

ssa/interp/interp_test.go

@@ -132,6 +132,7 @@ var testdataTests = []string{
 	"coverage.go",
 	"mrvchain.go",
 	"boundmeth.go",
+	"ifaceprom.go",
 }
 
 func run(t *testing.T, dir, input string) bool {

ssa/interp/testdata/ifaceprom.go

@@ -0,0 +1,35 @@
+package main
+
+// Test of promotion of methods of an interface embedded within a
+// struct.  In particular, this test excercises that the correct
+// method is called.
+
+type I interface {
+	one() int
+	two() string
+}
+
+type S struct {
+	I
+}
+
+type impl struct{}
+
+func (impl) one() int {
+	return 1
+}
+
+func (impl) two() string {
+	return "two"
+}
+
+func main() {
+	var s S
+	s.I = impl{}
+	if one := s.one(); one != 1 {
+		panic(one)
+	}
+	if two := s.two(); two != "two" {
+		panic(two)
+	}
+}

ssa/promote.go

@@ -314,11 +314,15 @@ func makeBridgeMethod(prog *Program, typ types.Type, cand *candidate) *Function
 		c.Call.Func = cand.concrete
 		c.Call.Args = append(c.Call.Args, v)
 	} else {
+		c.Call.Method = -1
+		iface := v.Type().Underlying().(*types.Interface)
+		for i, n := 0, iface.NumMethods(); i < n; i++ {
+			if iface.Method(i) == cand.method {
+				c.Call.Method = i
+				break
+			}
+		}
 		c.Call.Recv = v
-		// TODO(adonovan): fix: this looks wrong!  Need to
-		// find method index within
-		// v.Type().Underlying().(*types.Interface).Methods()
-		c.Call.Method = 0
 	}
 	for _, arg := range fn.Params[1:] {
 		c.Call.Args = append(c.Call.Args, arg)