go/types: add a test for Context deduplication of hash collisions

Add a test that exercises the fall-back logic in Context to handle hash
collisions by de-duplicating using Identical.

This has to be a somewhat invasive test because we don't know any actual
cases of hash collisions.

Change-Id: Idf00f7a6ab8c7517ed0f91fdc42d54f5e736b1b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/363517
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/types/context_test.go

@@ -0,0 +1,70 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"go/token"
+	"testing"
+)
+
+func TestContextHashCollisions(t *testing.T) {
+	if debug {
+		t.Skip("hash collisions are expected, and would fail debug assertions")
+	}
+	// Unit test the de-duplication fall-back logic in Context.
+	//
+	// We can't test this via Instantiate because this is only a fall-back in
+	// case our hash is imperfect.
+	//
+	// These lookups and updates use reasonable looking types in an attempt to
+	// make them robust to internal type assertions, but could equally well use
+	// arbitrary types.
+
+	// Create some distinct origin types. nullaryP and nullaryQ have no
+	// parameters and are identical (but have different type parameter names).
+	// unaryP has a parameter.
+	var nullaryP, nullaryQ, unaryP Type
+	{
+		// type nullaryP = func[P any]()
+		tparam := NewTypeParam(NewTypeName(token.NoPos, nil, "P", nil), &emptyInterface)
+		nullaryP = NewSignatureType(nil, nil, []*TypeParam{tparam}, nil, nil, false)
+	}
+	{
+		// type nullaryQ = func[Q any]()
+		tparam := NewTypeParam(NewTypeName(token.NoPos, nil, "Q", nil), &emptyInterface)
+		nullaryQ = NewSignatureType(nil, nil, []*TypeParam{tparam}, nil, nil, false)
+	}
+	{
+		// type unaryP = func[P any](_ P)
+		tparam := NewTypeParam(NewTypeName(token.NoPos, nil, "P", nil), &emptyInterface)
+		params := NewTuple(NewVar(token.NoPos, nil, "_", tparam))
+		unaryP = NewSignatureType(nil, nil, []*TypeParam{tparam}, params, nil, false)
+	}
+
+	ctxt := NewContext()
+
+	// Update the context with an instantiation of nullaryP.
+	inst := NewSignatureType(nil, nil, nil, nil, nil, false)
+	if got := ctxt.update("", nullaryP, []Type{Typ[Int]}, inst); got != inst {
+		t.Error("bad")
+	}
+
+	// unaryP is not identical to nullaryP, so we should not get inst when
+	// instantiated with identical type arguments.
+	if got := ctxt.lookup("", unaryP, []Type{Typ[Int]}); got != nil {
+		t.Error("bad")
+	}
+
+	// nullaryQ is identical to nullaryP, so we *should* get inst when
+	// instantiated with identical type arguments.
+	if got := ctxt.lookup("", nullaryQ, []Type{Typ[Int]}); got != inst {
+		t.Error("bad")
+	}
+
+	// ...but verify we don't get inst with different type arguments.
+	if got := ctxt.lookup("", nullaryQ, []Type{Typ[String]}); got != nil {
+		t.Error("bad")
+	}
+}