[dev.typeparams] cmd/compile/internal/types2: implement type sets with term lists

This CL resolves several known issues and TODOs.

- Represent type sets with term lists and using term list abstractions.

- Represent Unions internally as a list of (syntactical) terms.
  Use term operations to print terms and detect overlapping union
  entries.

- Compute type sets corresponding to unions lazily, on demand.

- Adjust code throughout.

- Adjusted error check in test/typeparam/mincheck.dir/main.go
  to make test pass.

Change-Id: Ib36fb7e1d343c2b6aec51d304f0f7d1ad415f999
Reviewed-on: https://go-review.googlesource.com/c/go/+/338310
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 36 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/examples/constraints.go2 b/src/cmd/compile/internal/types2/testdata/examples/constraints.go2
index 28aa19bb12..f40d18c63e 100644
--- a/src/cmd/compile/internal/types2/testdata/examples/constraints.go2
+++ b/src/cmd/compile/internal/types2/testdata/examples/constraints.go2
@@ -18,18 +18,25 @@ type (
 	}
 )
 
+type MyInt int
+
 type (
 	// Arbitrary types may be embedded like interfaces.
 	_ interface{int}
 	_ interface{~int}
 
 	// Types may be combined into a union.
-	_ interface{int|~string}
+	union interface{int|~string}
 
-	// Union terms must be unique independent of whether they are ~ or not.
-	_ interface{int|int /* ERROR duplicate term int */ }
-	_ interface{int|~ /* ERROR duplicate term int */ int }
-	_ interface{~int|~ /* ERROR duplicate term int */ int }
+	// Union terms must describe disjoint (non-overlapping) type sets.
+	_ interface{int|int /* ERROR overlapping terms int */ }
+	_ interface{int|~ /* ERROR overlapping terms ~int */ int }
+	_ interface{~int|~ /* ERROR overlapping terms ~int */ int }
+	_ interface{~int|MyInt /* ERROR overlapping terms p.MyInt and ~int */ }
+	_ interface{int|interface{}}
+	_ interface{int|~string|union}
+	_ interface{int|~string|interface{int}}
+	_ interface{union|union /* ERROR overlapping terms p.union and p.union */ }
 
 	// For now we do not permit interfaces with methods in unions.
 	_ interface{~ /* ERROR invalid use of ~ */ interface{}}
@@ -45,6 +52,15 @@ type (
 	_ interface{~ /* ERROR invalid use of ~ */ bar }
 )
 
+// Stand-alone type parameters are not permitted as elements or terms in unions.
+type (
+	_[T interface{ *T } ] struct{}        // ok
+	_[T interface{ int | *T } ] struct{}  // ok
+	_[T interface{ T /* ERROR cannot embed a type parameter */ } ] struct{}
+	_[T interface{ ~T /* ERROR cannot embed a type parameter */ } ] struct{}
+	_[T interface{ int|T /* ERROR cannot embed a type parameter */ }] struct{}
+)
+
 // Multiple embedded union elements are intersected. The order in which they
 // appear in the interface doesn't matter since intersection is a symmetric
 // operation.
@@ -58,3 +74,18 @@ func _[T interface{ ~int; myInt1|myInt2 }]() T { return T(0) }
 // Here the intersections are empty - there's no type that's in the type set of T.
 func _[T interface{ myInt1|myInt2; int }]() T { return T(0 /* ERROR cannot convert */ ) }
 func _[T interface{ int; myInt1|myInt2 }]() T { return T(0 /* ERROR cannot convert */ ) }
+
+// Union elements may be interfaces as long as they don't define
+// any methods or embed comparable.
+
+type (
+	Integer interface{ ~int|~int8|~int16|~int32|~int64 }
+	Unsigned interface{ ~uint|~uint8|~uint16|~uint32|~uint64 }
+	Floats interface{ ~float32|~float64 }
+	Complex interface{ ~complex64|~complex128 }
+	Number interface{ Integer|Unsigned|Floats|Complex }
+	Ordered interface{ Integer|Unsigned|Floats|~string }
+
+	_ interface{ Number | error /* ERROR cannot use error in union */ }
+	_ interface{ Ordered | comparable /* ERROR cannot use comparable in union */ }
+)