go/types: better error messages for field lookup errors

- follow wording of cmd/compile more closely
- only print base of a package path to avoid overly long error messages

Fixes #26234.

Change-Id: I47a8c64b3adcf73980cd296a24cf8ac721e5df06
Reviewed-on: https://go-review.googlesource.com/c/152764
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/call.go

@@ -374,11 +374,13 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 		switch {
 		case index != nil:
 			// TODO(gri) should provide actual type where the conflict happens
-			check.invalidOp(e.Sel.Pos(), "ambiguous selector %s", sel)
+			check.errorf(e.Sel.Pos(), "ambiguous selector %s", sel)
 		case indirect:
-			check.invalidOp(e.Sel.Pos(), "%s is not in method set of %s", sel, x.typ)
+			// TODO(gri) be more specific with this error message
+			check.errorf(e.Sel.Pos(), "%s is not in method set of %s", sel, x.typ)
 		default:
-			check.invalidOp(e.Sel.Pos(), "%s has no field or method %s", x, sel)
+			// TODO(gri) should check if capitalization of sel matters and provide better error message in that case
+			check.errorf(e.Sel.Pos(), "%s.%s undefined (type %s has no field or method %s)", x.expr, sel, x.typ, sel)
 		}
 		goto Error
 	}
@@ -392,7 +394,8 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 		// method expression
 		m, _ := obj.(*Func)
 		if m == nil {
-			check.invalidOp(e.Sel.Pos(), "%s has no method %s", x, sel)
+			// TODO(gri) should check if capitalization of sel matters and provide better error message in that case
+			check.errorf(e.Sel.Pos(), "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
 			goto Error
 		}
 

src/go/types/errors.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/token"
+	"path"
 	"strings"
 )
 
@@ -25,7 +26,7 @@ func unreachable() {
 
 func (check *Checker) qualifier(pkg *Package) string {
 	if pkg != check.pkg {
-		return pkg.path
+		return path.Base(pkg.path) // avoid excessively long path names in error messages
 	}
 	return ""
 }

src/go/types/testdata/issues.src

@@ -5,6 +5,7 @@
 package issues
 
 import "fmt"
+import syn "cmd/compile/internal/syntax"
 
 func issue7035() {
 	type T struct{ X int }
@@ -312,4 +313,28 @@ func issue28281d(... /* ERROR can only use ... with final parameter */ int, int)
 func issue28281e(a, b, c  ... /* ERROR can only use ... with final parameter */ int, d int)
 func issue28281f(... /* ERROR can only use ... with final parameter */ int, ... /* ERROR can only use ... with final parameter */ int, int)
 func (... /* ERROR expected type */ TT) f()
-func issue28281g() (... /* ERROR expected type */ TT)
\ No newline at end of file
+func issue28281g() (... /* ERROR expected type */ TT)
+
+// Issue #26234: Make various field/method lookup errors easier to read by matching cmd/compile's output
+func issue26234a(f *syn.File) {
+	// The error message below should refer to the actual package path base (syntax)
+	// not the local package name (syn).
+	f.foo /* ERROR f.foo undefined \(type \*syntax.File has no field or method foo\) */
+}
+
+type T struct {
+	x int
+	E1
+	E2
+}
+
+type E1 struct{ f int }
+type E2 struct{ f int }
+
+func issue26234b(x T) {
+	_ = x.f /* ERROR ambiguous selector f */
+}
+
+func issue26234c() {
+	T.x /* ERROR T.x undefined \(type T has no method x\) */ ()
+}