Revert "go/types: make Interface.Complete a no-op"

This reverts commit fda8ee8b07.

Reason for revert: Interface.Complete is still necessary for safe concurrency.

For #47726

Change-Id: I8b924ca5f4af8c7d7e2b5a27bb03a5a5ed9b1d22
Reviewed-on: https://go-review.googlesource.com/c/go/+/342710
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/internal/gcimporter/iimport.go

@@ -174,6 +174,10 @@ func iImportData(fset *token.FileSet, imports map[string]*types.Package, dataRea
 		p.doDecl(localpkg, name)
 	}
 
+	for _, typ := range p.interfaceList {
+		typ.Complete()
+	}
+
 	// record all referenced packages as imports
 	list := append(([]*types.Package)(nil), pkgList[1:]...)
 	sort.Sort(byPath(list))

src/go/types/interface.go

@@ -104,12 +104,22 @@ func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 // IsConstraint reports whether interface t is not just a method set.
 func (t *Interface) IsConstraint() bool { return !t.typeSet().IsMethodSet() }
 
-// Complete just returns its receiver. There's no other effect.
+// Complete computes the interface's type set. It must be called by users of
+// NewInterfaceType and NewInterface after the interface's embedded types are
+// fully defined and before using the interface type in any way other than to
+// form other types. The interface must not contain duplicate methods or a
+// panic occurs. Complete returns the receiver.
 //
-// Deprecated: Interfaces are now completed on demand; this function is only
+// Deprecated: Type sets are now computed lazily, on demand; this function
-// here for backward-compatibility. It does not have to be called explicitly
+//             is only here for backward-compatibility. It does not have to
-// anymore.
+//             be called explicitly anymore.
 func (t *Interface) Complete() *Interface {
+	// Some tests are still depending on the state change
+	// (string representation of an Interface not containing an
+	// /* incomplete */ marker) caused by the explicit Complete
+	// call, so we compute the type set eagerly here.
+	t.complete = true
+	t.typeSet()
 	return t
 }
 

src/go/types/issues_test.go

@@ -405,9 +405,8 @@ func TestIssue28282(t *testing.T) {
 	// create type interface { error }
 	et := Universe.Lookup("error").Type()
 	it := NewInterfaceType(nil, []Type{et})
+	it.Complete()
 	// verify that after completing the interface, the embedded method remains unchanged
-	// (interfaces are "completed" lazily now, so the completion happens implicitly when
-	// accessing Method(0))
 	want := et.Underlying().(*Interface).Method(0)
 	got := it.Method(0)
 	if got != want {