go/types, types2: in SetUnderlying, set Named.fromRHS if not set yet

This is necessary for cycle detection over imported types whose
underlying types are set by importers with SetUnderlying.

Preparation for fixing issue #48962.

Change-Id: I3218cda7feb06440fdb8345c94bcaa5f7d64e94e
Reviewed-on: https://go-review.googlesource.com/c/go/+/379694
Trust: Robert Griesemer <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: griesemer

src/cmd/compile/internal/types2/named.go

@@ -113,6 +113,9 @@ func (t *Named) SetUnderlying(underlying Type) {
 		panic("underlying type must not be *Named")
 	}
 	t.resolve(nil).underlying = underlying
+	if t.fromRHS == nil {
+		t.fromRHS = underlying // for cycle detection
+	}
 }
 
 // AddMethod adds method m unless it is already in the method list.

src/go/types/named.go

@@ -115,6 +115,9 @@ func (t *Named) SetUnderlying(underlying Type) {
 		panic("underlying type must not be *Named")
 	}
 	t.resolve(nil).underlying = underlying
+	if t.fromRHS == nil {
+		t.fromRHS = underlying // for cycle detection
+	}
 }
 
 // AddMethod adds method m unless it is already in the method list.