[dev.typeparams] cmd/compile: use types2.Constraint() rather than types2.Bound()

types2.Constraint() returns the top-level constraint type, including any
unions or other interface elements. Because of that, we needed to
add/fix some code in the type substituter and generic type instantiater
in the importer to deal with unions and non-method members of an
interface. Also, NewUnion was not correctly setting the HasTParam flag.

I also added a better error message when a symbol is not found in
(*deadcodePass).decodeIfaceMethod().

Change-Id: Id3668dc596dce63690fa05a9e5e42295b5e2bbb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/340670
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types/type.go b/src/cmd/compile/internal/types/type.go
index 099080f48f..875b0ba82f 100644
--- a/src/cmd/compile/internal/types/type.go
+++ b/src/cmd/compile/internal/types/type.go
@@ -1912,6 +1912,15 @@ func NewUnion(terms []*Type, tildes []bool) *Type {
 	}
 	t.Extra.(*Union).terms = terms
 	t.Extra.(*Union).tildes = tildes
+	nt := len(terms)
+	for i := 0; i < nt; i++ {
+		if terms[i].HasTParam() {
+			t.SetHasTParam(true)
+		}
+		if terms[i].HasShape() {
+			t.SetHasShape(true)
+		}
+	}
 	return t
 }