cmd/compile/internal/types2: no need to validate substituted instances

When substituting a type instance, we rely on the instance being
expanded and do not call validType, so there is need to depend on
subster.pos for error reporting or to use subst.check for creating the
new Named type. Errors will be reported for the unsubstituted instance.

This is a superficial change, but justifies some later simplification
where we don't have access to pos or check.

Change-Id: I1f3f12aa245d821512c6242ad829c940f20afae4
Reviewed-on: https://go-review.googlesource.com/c/go/+/342150
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index ed3fd654a0..2c0fc6e391 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -222,10 +222,15 @@ func (subst *subster) typ(typ Type) Type {
 			return named
 		}
 
-		// create a new named type and populate typMap to avoid endless recursion
+		// Create a new named type and populate typMap to avoid endless recursion.
+		// The position used here is irrelevant because validation only occurs on t
+		// (we don't call validType on named), but we use subst.pos to help with
+		// debugging.
 		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
 		t.load()
-		named := subst.check.newNamed(tname, t.orig, t.underlying, t.TParams(), t.methods) // method signatures are updated lazily
+		// It's ok to provide a nil *Checker because the newly created type
+		// doesn't need to be (lazily) expanded; it's expanded below.
+		named := (*Checker)(nil).newNamed(tname, t.orig, nil, t.tparams, t.methods) // t is loaded, so tparams and methods are available
 		named.targs = newTArgs
 		subst.typMap[h] = named
 		t.expand(subst.typMap) // must happen after typMap update to avoid infinite recursion
@@ -235,7 +240,7 @@ func (subst *subster) typ(typ Type) Type {
 		named.underlying = subst.typOrNil(t.underlying)
 		dump(">>> underlying: %v", named.underlying)
 		assert(named.underlying != nil)
-		named.fromRHS = named.underlying // for cycle detection (Checker.validType)
+		named.fromRHS = named.underlying // for consistency, though no cycle detection is necessary
 
 		return named