go/types: tweaks to ArgumentError to be more idiomatic

This CL makes a few changes to the new ArgumentError type to be more
idiomatic:
 - Use a pointer receiver for methods.
 - Export fields, similarly to Error. ArgumentError has a clear meaning
   (an error associated with an index), so there is no need to hide its
   representation.
 - Add an Unwrap method to access the underlying error.
 - Say explicitly that the error returned from Instantiate may wrap
   *ArgumentError. There is no need to commit to an API that always
   returns an error with dynamic type *ArgumentError.

Updates #47916

Change-Id: Ib1a43e921f247794e7155280ccbf5a6775ed3978
Reviewed-on: https://go-review.googlesource.com/c/go/+/351335
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/api.go

@@ -64,15 +64,12 @@ func (err Error) Error() string {
 
 // An ArgumentError holds an error associated with an argument index.
 type ArgumentError struct {
-	index int
-	error
+	Index int
+	Err   error
 }
 
-// Index returns the positional index of the argument associated with the
-// error.
-func (e ArgumentError) Index() int {
-	return e.index
-}
+func (e *ArgumentError) Error() string { return e.Err.Error() }
+func (e *ArgumentError) Unwrap() error { return e.Err }
 
 // An Importer resolves import paths to Packages.
 //

src/go/types/api_test.go

@@ -6,6 +6,7 @@ package types_test
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"go/ast"
 	"go/importer"
@@ -2000,9 +2001,13 @@ func TestInstantiateErrors(t *testing.T) {
 			t.Fatalf("Instantiate(%v, %v) returned nil error, want non-nil", T, test.targs)
 		}
 
-		gotAt := err.(ArgumentError).Index()
-		if gotAt != test.wantAt {
-			t.Errorf("Instantate(%v, %v): error at index %d, want index %d", T, test.targs, gotAt, test.wantAt)
+		var argErr *ArgumentError
+		if !errors.As(err, &argErr) {
+			t.Fatalf("Instantiate(%v, %v): error is not an *ArgumentError", T, test.targs)
+		}
+
+		if argErr.Index != test.wantAt {
+			t.Errorf("Instantate(%v, %v): error at index %d, want index %d", T, test.targs, argErr.Index, test.wantAt)
 		}
 	}
 }

src/go/types/instantiate.go

@@ -25,8 +25,8 @@ import (
 // unimplemented.
 //
 // If verify is set and constraint satisfaction fails, the returned error may
-// be of dynamic type ArgumentError indicating which type argument did not
-// satisfy its corresponding type parameter constraint, and why.
+// wrap an *ArgumentError indicating which type argument did not satisfy its
+// corresponding type parameter constraint, and why.
 //
 // TODO(rfindley): change this function to also return an error if lengths of
 // tparams and targs do not match.
@@ -43,7 +43,7 @@ func Instantiate(env *Environment, typ Type, targs []Type, validate bool) (Type,
 			tparams = t.TypeParams().list()
 		}
 		if i, err := (*Checker)(nil).verify(token.NoPos, tparams, targs); err != nil {
-			return inst, ArgumentError{i, err}
+			return inst, &ArgumentError{i, err}
 		}
 	}