[dev.typeparams] cmd/compile/internal/types2: merge instance and Named to eliminate sanitization

This is a port of CL 335929 to types2. It differs significantly from
that CL to handle lazy loading, which wasn't tested in go/types.
Additionally, the *Checker field was moved out of instance and back
onto Named. This way we can tell whether a Named type is uninstantiated
simply by checking whether Named.instance is non-nil, which simplified
the code considerably.

Fixes #46151

Change-Id: I617263bcfaa768ac5442213cecad8d567c2749fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/336252
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 5 insertions, 10 deletions

diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 63b234a60e..87e3e3018e 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -26,12 +26,7 @@ func makeSubstMap(tpars []*TypeName, targs []Type) *substMap {
 	assert(len(tpars) == len(targs))
 	proj := make(map[*TypeParam]Type, len(tpars))
 	for i, tpar := range tpars {
-		// We must expand type arguments otherwise *instance
-		// types end up as components in composite types.
-		// TODO(gri) explain why this causes problems, if it does
-		targ := expand(targs[i]) // possibly nil
-		targs[i] = targ
-		proj[tpar.typ.(*TypeParam)] = targ
+		proj[tpar.typ.(*TypeParam)] = targs[i]
 	}
 	return &substMap{targs, proj}
 }
@@ -83,6 +78,7 @@ func (check *Checker) subst(pos syntax.Pos, typ Type, smap *substMap) Type {
 		// for recursive types (example: type T[P any] *T[P]).
 		subst.typMap = make(map[string]*Named)
 	}
+
 	return subst.typ(typ)
 }
 
@@ -241,10 +237,13 @@ func (subst *subster) typ(typ Type) Type {
 		named := subst.check.newNamed(tname, t, t.Underlying(), t.TParams(), t.methods) // method signatures are updated lazily
 		named.targs = new_targs
 		subst.typMap[h] = named
+		t.expand() // must happen after typMap update to avoid infinite recursion
 
 		// do the substitution
 		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, new_targs)
 		named.underlying = subst.typOrNil(t.Underlying())
+		dump(">>> underlying: %v", named.underlying)
+		assert(named.underlying != nil)
 		named.fromRHS = named.underlying // for cycle detection (Checker.validType)
 
 		return named
@@ -252,10 +251,6 @@ func (subst *subster) typ(typ Type) Type {
 	case *TypeParam:
 		return subst.smap.lookup(t)
 
-	case *instance:
-		// TODO(gri) can we avoid the expansion here and just substitute the type parameters?
-		return subst.typ(t.expand())
-
 	default:
 		unimplemented()
 	}