cmd/compile/internal/types2: add a test for Context deduplication of hash collisions

This CL is a clean port of CL 363517 from go/types to types2,
with the exception that types_test.go was not removed because
it's still needed to set a types2-specific test flag.

Change-Id: I12177866537c0f95f3fa36fa0f4aa02016609ca9
Reviewed-on: https://go-review.googlesource.com/c/go/+/364494
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -0,0 +1,69 @@
+// 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 types2
+
+import (
+	"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(nopos, nil, "P", nil), &emptyInterface)
+		nullaryP = NewSignatureType(nil, nil, []*TypeParam{tparam}, nil, nil, false)
+	}
+	{
+		// type nullaryQ = func[Q any]()
+		tparam := NewTypeParam(NewTypeName(nopos, nil, "Q", nil), &emptyInterface)
+		nullaryQ = NewSignatureType(nil, nil, []*TypeParam{tparam}, nil, nil, false)
+	}
+	{
+		// type unaryP = func[P any](_ P)
+		tparam := NewTypeParam(NewTypeName(nopos, nil, "P", nil), &emptyInterface)
+		params := NewTuple(NewVar(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")
+	}
+}

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

@@ -7,6 +7,3 @@ package types2
 func init() {
 	acceptMethodTypeParams = true
 }
-
-// Debug is set if types2 is built with debug mode enabled.
-const Debug = debug