go/types, types2: better error message when using *interface instead of interface

- detect *interface case and report specific error
- replaced switch with sequence of if's for more clarity
- fixed isInterfacePtr: it applies to all interfaces, incl.
  type parameters
- reviewed/fixed all uses of isInterfacePtr
- adjusted error messages to be consistently of the format
  "type %s is pointer to interface, not interface"

Fixes #48312.

Change-Id: Ic3c8cfcf93ad57ecdb60f6a727cce9e1aa4afb5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/376914
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

6 files changed, 71 insertions, 41 deletions

diff --git a/src/cmd/compile/internal/types2/call.go b/src/cmd/compile/internal/types2/call.go
index d93805e9c7..bd62e825af 100644
--- a/src/cmd/compile/internal/types2/call.go
+++ b/src/cmd/compile/internal/types2/call.go
@@ -531,41 +531,51 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 
 	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
 	if obj == nil {
-		switch {
-		case index != nil:
+		if index != nil {
 			// TODO(gri) should provide actual type where the conflict happens
 			check.errorf(e.Sel, "ambiguous selector %s.%s", x.expr, sel)
-		case indirect:
+			goto Error
+		}
+
+		if indirect {
 			check.errorf(e.Sel, "cannot call pointer method %s on %s", sel, x.typ)
-		default:
-			var why string
-			if tpar, _ := x.typ.(*TypeParam); tpar != nil {
-				// Type parameter bounds don't specify fields, so don't mention "field".
-				if tname := tpar.iface().obj; tname != nil {
-					why = check.sprintf("interface %s has no method %s", tname.name, sel)
-				} else {
-					why = check.sprintf("type bound for %s has no method %s", x.typ, sel)
-				}
+			goto Error
+		}
+
+		if isInterfacePtr(x.typ) {
+			check.errorf(e.Sel, "%s.%s undefined (type %s is pointer to interface, not interface)", x.expr, sel, x.typ)
+			goto Error
+		}
+
+		var why string
+		if tpar, _ := x.typ.(*TypeParam); tpar != nil {
+			// Type parameter bounds don't specify fields, so don't mention "field".
+			// TODO(gri) Type constraints may have accessible fields now. Revisit this.
+			if tname := tpar.iface().obj; tname != nil {
+				why = check.sprintf("interface %s has no method %s", tname.name, sel)
 			} else {
-				why = check.sprintf("type %s has no field or method %s", x.typ, sel)
+				why = check.sprintf("type bound for %s has no method %s", x.typ, sel)
 			}
+		} else {
+			why = check.sprintf("type %s has no field or method %s", x.typ, sel)
+		}
 
-			// Check if capitalization of sel matters and provide better error message in that case.
-			if len(sel) > 0 {
-				var changeCase string
-				if r := rune(sel[0]); unicode.IsUpper(r) {
-					changeCase = string(unicode.ToLower(r)) + sel[1:]
-				} else {
-					changeCase = string(unicode.ToUpper(r)) + sel[1:]
-				}
-				if obj, _, _ = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, changeCase); obj != nil {
-					why += ", but does have " + changeCase
-				}
+		// Check if capitalization of sel matters and provide better error message in that case.
+		// TODO(gri) This code only looks at the first character but LookupFieldOrMethod has an
+		//           (internal) mechanism for case-insensitive lookup. Should use that instead.
+		if len(sel) > 0 {
+			var changeCase string
+			if r := rune(sel[0]); unicode.IsUpper(r) {
+				changeCase = string(unicode.ToLower(r)) + sel[1:]
+			} else {
+				changeCase = string(unicode.ToUpper(r)) + sel[1:]
+			}
+			if obj, _, _ = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, changeCase); obj != nil {
+				why += ", but does have " + changeCase
 			}
-
-			check.errorf(e.Sel, "%s.%s undefined (%s)", x.expr, sel, why)
-
 		}
+
+		check.errorf(e.Sel, "%s.%s undefined (%s)", x.expr, sel, why)
 		goto Error
 	}
 
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index 0cce3fdc3f..aa1ab8ac98 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -448,12 +448,12 @@ func (check *Checker) missingMethodReason(V, T Type, m, wrongType *Func) string
 		// an extra formatting option for types2.Type that doesn't print out
 		// 'func'.
 		r = strings.Replace(r, "^^func", "", -1)
-	} else if IsInterface(T) && !isTypeParam(T) {
+	} else if IsInterface(T) {
 		if isInterfacePtr(V) {
-			r = fmt.Sprintf("(%s is pointer to interface, not interface)", V)
+			r = fmt.Sprintf("(type %s is pointer to interface, not interface)", V)
 		}
-	} else if isInterfacePtr(T) && !isTypeParam(T) {
-		r = fmt.Sprintf("(%s is pointer to interface, not interface)", T)
+	} else if isInterfacePtr(T) {
+		r = fmt.Sprintf("(type %s is pointer to interface, not interface)", T)
 	}
 	if r == "" {
 		r = fmt.Sprintf("(missing %s)", mname)
@@ -463,7 +463,7 @@ func (check *Checker) missingMethodReason(V, T Type, m, wrongType *Func) string
 
 func isInterfacePtr(T Type) bool {
 	p, _ := under(T).(*Pointer)
-	return p != nil && IsInterface(p.base) && !isTypeParam(p.base)
+	return p != nil && IsInterface(p.base)
 }
 
 // assertableTo reports whether a value of type V can be asserted to have type T.
diff --git a/src/cmd/compile/internal/types2/operand.go b/src/cmd/compile/internal/types2/operand.go
index f6bd0291ec..69e3a0a832 100644
--- a/src/cmd/compile/internal/types2/operand.go
+++ b/src/cmd/compile/internal/types2/operand.go
@@ -317,7 +317,7 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 	if check != nil && check.conf.CompilerErrorMessages {
 		if isInterfacePtr(Tu) {
 			if reason != nil {
-				*reason = check.sprintf("%s does not implement %s (%s is pointer to interface, not interface)", x.typ, T, T)
+				*reason = check.sprintf("%s does not implement %s (type %s is pointer to interface, not interface)", x.typ, T, T)
 			}
 			return false, _InvalidIfaceAssign
 		}
diff --git a/src/cmd/compile/internal/types2/testdata/check/methodsets.src b/src/cmd/compile/internal/types2/testdata/check/methodsets.src
index 9fb10deb9a..b0eb14cf50 100644
--- a/src/cmd/compile/internal/types2/testdata/check/methodsets.src
+++ b/src/cmd/compile/internal/types2/testdata/check/methodsets.src
@@ -196,9 +196,9 @@ func issue5918() {
 		_ func(error) string = error.Error
 
 		perr = &err
-		_ = perr.Error /* ERROR "no field or method" */ ()
-		_ func() string = perr.Error /* ERROR "no field or method" */
-		_ func(*error) string = (*error).Error /* ERROR "no field or method" */
+		_ = perr.Error /* ERROR "type \*error is pointer to interface, not interface" */ ()
+		_ func() string = perr.Error /* ERROR "type \*error is pointer to interface, not interface" */
+		_ func(*error) string = (*error).Error /* ERROR "type \*error is pointer to interface, not interface" */
 	)
 
 	type T *interface{ m() int }
@@ -207,8 +207,8 @@ func issue5918() {
 		_ = (*x).m()
 		_ = (*x).m
 
-		_ = x.m /* ERROR "no field or method" */ ()
-		_ = x.m /* ERROR "no field or method" */
-		_ = T.m /* ERROR "no field or method" */
+		_ = x.m /* ERROR "type T is pointer to interface, not interface" */ ()
+		_ = x.m /* ERROR "type T is pointer to interface, not interface" */
+		_ = T.m /* ERROR "type T is pointer to interface, not interface" */
 	)
 }
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2
index 6a2e787bf9..edde497f5a 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47747.go2
@@ -20,7 +20,7 @@ func _[P interface{ m() }](x P) {
         x.m()
         // (&x).m doesn't exist because &x is of type *P
         // and pointers to type parameters don't have methods
-        (&x).m /* ERROR \*P has no field or method m */ ()
+        (&x).m /* ERROR type \*P is pointer to interface, not interface */ ()
 }
 
 
@@ -29,7 +29,7 @@ type T2 interface{ m() }
 func _(x *T2) {
         // x.m doesn't exists because x is of type *T2
         // and pointers to interfaces don't have methods
-        x.m /* ERROR \*T2 has no field or method m */()
+        x.m /* ERROR type \*T2 is pointer to interface, not interface */()
 }
 
 // Test case 1 from issue
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue48312.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue48312.go2
new file mode 100644
index 0000000000..6e5911d0aa
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue48312.go2
@@ -0,0 +1,20 @@
+// Copyright 2022 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
+
+type T interface{ m() }
+type P *T
+
+func _(p *T) {
+	p.m /* ERROR type \*T is pointer to interface, not interface */ ()
+}
+
+func _(p P) {
+	p.m /* ERROR type P is pointer to interface, not interface */ ()
+}
+
+func _[P T](p *P) {
+	p.m /* ERROR type \*P is pointer to interface, not interface */ ()
+}