cmd/internal/gc: improve "type *X has no field or method M" message

Try to provide hints for common areas, either *interface
were interface would have been better, and note incorrect
capitalization (but don't be more ambitious than that, at
least not today).

Added code and test for cases

  ptrInterface.ExistingMethod
  ptrInterface.unexportedMethod
  ptrInterface.MissingMethod
  ptrInterface.withwRongcASEdMethod
  interface.withwRongcASEdMethod
  ptrStruct.withwRongcASEdMethod
  struct.withwRongcASEdMethod

also included tests for related errors to check for
unintentional changes and consistent wording.

Somewhat simplified from previous versions to avoid second-
guessing user errors, yet also biased to point out most-likely
root cause.

Fixes #10700

Change-Id: I16693e93cc8d8ca195e7742a222d640c262105b4
Reviewed-on: https://go-review.googlesource.com/9731
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 14 insertions, 9 deletions

diff --git a/test/interface/embed2.go b/test/interface/embed2.go
index 1636db78eb..df3e2e435b 100644
--- a/test/interface/embed2.go
+++ b/test/interface/embed2.go
@@ -12,20 +12,25 @@ import "os"
 
 const Value = 1e12
 
-type Inter interface { M() int64 }
+type Inter interface {
+	M() int64
+}
 
 type T int64
+
 func (t T) M() int64 { return int64(t) }
+
 var t = T(Value)
 var pt = &t
 var ti Inter = t
 var pti = &ti
 
-type S struct { Inter }
-var s = S{ ti }
+type S struct{ Inter }
+
+var s = S{ti}
 var ps = &s
 
-type SP struct { *Inter }	// ERROR "interface"
+type SP struct{ *Inter } // ERROR "interface"
 
 var i Inter
 var pi = &i
@@ -43,25 +48,25 @@ func main() {
 	check("t.M()", t.M())
 	check("pt.M()", pt.M())
 	check("ti.M()", ti.M())
-	check("pti.M()", pti.M())	// ERROR "method"
+	check("pti.M()", pti.M()) // ERROR "pointer to interface, not interface"
 	check("s.M()", s.M())
 	check("ps.M()", ps.M())
 
 	i = t
 	check("i = t; i.M()", i.M())
-	check("i = t; pi.M()", pi.M())	// ERROR "method"
+	check("i = t; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	i = pt
 	check("i = pt; i.M()", i.M())
-	check("i = pt; pi.M()", pi.M())	// ERROR "method"
+	check("i = pt; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	i = s
 	check("i = s; i.M()", i.M())
-	check("i = s; pi.M()", pi.M())	// ERROR "method"
+	check("i = s; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	i = ps
 	check("i = ps; i.M()", i.M())
-	check("i = ps; pi.M()", pi.M())	// ERROR "method"
+	check("i = ps; pi.M()", pi.M()) // ERROR "pointer to interface, not interface"
 
 	if !ok {
 		println("BUG: interface10")