go/types, types2: better error messages for invalid qualified identifiers

This change borrows code from CL 631356 by Emmanuel Odeke (thanks!).

Fixes #70549.

Change-Id: Id6f794ea2a95b4297999456f22c6e02890fce13b
Reviewed-on: https://go-review.googlesource.com/c/go/+/662775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <mark@golang.org>

src/cmd/compile/internal/types2/call.go

@@ -720,7 +720,14 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *TypeName
 				exp = pkg.scope.Lookup(sel)
 				if exp == nil {
 					if !pkg.fake && isValidName(sel) {
-						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", syntax.Expr(e))
+						// Try to give a better error message when selector matches an object name ignoring case.
+						exps := pkg.scope.lookupIgnoringCase(sel, true)
+						if len(exps) >= 1 {
+							// report just the first one
+							check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s (but have %s)", syntax.Expr(e), exps[0].Name())
+						} else {
+							check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", syntax.Expr(e))
+						}
 					}
 					goto Error
 				}

src/cmd/compile/internal/types2/scope.go

@@ -83,6 +83,19 @@ func (s *Scope) Lookup(name string) Object {
 	return obj
 }
 
+// lookupIgnoringCase returns the objects in scope s whose names match
+// the given name ignoring case. If exported is set, only exported names
+// are returned.
+func (s *Scope) lookupIgnoringCase(name string, exported bool) []Object {
+	var matches []Object
+	for _, n := range s.Names() {
+		if (!exported || isExported(n)) && strings.EqualFold(n, name) {
+			matches = append(matches, s.Lookup(n))
+		}
+	}
+	return matches
+}
+
 // Insert attempts to insert an object obj into scope s.
 // If s already contains an alternative object alt with
 // the same name, Insert leaves s unchanged and returns alt.

src/go/types/call.go

@@ -722,7 +722,14 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *TypeName, w
 				exp = pkg.scope.Lookup(sel)
 				if exp == nil {
 					if !pkg.fake && isValidName(sel) {
-						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e))
+						// Try to give a better error message when selector matches an object name ignoring case.
+						exps := pkg.scope.lookupIgnoringCase(sel, true)
+						if len(exps) >= 1 {
+							// report just the first one
+							check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s (but have %s)", ast.Expr(e), exps[0].Name())
+						} else {
+							check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e))
+						}
 					}
 					goto Error
 				}

src/go/types/scope.go

@@ -86,6 +86,19 @@ func (s *Scope) Lookup(name string) Object {
 	return obj
 }
 
+// lookupIgnoringCase returns the objects in scope s whose names match
+// the given name ignoring case. If exported is set, only exported names
+// are returned.
+func (s *Scope) lookupIgnoringCase(name string, exported bool) []Object {
+	var matches []Object
+	for _, n := range s.Names() {
+		if (!exported || isExported(n)) && strings.EqualFold(n, name) {
+			matches = append(matches, s.Lookup(n))
+		}
+	}
+	return matches
+}
+
 // Insert attempts to insert an object obj into scope s.
 // If s already contains an alternative object alt with
 // the same name, Insert leaves s unchanged and returns alt.

src/internal/types/testdata/fixedbugs/issue70549.go

@@ -0,0 +1,15 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "math"
+
+var (
+	_ = math.Sin
+	_ = math.SIn /* ERROR "undefined: math.SIn (but have Sin)" */
+	_ = math.sin /* ERROR "name sin not exported by package math" */
+	_ = math.Foo /* ERROR "undefined: math.Foo" */
+	_ = math.foo /* ERROR "undefined: math.foo" */
+)