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>

diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index ed3fd654a0e0e31ea8091437ed77ab99f0b05f02..2c0fc6e39100cee02580a62eace828a734b96813 100644 (file)
--- 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