[release-branch.go1.18] go/types, types2: use correct underlying type in union set computation

Fixes #51665.

Change-Id: Ibf415d7e12849b8f50b58d74713613d4e65bc347
Reviewed-on: https://go-review.googlesource.com/c/go/+/392575
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/392577
TryBot-Result: Gopher Robot <gobot@golang.org>

4 files changed, 80 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue51658.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue51658.go2
new file mode 100644
index 0000000000..c437c92d29
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue51658.go2
@@ -0,0 +1,39 @@
+// Copyright 2022 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 p
+
+type F { // ERROR syntax error
+	float64
+} // ERROR syntax error
+
+func _[T F | int](x T) {
+	_ = x == 0 // don't crash when recording type of 0
+}
+
+// test case from issue
+
+type FloatType { // ERROR syntax error
+	float32 | float64
+} // ERROR syntax error
+
+type IntegerType interface {
+	int8 | int16 | int32 | int64 | int |
+		uint8 | uint16 | uint32 | uint64 | uint
+}
+
+type ComplexType interface {
+	complex64 | complex128
+}
+
+type Number interface {
+	FloatType | IntegerType | ComplexType
+}
+
+func GetDefaultNumber[T Number](value, defaultValue T) T {
+	if value == 0 {
+		return defaultValue
+	}
+	return value
+}
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index 8df8949435..646b436685 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -406,7 +406,7 @@ func computeUnionTypeSet(check *Checker, unionSets map[*Union]*_TypeSet, pos syn
 			// For now we don't permit type parameters as constraints.
 			assert(!isTypeParam(t.typ))
 			terms = computeInterfaceTypeSet(check, pos, ui).terms
-		} else if t.typ == Typ[Invalid] {
+		} else if u == Typ[Invalid] {
 			continue
 		} else {
 			if t.tilde && !Identical(t.typ, u) {
diff --git a/src/go/types/testdata/fixedbugs/issue51658.go2 b/src/go/types/testdata/fixedbugs/issue51658.go2
new file mode 100644
index 0000000000..04ce6a9760
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue51658.go2
@@ -0,0 +1,39 @@
+// Copyright 2022 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 p
+
+type F { // ERROR expected type
+	float64
+} // ERROR expected declaration
+
+func _[T F | int](x T) {
+	_ = x == 0 // don't crash when recording type of 0
+}
+
+// test case from issue
+
+type FloatType { // ERROR expected type
+	float32 | float64
+} // ERROR expected declaration
+
+type IntegerType interface {
+	int8 | int16 | int32 | int64 | int |
+		uint8 | uint16 | uint32 | uint64 | uint
+}
+
+type ComplexType interface {
+	complex64 | complex128
+}
+
+type Number interface {
+	FloatType | IntegerType | ComplexType
+}
+
+func GetDefaultNumber[T Number](value, defaultValue T) T {
+	if value == 0 {
+		return defaultValue
+	}
+	return value
+}
diff --git a/src/go/types/typeset.go b/src/go/types/typeset.go
index 6603383ea3..b33141ec32 100644
--- a/src/go/types/typeset.go
+++ b/src/go/types/typeset.go
@@ -406,7 +406,7 @@ func computeUnionTypeSet(check *Checker, unionSets map[*Union]*_TypeSet, pos tok
 			// For now we don't permit type parameters as constraints.
 			assert(!isTypeParam(t.typ))
 			terms = computeInterfaceTypeSet(check, pos, ui).terms
-		} else if t.typ == Typ[Invalid] {
+		} else if u == Typ[Invalid] {
 			continue
 		} else {
 			if t.tilde && !Identical(t.typ, u) {