go/types, types2: better error message when missing method is unexported

Change lookupMethod such that "foldCase" means "ignore case
and package" and analyze a lookup result further to determine
if a method name was not exported, and report a better error
message in that case.

Fixes #59831.

Change-Id: Ice6222e1fc00dba13caeda6c48971e8473d12da5
Reviewed-on: https://go-review.googlesource.com/c/go/+/549298
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

4 files changed, 172 insertions, 10 deletions

diff --git a/src/cmd/compile/internal/types2/issues_test.go b/src/cmd/compile/internal/types2/issues_test.go
index a8b70b8964..a8893cf6de 100644
--- a/src/cmd/compile/internal/types2/issues_test.go
+++ b/src/cmd/compile/internal/types2/issues_test.go
@@ -1003,3 +1003,76 @@ type S struct{ A }
 		t.Fatalf("got %q; want %q", got, want)
 	}
 }
+
+func TestIssue59831(t *testing.T) {
+	// Package a exports a type S with an unexported method m;
+	// the tests check the error messages when m is not found.
+	const asrc = `package a; type S struct{}; func (S) m() {}`
+	apkg := mustTypecheck(asrc, nil, nil)
+
+	// Package b exports a type S with an exported method m;
+	// the tests check the error messages when M is not found.
+	const bsrc = `package b; type S struct{}; func (S) M() {}`
+	bpkg := mustTypecheck(bsrc, nil, nil)
+
+	tests := []struct {
+		imported *Package
+		src, err string
+	}{
+		// tests importing a (or nothing)
+		{apkg, `package a1; import "a"; var _ interface { M() } = a.S{}`,
+			"a.S does not implement interface{M()} (missing method M) have m() want M()"},
+
+		{apkg, `package a2; import "a"; var _ interface { m() } = a.S{}`,
+			"a.S does not implement interface{m()} (unexported method m)"}, // test for issue
+
+		{nil, `package a3; type S struct{}; func (S) m(); var _ interface { M() } = S{}`,
+			"S does not implement interface{M()} (missing method M) have m() want M()"},
+
+		{nil, `package a4; type S struct{}; func (S) m(); var _ interface { m() } = S{}`,
+			""}, // no error expected
+
+		{nil, `package a5; type S struct{}; func (S) m(); var _ interface { n() } = S{}`,
+			"S does not implement interface{n()} (missing method n)"},
+
+		// tests importing b (or nothing)
+		{bpkg, `package b1; import "b"; var _ interface { m() } = b.S{}`,
+			"b.S does not implement interface{m()} (missing method m) have M() want m()"},
+
+		{bpkg, `package b2; import "b"; var _ interface { M() } = b.S{}`,
+			""}, // no error expected
+
+		{nil, `package b3; type S struct{}; func (S) M(); var _ interface { M() } = S{}`,
+			""}, // no error expected
+
+		{nil, `package b4; type S struct{}; func (S) M(); var _ interface { m() } = S{}`,
+			"S does not implement interface{m()} (missing method m) have M() want m()"},
+
+		{nil, `package b5; type S struct{}; func (S) M(); var _ interface { n() } = S{}`,
+			"S does not implement interface{n()} (missing method n)"},
+	}
+
+	for _, test := range tests {
+		// typecheck test source
+		conf := Config{Importer: importHelper{pkg: test.imported}}
+		pkg, err := typecheck(test.src, &conf, nil)
+		if err == nil {
+			if test.err != "" {
+				t.Errorf("package %s: got no error, want %q", pkg.Name(), test.err)
+			}
+			continue
+		}
+		if test.err == "" {
+			t.Errorf("package %s: got %q, want not error", pkg.Name(), err.Error())
+		}
+
+		// flatten reported error message
+		errmsg := strings.ReplaceAll(err.Error(), "\n", " ")
+		errmsg = strings.ReplaceAll(errmsg, "\t", "")
+
+		// verify error message
+		if !strings.Contains(errmsg, test.err) {
+			t.Errorf("package %s: got %q, want %q", pkg.Name(), errmsg, test.err)
+		}
+	}
+}
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index 014a5489cd..bc47c15060 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -96,7 +96,7 @@ func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 // and missingMethod (the latter doesn't care about struct fields).
 //
 // If foldCase is true, method names are considered equal if they are equal
-// with case folding.
+// with case folding, irrespective of which package they are in.
 //
 // The resulting object may not be fully type-checked.
 func lookupFieldOrMethodImpl(T Type, addressable bool, pkg *Package, name string, foldCase bool) (obj Object, index []int, indirect bool) {
@@ -343,6 +343,7 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 		ok = iota
 		notFound
 		wrongName
+		unexported
 		wrongSig
 		ambigSel
 		ptrRecv
@@ -388,6 +389,11 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 					f, _ = obj.(*Func)
 					if f != nil {
 						state = wrongName
+						if f.name == m.name {
+							// If the names are equal, f must be unexported
+							// (otherwise the package wouldn't matter).
+							state = unexported
+						}
 					}
 				}
 				break
@@ -436,8 +442,9 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 			}
 		case wrongName:
 			fs, ms := check.funcString(f, false), check.funcString(m, false)
-			*cause = check.sprintf("(missing method %s)\n\t\thave %s\n\t\twant %s",
-				m.Name(), fs, ms)
+			*cause = check.sprintf("(missing method %s)\n\t\thave %s\n\t\twant %s", m.Name(), fs, ms)
+		case unexported:
+			*cause = check.sprintf("(unexported method %s)", m.Name())
 		case wrongSig:
 			fs, ms := check.funcString(f, false), check.funcString(m, false)
 			if fs == ms {
@@ -582,11 +589,12 @@ func fieldIndex(fields []*Var, pkg *Package, name string) int {
 }
 
 // lookupMethod returns the index of and method with matching package and name, or (-1, nil).
-// If foldCase is true, method names are considered equal if they are equal with case folding.
+// If foldCase is true, method names are considered equal if they are equal with case folding
+// and their packages are ignored (e.g., pkg1.m, pkg1.M, pkg2.m, and pkg2.M are all equal).
 func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
 	if name != "_" {
 		for i, m := range methods {
-			if (m.name == name || foldCase && strings.EqualFold(m.name, name)) && m.sameId(pkg, m.name) {
+			if m.sameId(pkg, name) || foldCase && strings.EqualFold(m.name, name) {
 				return i, m
 			}
 		}
diff --git a/src/go/types/issues_test.go b/src/go/types/issues_test.go
index 91631fe9c7..b4c8218bc4 100644
--- a/src/go/types/issues_test.go
+++ b/src/go/types/issues_test.go
@@ -1013,3 +1013,76 @@ type S struct{ A }
 		t.Fatalf("got %q; want %q", got, want)
 	}
 }
+
+func TestIssue59831(t *testing.T) {
+	// Package a exports a type S with an unexported method m;
+	// the tests check the error messages when m is not found.
+	const asrc = `package a; type S struct{}; func (S) m() {}`
+	apkg := mustTypecheck(asrc, nil, nil)
+
+	// Package b exports a type S with an exported method m;
+	// the tests check the error messages when M is not found.
+	const bsrc = `package b; type S struct{}; func (S) M() {}`
+	bpkg := mustTypecheck(bsrc, nil, nil)
+
+	tests := []struct {
+		imported *Package
+		src, err string
+	}{
+		// tests importing a (or nothing)
+		{apkg, `package a1; import "a"; var _ interface { M() } = a.S{}`,
+			"a.S does not implement interface{M()} (missing method M) have m() want M()"},
+
+		{apkg, `package a2; import "a"; var _ interface { m() } = a.S{}`,
+			"a.S does not implement interface{m()} (unexported method m)"}, // test for issue
+
+		{nil, `package a3; type S struct{}; func (S) m(); var _ interface { M() } = S{}`,
+			"S does not implement interface{M()} (missing method M) have m() want M()"},
+
+		{nil, `package a4; type S struct{}; func (S) m(); var _ interface { m() } = S{}`,
+			""}, // no error expected
+
+		{nil, `package a5; type S struct{}; func (S) m(); var _ interface { n() } = S{}`,
+			"S does not implement interface{n()} (missing method n)"},
+
+		// tests importing b (or nothing)
+		{bpkg, `package b1; import "b"; var _ interface { m() } = b.S{}`,
+			"b.S does not implement interface{m()} (missing method m) have M() want m()"},
+
+		{bpkg, `package b2; import "b"; var _ interface { M() } = b.S{}`,
+			""}, // no error expected
+
+		{nil, `package b3; type S struct{}; func (S) M(); var _ interface { M() } = S{}`,
+			""}, // no error expected
+
+		{nil, `package b4; type S struct{}; func (S) M(); var _ interface { m() } = S{}`,
+			"S does not implement interface{m()} (missing method m) have M() want m()"},
+
+		{nil, `package b5; type S struct{}; func (S) M(); var _ interface { n() } = S{}`,
+			"S does not implement interface{n()} (missing method n)"},
+	}
+
+	for _, test := range tests {
+		// typecheck test source
+		conf := Config{Importer: importHelper{pkg: test.imported}}
+		pkg, err := typecheck(test.src, &conf, nil)
+		if err == nil {
+			if test.err != "" {
+				t.Errorf("package %s: got no error, want %q", pkg.Name(), test.err)
+			}
+			continue
+		}
+		if test.err == "" {
+			t.Errorf("package %s: got %q, want not error", pkg.Name(), err.Error())
+		}
+
+		// flatten reported error message
+		errmsg := strings.ReplaceAll(err.Error(), "\n", " ")
+		errmsg = strings.ReplaceAll(errmsg, "\t", "")
+
+		// verify error message
+		if !strings.Contains(errmsg, test.err) {
+			t.Errorf("package %s: got %q, want %q", pkg.Name(), errmsg, test.err)
+		}
+	}
+}
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index 05d30c178a..7723c43565 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -98,7 +98,7 @@ func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 // and missingMethod (the latter doesn't care about struct fields).
 //
 // If foldCase is true, method names are considered equal if they are equal
-// with case folding.
+// with case folding, irrespective of which package they are in.
 //
 // The resulting object may not be fully type-checked.
 func lookupFieldOrMethodImpl(T Type, addressable bool, pkg *Package, name string, foldCase bool) (obj Object, index []int, indirect bool) {
@@ -345,6 +345,7 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 		ok = iota
 		notFound
 		wrongName
+		unexported
 		wrongSig
 		ambigSel
 		ptrRecv
@@ -390,6 +391,11 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 					f, _ = obj.(*Func)
 					if f != nil {
 						state = wrongName
+						if f.name == m.name {
+							// If the names are equal, f must be unexported
+							// (otherwise the package wouldn't matter).
+							state = unexported
+						}
 					}
 				}
 				break
@@ -438,8 +444,9 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 			}
 		case wrongName:
 			fs, ms := check.funcString(f, false), check.funcString(m, false)
-			*cause = check.sprintf("(missing method %s)\n\t\thave %s\n\t\twant %s",
-				m.Name(), fs, ms)
+			*cause = check.sprintf("(missing method %s)\n\t\thave %s\n\t\twant %s", m.Name(), fs, ms)
+		case unexported:
+			*cause = check.sprintf("(unexported method %s)", m.Name())
 		case wrongSig:
 			fs, ms := check.funcString(f, false), check.funcString(m, false)
 			if fs == ms {
@@ -584,11 +591,12 @@ func fieldIndex(fields []*Var, pkg *Package, name string) int {
 }
 
 // lookupMethod returns the index of and method with matching package and name, or (-1, nil).
-// If foldCase is true, method names are considered equal if they are equal with case folding.
+// If foldCase is true, method names are considered equal if they are equal with case folding
+// and their packages are ignored (e.g., pkg1.m, pkg1.M, pkg2.m, and pkg2.M are all equal).
 func lookupMethod(methods []*Func, pkg *Package, name string, foldCase bool) (int, *Func) {
 	if name != "_" {
 		for i, m := range methods {
-			if (m.name == name || foldCase && strings.EqualFold(m.name, name)) && m.sameId(pkg, m.name) {
+			if m.sameId(pkg, name) || foldCase && strings.EqualFold(m.name, name) {
 				return i, m
 			}
 		}