go/types, types2: refer to type parameter if so for interface pointer errors

Follow-up on comment in CL 376914.

Also:
- add missing check != nil test in assignableTo
- use check.sprintf rather than fmt.Sprintf in missingMethodReason

For #48312.

Change-Id: Ie209b4101a7f2c279e42a59987d0068079c8b69f
Reviewed-on: https://go-review.googlesource.com/c/go/+/377375
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

5 files changed, 19 insertions, 12 deletions

diff --git a/src/go/types/call.go b/src/go/types/call.go
index 4a31ec2586..d5b83451c4 100644
--- a/src/go/types/call.go
+++ b/src/go/types/call.go
@@ -546,7 +546,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 
 		var why string
 		if isInterfacePtr(x.typ) {
-			why = check.sprintf("type %s is pointer to interface, not interface", x.typ)
+			why = check.interfacePtrError(x.typ)
 		} 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.
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index 1b820d5403..598f615247 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -7,7 +7,6 @@
 package types
 
 import (
-	"fmt"
 	"strings"
 )
 
@@ -403,17 +402,17 @@ func (check *Checker) missingMethodReason(V, T Type, m, wrongType *Func) string
 	if wrongType != nil {
 		if Identical(m.typ, wrongType.typ) {
 			if m.Name() == wrongType.Name() {
-				r = fmt.Sprintf("(%s has pointer receiver)", mname)
+				r = check.sprintf("(%s has pointer receiver)", mname)
 			} else {
-				r = fmt.Sprintf("(missing %s)\n\t\thave %s^^%s\n\t\twant %s^^%s",
+				r = check.sprintf("(missing %s)\n\t\thave %s^^%s\n\t\twant %s^^%s",
 					mname, wrongType.Name(), wrongType.typ, m.Name(), m.typ)
 			}
 		} else {
 			if compilerErrorMessages {
-				r = fmt.Sprintf("(wrong type for %s)\n\t\thave %s^^%s\n\t\twant %s^^%s",
+				r = check.sprintf("(wrong type for %s)\n\t\thave %s^^%s\n\t\twant %s^^%s",
 					mname, wrongType.Name(), wrongType.typ, m.Name(), m.typ)
 			} else {
-				r = fmt.Sprintf("(wrong type for %s: have %s, want %s)",
+				r = check.sprintf("(wrong type for %s: have %s, want %s)",
 					mname, wrongType.typ, m.typ)
 			}
 		}
@@ -424,13 +423,13 @@ func (check *Checker) missingMethodReason(V, T Type, m, wrongType *Func) string
 		r = strings.Replace(r, "^^func", "", -1)
 	} else if IsInterface(T) {
 		if isInterfacePtr(V) {
-			r = fmt.Sprintf("(type %s is pointer to interface, not interface)", V)
+			r = "(" + check.interfacePtrError(V) + ")"
 		}
 	} else if isInterfacePtr(T) {
-		r = fmt.Sprintf("(type %s is pointer to interface, not interface)", T)
+		r = "(" + check.interfacePtrError(T) + ")"
 	}
 	if r == "" {
-		r = fmt.Sprintf("(missing %s)", mname)
+		r = check.sprintf("(missing %s)", mname)
 	}
 	return r
 }
@@ -440,6 +439,14 @@ func isInterfacePtr(T Type) bool {
 	return p != nil && IsInterface(p.base)
 }
 
+func (check *Checker) interfacePtrError(T Type) string {
+	assert(isInterfacePtr(T))
+	if p, _ := under(T).(*Pointer); isTypeParam(p.base) {
+		return check.sprintf("type %s is pointer to type parameter, not type parameter", T)
+	}
+	return check.sprintf("type %s is pointer to interface, not interface", T)
+}
+
 // assertableTo reports whether a value of type V can be asserted to have type T.
 // It returns (nil, false) as affirmative answer. Otherwise it returns a missing
 // method required by V and whether it is missing or just has the wrong type.
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index d669981cf2..d119b5ee7b 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -282,7 +282,7 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 	if Ti, ok := Tu.(*Interface); ok && Tp == nil {
 		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
 			if reason != nil {
-				if compilerErrorMessages {
+				if check != nil && compilerErrorMessages {
 					*reason = check.sprintf("%s does not implement %s %s", x.typ, T,
 						check.missingMethodReason(x.typ, T, m, wrongType))
 				} else {
diff --git a/src/go/types/testdata/fixedbugs/issue47747.go2 b/src/go/types/testdata/fixedbugs/issue47747.go2
index edde497f5a..6f09fc2f57 100644
--- a/src/go/types/testdata/fixedbugs/issue47747.go2
+++ b/src/go/types/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 type \*P is pointer to interface, not interface */ ()
+        (&x).m /* ERROR type \*P is pointer to type parameter, not type parameter */ ()
 }
 
 
diff --git a/src/go/types/testdata/fixedbugs/issue48312.go2 b/src/go/types/testdata/fixedbugs/issue48312.go2
index 6e5911d0aa..2fdb7cad94 100644
--- a/src/go/types/testdata/fixedbugs/issue48312.go2
+++ b/src/go/types/testdata/fixedbugs/issue48312.go2
@@ -16,5 +16,5 @@ func _(p P) {
 }
 
 func _[P T](p *P) {
-	p.m /* ERROR type \*P is pointer to interface, not interface */ ()
+	p.m /* ERROR type \*P is pointer to type parameter, not type parameter */ ()
 }