diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index 15e80a0b1b..5aa8091a5c 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -590,9 +590,9 @@ func fieldIndex(fields []*Var, pkg *Package, name string, foldCase bool) int {
 	return -1
 }
 
-// lookupMethod returns the index of and method with matching package and name, or (-1, nil).
+// methodIndex returns the index of and method with matching package and name, or (-1, nil).
 // See Object.sameId for the meaning of foldCase.
-func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
+func methodIndex(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
 	if name != "_" {
 		for i, m := range methods {
 			if m.sameId(pkg, name, foldCase) {
diff --git a/src/cmd/compile/internal/types2/named.go b/src/cmd/compile/internal/types2/named.go
index 893247de35..57caef123f 100644
--- a/src/cmd/compile/internal/types2/named.go
+++ b/src/cmd/compile/internal/types2/named.go
@@ -6,6 +6,7 @@ package types2
 
 import (
 	"cmd/compile/internal/syntax"
+	"strings"
 	"sync"
 	"sync/atomic"
 )
@@ -444,15 +445,40 @@ func (t *Named) SetUnderlying(underlying Type) {
 }
 
 // AddMethod adds method m unless it is already in the method list.
-// t must not have type arguments.
+// The method must be in the same package as t, and t must not have
+// type arguments.
 func (t *Named) AddMethod(m *Func) {
+	assert(samePkg(t.obj.pkg, m.pkg))
 	assert(t.inst == nil)
 	t.resolve()
-	if i, _ := lookupMethod(t.methods, m.pkg, m.name, false); i < 0 {
+	if t.methodIndex(m.name, false) < 0 {
 		t.methods = append(t.methods, m)
 	}
 }
 
+// methodIndex returns the index of the method with the given name.
+// If foldCase is set, capitalization in the name is ignored.
+// The result is negative if no such method exists.
+func (t *Named) methodIndex(name string, foldCase bool) int {
+	if name == "_" {
+		return -1
+	}
+	if foldCase {
+		for i, m := range t.methods {
+			if strings.EqualFold(m.name, name) {
+				return i
+			}
+		}
+	} else {
+		for i, m := range t.methods {
+			if m.name == name {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
 // TODO(gri) Investigate if Unalias can be moved to where underlying is set.
 func (t *Named) Underlying() Type { return Unalias(t.resolve().underlying) }
 func (t *Named) String() string   { return TypeString(t, nil) }
@@ -553,15 +579,16 @@ loop:
 
 func (n *Named) lookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
 	n.resolve()
-	// If n is an instance, we may not have yet instantiated all of its methods.
-	// Look up the method index in orig, and only instantiate method at the
-	// matching index (if any).
-	i, _ := lookupMethod(n.Origin().methods, pkg, name, foldCase)
-	if i < 0 {
-		return -1, nil
+	if samePkg(n.obj.pkg, pkg) || isExported(name) || foldCase {
+		// If n is an instance, we may not have yet instantiated all of its methods.
+		// Look up the method index in orig, and only instantiate method at the
+		// matching index (if any).
+		if i := n.Origin().methodIndex(name, foldCase); i >= 0 {
+			// For instances, m.Method(i) will be different from the orig method.
+			return i, n.Method(i)
+		}
 	}
-	// For instances, m.Method(i) will be different from the orig method.
-	return i, n.Method(i)
+	return -1, nil
 }
 
 // context returns the type-checker context.
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index a6ccfdb80c..bf07162a21 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -57,7 +57,7 @@ func (s *_TypeSet) Method(i int) *Func { return s.methods[i] }
 
 // LookupMethod returns the index of and method with matching package and name, or (-1, nil).
 func (s *_TypeSet) LookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
-	return lookupMethod(s.methods, pkg, name, foldCase)
+	return methodIndex(s.methods, pkg, name, foldCase)
 }
 
 func (s *_TypeSet) String() string {
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index 82425f64a8..436a7afbaa 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -592,9 +592,9 @@ func fieldIndex(fields []*Var, pkg *Package, name string, foldCase bool) int {
 	return -1
 }
 
-// lookupMethod returns the index of and method with matching package and name, or (-1, nil).
+// methodIndex returns the index of and method with matching package and name, or (-1, nil).
 // See Object.sameId for the meaning of foldCase.
-func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
+func methodIndex(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
 	if name != "_" {
 		for i, m := range methods {
 			if m.sameId(pkg, name, foldCase) {
diff --git a/src/go/types/named.go b/src/go/types/named.go
index 21c0de255d..e053fed76b 100644
--- a/src/go/types/named.go
+++ b/src/go/types/named.go
@@ -8,6 +8,7 @@ package types
 
 import (
 	"go/token"
+	"strings"
 	"sync"
 	"sync/atomic"
 )
@@ -446,15 +447,40 @@ func (t *Named) SetUnderlying(underlying Type) {
 }
 
 // AddMethod adds method m unless it is already in the method list.
-// t must not have type arguments.
+// The method must be in the same package as t, and t must not have
+// type arguments.
 func (t *Named) AddMethod(m *Func) {
+	assert(samePkg(t.obj.pkg, m.pkg))
 	assert(t.inst == nil)
 	t.resolve()
-	if i, _ := lookupMethod(t.methods, m.pkg, m.name, false); i < 0 {
+	if t.methodIndex(m.name, false) < 0 {
 		t.methods = append(t.methods, m)
 	}
 }
 
+// methodIndex returns the index of the method with the given name.
+// If foldCase is set, capitalization in the name is ignored.
+// The result is negative if no such method exists.
+func (t *Named) methodIndex(name string, foldCase bool) int {
+	if name == "_" {
+		return -1
+	}
+	if foldCase {
+		for i, m := range t.methods {
+			if strings.EqualFold(m.name, name) {
+				return i
+			}
+		}
+	} else {
+		for i, m := range t.methods {
+			if m.name == name {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
 // TODO(gri) Investigate if Unalias can be moved to where underlying is set.
 func (t *Named) Underlying() Type { return Unalias(t.resolve().underlying) }
 func (t *Named) String() string   { return TypeString(t, nil) }
@@ -555,15 +581,16 @@ loop:
 
 func (n *Named) lookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
 	n.resolve()
-	// If n is an instance, we may not have yet instantiated all of its methods.
-	// Look up the method index in orig, and only instantiate method at the
-	// matching index (if any).
-	i, _ := lookupMethod(n.Origin().methods, pkg, name, foldCase)
-	if i < 0 {
-		return -1, nil
+	if samePkg(n.obj.pkg, pkg) || isExported(name) || foldCase {
+		// If n is an instance, we may not have yet instantiated all of its methods.
+		// Look up the method index in orig, and only instantiate method at the
+		// matching index (if any).
+		if i := n.Origin().methodIndex(name, foldCase); i >= 0 {
+			// For instances, m.Method(i) will be different from the orig method.
+			return i, n.Method(i)
+		}
 	}
-	// For instances, m.Method(i) will be different from the orig method.
-	return i, n.Method(i)
+	return -1, nil
 }
 
 // context returns the type-checker context.
diff --git a/src/go/types/typeset.go b/src/go/types/typeset.go
index d164749996..4e3865ddc4 100644
--- a/src/go/types/typeset.go
+++ b/src/go/types/typeset.go
@@ -57,7 +57,7 @@ func (s *_TypeSet) Method(i int) *Func { return s.methods[i] }
 
 // LookupMethod returns the index of and method with matching package and name, or (-1, nil).
 func (s *_TypeSet) LookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
-	return lookupMethod(s.methods, pkg, name, foldCase)
+	return methodIndex(s.methods, pkg, name, foldCase)
 }
 
 func (s *_TypeSet) String() string {