cmd/compile: deal with helper generic types that add methods to T

Deal with cases like: 'type P[T any] T' (used to add methods to an
arbitrary type T), In this case, P[T] has kind types.TTYPEPARAM (as does
T itself), but requires more code to substitute than a simple TTYPEPARAM
T. See the comment near the beginning of subster.typ() in stencil.go.

Add new test absdiff.go. This test has a case for complex types (which
I've commented out) that will only work when we deal better with Go
builtins in generic functions (like real and imag).

Remove change in fmt.go for TTYPEPARAMS that is no longer needed (since
all TTYPEPARAMS have a sym) and was sometimes causing an extra prefix
when formatting method names.

Separate out the setting of a TTYPEPARAM bound, since it can reference
the TTYPEPARAM being defined, so must be done separately. Also, we don't
currently (and may not ever) need bounds after types2 typechecking.

Change-Id: Id173057e0c4563b309b95e665e9c1151ead4ba77
Reviewed-on: https://go-review.googlesource.com/c/go/+/300049
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 2 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/types/type.go b/src/cmd/compile/internal/types/type.go
index d76d9b409f..ffaf755345 100644
--- a/src/cmd/compile/internal/types/type.go
+++ b/src/cmd/compile/internal/types/type.go
@@ -1742,12 +1742,9 @@ func NewInterface(pkg *Pkg, methods []*Field) *Type {
 	return t
 }
 
-// NewTypeParam returns a new type param with the given constraint (which may
-// not really be needed except for the type checker).
-func NewTypeParam(pkg *Pkg, constraint *Type) *Type {
+// NewTypeParam returns a new type param.
+func NewTypeParam(pkg *Pkg) *Type {
 	t := New(TTYPEPARAM)
-	constraint.wantEtype(TINTER)
-	t.methods = constraint.methods
 	t.Extra.(*Interface).pkg = pkg
 	t.SetHasTParam(true)
 	return t