From 13893bbf9e40a41bd66e8a8b9f5b1de8c9d29ef1 Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Mon, 9 Dec 2019 14:08:26 -0800
Subject: [PATCH] go/types: type instantiation to verify type bounds

Change-Id: Ia4670a0d6bbabf4a1ac71410c7bfccc99ba61c9d
---
 src/go/types/NOTES                  | 13 ------
 src/go/types/subst.go               | 71 +++++++++++++++++++++++++----
 src/go/types/testdata/contracts.go2 | 14 ++++++
 src/go/types/type.go                |  2 +-
 4 files changed, 78 insertions(+), 22 deletions(-)

diff --git a/src/go/types/NOTES b/src/go/types/NOTES
index 1b02386a8b..39620f4031 100644
--- a/src/go/types/NOTES
+++ b/src/go/types/NOTES
@@ -9,19 +9,6 @@ TODO
 
 
 OPEN ISSUES
-- type constraints are not verified in some cases:
-
-package p
-
-type B interface{type int}
-
-func f(type P B)(x P) {}
-
-func _() {
-	f(string)("foo") // this should be invalid
-	f("foo")         // this should be invalid
-}
-
 - incorrect constraint errors:
 
 package p
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index dce998f2c7..3eac9ba021 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -28,24 +28,79 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type) (res Ty
 		}()
 	}
 
-	// TODO(gri) need to do a satisfy bounds check here
-
+	// TODO(gri) What is better here: work with TypeParams, or work with TypeNames?
+	var tparams []*TypeName
 	switch typ := typ.(type) {
 	case *Named:
-		// TODO(gri) What is better here: work with TypeParams, or work with TypeNames?
-		return check.subst(pos, typ, typ.obj.tparams, targs)
-
+		tparams = typ.obj.tparams
 	case *Signature:
-		return check.subst(pos, typ, typ.tparams, targs)
+		tparams = typ.tparams
+	default:
+		check.dump(">>> trying to instantiate %s", typ)
+		unreachable() // only defined types and (defined) functions can be generic
+
 	}
 
-	panic("unreachable") // only defined types and (defined) functions can be generic
+	// check bounds
+	for i, tname := range tparams {
+		tpar := tname.typ.(*TypeParam)
+		if tpar.bound == nil {
+			continue // no bound
+		}
+
+		// determine type parameter bound
+		var iface *Interface
+		switch bound := tpar.bound.Underlying().(type) {
+		case *Interface:
+			iface = bound
+		case *Contract:
+			iface = bound.ifaceAt(i)
+		default:
+			unreachable()
+		}
+
+		// use interface type of type parameter, if any
+		// targ must implement iface
+		targ := targs[i]
+		if m, _ := check.missingMethod(targ, iface, true); m != nil {
+			check.softErrorf(pos, "%s does not satisfy %s (missing method %s)", targ, tpar.bound, m)
+			break
+		}
+
+		// targ's underlying type must also be one of the interface types listed, if any
+		if len(iface.types) > 0 {
+			utyp := targ.Underlying()
+
+			// TODO(gri) Cannot handle a type argument that is itself parameterized for now
+			switch utyp.(type) {
+			case *Interface, *Contract:
+				panic("unimplemented")
+			}
+
+			// TODO(gri) factor this out as sep. function?
+			ok := false
+			for _, t := range iface.types {
+				// if we find one matching type, we're ok
+				if Identical(utyp, t) {
+					ok = true
+					break
+				}
+			}
+
+			if !ok {
+				check.softErrorf(pos, "%s does not satisfy %s (%s not found in %s)", targ, tpar.bound, targ, iface)
+				break
+			}
+		}
+	}
+
+	return check.subst(pos, typ, tparams, targs)
 }
 
 // subst returns the type typ with its type parameters tparams replaced by
 // the corresponding type arguments targs, recursively.
 func (check *Checker) subst(pos token.Pos, typ Type, tpars []*TypeName, targs []Type) Type {
-	assert(len(tpars) == len(targs))
+	//assert(len(tpars) == len(targs)) // bounds may have only some type parameters
 	if len(tpars) == 0 {
 		return typ
 	}
diff --git a/src/go/types/testdata/contracts.go2 b/src/go/types/testdata/contracts.go2
index ab11a6297b..81a55650aa 100644
--- a/src/go/types/testdata/contracts.go2
+++ b/src/go/types/testdata/contracts.go2
@@ -183,3 +183,17 @@ func adderSum(type T Adder(T))(data []T) T {
 }
 
 func _(type T Adder /* ERROR cannot use generic type Adder */)(data []T) T
+
+// --------------------------------------------------------------------------------------
+// Instantiations require bounds to be satisfied
+
+type B1 interface{type int}
+
+func f1(type P B1)(x P) {}
+
+func _() {
+	f1 /* ERROR string not found */ (string)("foo")
+        f1 /* ERROR string not found */ ("foo")
+        f1 /* ERROR float64 not found */ (1.2)
+        f1(42)
+}
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 86dadb4a8a..c8b91028a0 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -540,7 +540,7 @@ type TypeParam struct {
 	id    uint64 // unique id (TODO should this be with the object? all objects?)
 	obj   *TypeName
 	index int  // parameter index
-	bound Type // either an *Interface or a *Contract
+	bound Type // either an *Interface or a *Contract // TODO(gri) make this *Interface or nil only?
 }
 
 // NewTypeParam returns a new TypeParam.