[dev.typeparams] cmd/compile/internal/types: adjust some error messages to match the compiler

Change-Id: I04bd7b294de4ed0fb01bc0609e09debea2d797bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/274974
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -1867,7 +1867,11 @@ func (check *Checker) typeAssertion(pos syntax.Pos, x *operand, xtyp *Interface,
 	} else {
 		msg = "missing method " + method.name
 	}
-	check.errorf(pos, "%s cannot have dynamic type %s (%s)", x, T, msg)
+	if check.conf.CompilerErrorMessages {
+		check.errorf(pos, "impossible type assertion: %s (%s)", x, msg)
+	} else {
+		check.errorf(pos, "%s cannot have dynamic type %s (%s)", x, T, msg)
+	}
 }
 
 // expr typechecks expression e and initializes x with the expression value.

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

@@ -886,7 +886,7 @@ func rangeKeyVal(typ Type, wantKey, wantVal bool) (Type, Type, string) {
 	case *Chan:
 		var msg string
 		if typ.dir == SendOnly {
-			msg = "send-only channel"
+			msg = "receive from send-only channel"
 		}
 		return typ.elem, Typ[Invalid], msg
 	case *Sum:

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

@@ -806,7 +806,11 @@ func (check *Checker) interfaceType(ityp *Interface, iface *syntax.InterfaceType
 			// of a type list (f.Name.Value == "type").
 			name := f.Name.Value
 			if name == "_" {
-				check.errorf(f.Name, "invalid method name _")
+				if check.conf.CompilerErrorMessages {
+					check.errorf(f.Name, "methods must have a unique non-blank name")
+				} else {
+					check.errorf(f.Name, "invalid method name _")
+				}
 				continue // ignore
 			}