[dev.typeparams] cmd/compile/internal/types2: accept embedded interface elements

Accept embedded interface elements of the form ~T or A|B and
treat them like type lists: for now the elements of a union
cannot be interfaces. Also, translate existing style "type"-
lists in interfaces into interface elements: "type a, b, c"
becomes a union element "~a|~b|~c" which in turn is handled
internally like a type list.

For now, "~" is still ignored and type lists are mapped to
Sum types as before, thus ensuring that all existing tests
work as before (with some minor adjustments).

Introduced a new Union type to represent union elements.
For now they don't make it past interface completion where
they are represented as a Sum type. Thus, except for printing
(and the respective tests) and substitution for interfaces,
the various type switches ignore Union types. In a next step,
we'll replace Sum types with union types and then consider
the ~ functionality as well.

Because union elements are no different from embedded interfaces
we don't need a separate Interface.types field anymore. Removed.

For #45346.

Change-Id: I98ac3286aea9d706e98aee80241d4712ed99af08
Reviewed-on: https://go-review.googlesource.com/c/go/+/321689
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index b9e178dd57..94fb506d80 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -769,7 +769,7 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		tpar := NewTypeName(nopos, nil /* = Universe pkg */, "<type parameter>", nil)
 		ptyp := check.NewTypeParam(tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
 		tsum := NewSum(rtypes)
-		ptyp.bound = &Interface{types: tsum, allMethods: markComplete, allTypes: tsum}
+		ptyp.bound = &Interface{allMethods: markComplete, allTypes: tsum}
 
 		return ptyp
 	}