go/types: do not permit parenthesized uninstantiated generic types

Document the design decision.

Change-Id: Ied5d77bc576653b49edcde4cdb3f6b95623a9045

src/go/types/NOTES

@@ -7,4 +7,10 @@ TODO
   (need an allTypes list, like we have an allMethods list?)
 
 OPEN ISSUES
-- do we allow parenthesized generic uninstantiated types?
+
+DESIGN DECISIONS
+- 12/4/2019: do not allow parenthesized generic uninstantiated types (unless instantiated implicitly)
+  In other words: generic types must always be instantiated before they can be used in any form
+  More generally: Only permit type instantiation T(x) in type context, when the type is a named type.
+  Do not permit in in general in type context: e.g., disallow []T(x) because we consider that a
+  conversion, in general. Same for ([]T)(x).

src/go/types/contracts.go

@@ -249,7 +249,7 @@ func (check *Checker) satisfyContract(contr *Contract, targs []Type) bool {
 
 	for i, targ := range targs {
 		iface := contr.ifaceAt(i)
-		// If iface is parameterized, we need to replace the type parameters
+		// If iface is generic, we need to replace the type parameters
 		// with the respective type arguments.
 		// TODO(gri) fix this
 		if IsParameterized(iface) {

src/go/types/examples/types.go2

@@ -83,7 +83,7 @@ func _() {
 	x1a.f = x2a.f
 }
 
-// Another interesting corner case is parameterized types that don't use
+// Another interesting corner case are generic types that don't use
 // their type arguments. For instance:
 type T(type P) struct{}
 
@@ -101,8 +101,9 @@ func _() {
 	xint = xbool // ERROR assignment
 }
 
-// Parameterized types cannot be used without instantiation.
+// Generic types cannot be used without instantiation.
 var _ T // ERROR cannot use generic type T
 
-// TODO(gri) Should we allow parentheses around generic, uninstantiated types?
-// var _ (T)(int)
+// In type context, parameterized (generic) types cannot use instantiation syntax.
+// See also NOTES entry from 12/4/2019.
+var _ (T /* ERROR cannot use generic type T */ )( /* ERROR expected ';' */ int)

src/go/types/typexpr.go

@@ -321,7 +321,8 @@ func (check *Checker) typInternal(e ast.Expr, def *Named) (T Type) {
 		return typ
 
 	case *ast.ParenExpr:
-		// TODO(gri) should we allow parenthesized generic (uninstantiated) types?
+		// Generic types must be instantiated before they can be used in any form.
+		// Consequently, generic types cannot be parenthesized.
 		return check.definedType(e.X, def)
 
 	case *ast.ArrayType: