go/types: implement conversions to generic types

Change-Id: Ifa878fddea3548a27579b3196921c6816f2d9363

src/go/types/NOTES

@@ -1,6 +1,6 @@
 TODO
 - allow recursive type parameterization without need to repeat type parameters
-- if type parameters are repeated in recursive instantiation, they must be the same order
+- if type parameters are repeated in recursive instantiation, they must be the same order (not yet checked)
 - implement contract embedding
 - interface embedding doesn't take care of literal type constraints yet
   (need an allTypes list, like we have an allMethods list?)
@@ -8,7 +8,6 @@ TODO
 OPEN ISSUES
 - using a contract and enumerating type arguments currently leads to an error (e.g. func f(type T C(T)) (x T) ... )
 git add - contracts slip through in places where only types are permitted
-- conversions against parameterized types are not implemented
 - parameterized interface methods (of type bounds) need to be customized (subst) for context
 
 DESIGN DECISIONS

src/go/types/expr.go

@@ -510,6 +510,39 @@ func (check *Checker) convertUntyped(x *operand, target Type) {
 		return
 	}
 
+	// In case of a type parameter, conversion must succeed against
+	// all types enumerated by the the type parameter bound.
+	if t, _ := target.Underlying().(*TypeParam); t != nil {
+		types := t.Interface().types
+		if len(types) == 0 {
+			goto Error
+		}
+
+		for _, t := range t.Interface().types {
+			check.convertUntypedInternal(x, t)
+			if x.mode == invalid {
+				goto Error
+			}
+		}
+
+		x.typ = target
+		check.updateExprType(x.expr, target, true) // UntypedNils are final
+		return
+	}
+
+	check.convertUntypedInternal(x, target)
+	return
+
+Error:
+	// TODO(gri) better error message (explain cause)
+	check.errorf(x.pos(), "cannot convert %s to %s", x, target)
+	x.mode = invalid
+}
+
+// convertUntypedInternal should only be called by convertUntyped.
+func (check *Checker) convertUntypedInternal(x *operand, target Type) {
+	assert(isTyped(target))
+
 	// typed target
 	switch t := target.Underlying().(type) {
 	case *Basic:

src/go/types/testdata/linalg.go2

@@ -62,10 +62,9 @@ contract Complex(T) {
 type OrderedAbs(type T OrderedNumeric) T
 
 func (a OrderedAbs(T)) Abs() T {
-	// TODO(gri) implement conversions against parameterized types
-	// if a < 0 {
-	// 	return -a
-	// }
+	if a < 0 {
+		return -a
+	}
 	return a
 }