cmd/compile/internal/types2: tweaks to ArgumentError to be more idiomatic

This CL is a clean port of CL 351335 from go/types to types2.

Updates #47916

Change-Id: Idc377fb71d480a51d5e93a348f3a880346011974
Reviewed-on: https://go-review.googlesource.com/c/go/+/364535
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -55,17 +55,14 @@ func (err Error) FullError() string {
 	return fmt.Sprintf("%s: %s", err.Pos, err.Full)
 }
 
-// An ArgumentError holds an error that is associated with an argument.
+// 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/cmd/compile/internal/types2/api_test.go

@@ -7,6 +7,7 @@ package types2_test
 import (
 	"bytes"
 	"cmd/compile/internal/syntax"
+	"errors"
 	"fmt"
 	"internal/testenv"
 	"reflect"
@@ -2002,9 +2003,13 @@ 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/cmd/compile/internal/types2/instantiate.go

@@ -24,8 +24,8 @@ import (
 // instances with the same identity.
 //
 // 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.
@@ -42,7 +42,7 @@ func Instantiate(ctxt *Context, typ Type, targs []Type, validate bool) (Type, er
 			tparams = t.TypeParams().list()
 		}
 		if i, err := (*Checker)(nil).verify(nopos, tparams, targs); err != nil {
-			return inst, ArgumentError{i, err}
+			return inst, &ArgumentError{i, err}
 		}
 	}