go/types, types2: remove coreType call in lookup

For #70128.

Change-Id: I7d16ad7fdc6b07a2632b4eaefaedfa2bcceffe1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/652215
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>

Author: griesemer

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

@@ -67,13 +67,13 @@ func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string, fo
 
 	obj, index, indirect = lookupFieldOrMethodImpl(T, addressable, pkg, name, foldCase)
 
-	// If we didn't find anything and if we have a type parameter with a core type,
-	// see if there is a matching field (but not a method, those need to be declared
-	// explicitly in the constraint). If the constraint is a named pointer type (see
-	// above), we are ok here because only fields are accepted as results.
+	// If we didn't find anything and if we have a type parameter with a shared underlying
+	// type, see if there is a matching field (but not a method, those need to be declared
+	// explicitly in the constraint). If the constraint is a named pointer type (see above),
+	// we are ok here because only fields are accepted as results.
 	const enableTParamFieldLookup = false // see go.dev/issue/51576
 	if enableTParamFieldLookup && obj == nil && isTypeParam(T) {
-		if t := coreType(T); t != nil {
+		if t := sharedUnder(nil, T, nil); t != nil {
 			obj, index, indirect = lookupFieldOrMethodImpl(t, addressable, pkg, name, foldCase)
 			if _, ok := obj.(*Var); !ok {
 				obj, index, indirect = nil, nil, false // accept fields (variables) only