[dev.typeparams] cmd/compile/internal/types2: trigger verification while resolving instance

This is a straight port of CL 335978 with minor adjustements to
white space and an error message.

Change-Id: Icfcb562f75802a119ce5d02427bffecf7e279b2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/338097
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

3 files changed, 37 insertions, 28 deletions

diff --git a/src/cmd/compile/internal/types2/instance.go b/src/cmd/compile/internal/types2/instance.go
index df0fc17ba7..711d7de53c 100644
--- a/src/cmd/compile/internal/types2/instance.go
+++ b/src/cmd/compile/internal/types2/instance.go
@@ -26,7 +26,7 @@ func (n *Named) expand() {
 		// tparams. This is done implicitly by the call to n.TParams, but making it
 		// explicit is harmless: load is idempotent.
 		n.load()
-		inst := n.check.instantiate(n.instance.pos, n.orig.underlying, n.TParams(), n.targs, n.instance.posList, n.instance.verify)
+		inst := n.check.instantiate(n.instance.pos, n.orig.underlying, n.TParams(), n.targs, n.instance.posList)
 		n.underlying = inst
 		n.fromRHS = inst
 		n.instance = nil
diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index 1294b08490..7a40cea889 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -54,10 +54,15 @@ func (check *Checker) Instantiate(pos syntax.Pos, typ Type, targs []Type, posLis
 		// only types and functions can be generic
 		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
 	}
-	return check.instantiate(pos, typ, tparams, targs, posList, verify)
+
+	inst := check.instantiate(pos, typ, tparams, targs, posList)
+	if verify && len(tparams) == len(targs) {
+		check.verify(pos, tparams, targs, posList)
+	}
+	return inst
 }
 
-func (check *Checker) instantiate(pos syntax.Pos, typ Type, tparams []*TypeName, targs []Type, posList []syntax.Pos, verify bool) (res Type) {
+func (check *Checker) instantiate(pos syntax.Pos, typ Type, tparams []*TypeName, targs []Type, posList []syntax.Pos) (res Type) {
 	// the number of supplied types must match the number of type parameters
 	if len(targs) != len(tparams) {
 		// TODO(gri) provide better error message
@@ -67,9 +72,6 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, tparams []*TypeName,
 		}
 		panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, len(targs), len(tparams)))
 	}
-	if verify && check == nil {
-		panic("cannot have nil receiver if verify is set")
-	}
 
 	if check != nil && check.conf.Trace {
 		check.trace(pos, "-- instantiating %s with %s", typ, typeListString(targs))
@@ -93,24 +95,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, tparams []*TypeName,
 		return typ // nothing to do (minor optimization)
 	}
 
-	smap := makeSubstMap(tparams, targs)
-
-	// check bounds
-	if verify {
-		for i, tname := range tparams {
-			// best position for error reporting
-			pos := pos
-			if i < len(posList) {
-				pos = posList[i]
-			}
-			// stop checking bounds after the first failure
-			if !check.satisfies(pos, targs[i], tname.typ.(*TypeParam), smap) {
-				break
-			}
-		}
-	}
-
-	return check.subst(pos, typ, smap)
+	return check.subst(pos, typ, makeSubstMap(tparams, targs))
 }
 
 // InstantiateLazy is like Instantiate, but avoids actually
@@ -120,10 +105,16 @@ func (check *Checker) InstantiateLazy(pos syntax.Pos, typ Type, targs []Type, po
 	// Don't use asNamed here: we don't want to expand the base during lazy
 	// instantiation.
 	base := typ.(*Named)
-
 	if base == nil {
 		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
 	}
+
+	if verify && len(base.tparams) == len(targs) {
+		check.later(func() {
+			check.verify(pos, base.tparams, targs, posList)
+		})
+	}
+
 	h := instantiatedHash(base, targs)
 	if check != nil {
 		// typ may already have been instantiated with identical type arguments. In
@@ -148,6 +139,26 @@ func (check *Checker) InstantiateLazy(pos syntax.Pos, typ Type, targs []Type, po
 	return named
 }
 
+func (check *Checker) verify(pos syntax.Pos, tparams []*TypeName, targs []Type, posList []syntax.Pos) {
+	if check == nil {
+		panic("cannot have nil Checker if verifying constraints")
+	}
+
+	smap := makeSubstMap(tparams, targs)
+	for i, tname := range tparams {
+		// best position for error reporting
+		pos := pos
+		if i < len(posList) {
+			pos = posList[i]
+		}
+
+		// stop checking bounds after the first failure
+		if !check.satisfies(pos, targs[i], tname.typ.(*TypeParam), smap) {
+			break
+		}
+	}
+}
+
 // satisfies reports whether the type argument targ satisfies the constraint of type parameter
 // parameter tpar (after any of its type parameters have been substituted through smap).
 // A suitable error is reported if the result is false.
diff --git a/src/cmd/compile/internal/types2/testdata/check/issues.go2 b/src/cmd/compile/internal/types2/testdata/check/issues.go2
index e29357de0b..1ede383ebe 100644
--- a/src/cmd/compile/internal/types2/testdata/check/issues.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/issues.go2
@@ -74,10 +74,8 @@ func (u T2[U]) Add1() U {
     return u.s + 1
 }
 
-// TODO(rfindley): we should probably report an error here as well, not
-//                 just when the type is first instantiated.
 func NewT2[U any]() T2[U /* ERROR U has no constraints */ ] {
-    return T2[U]{}
+    return T2[U /* ERROR U has no constraints */ ]{}
 }
 
 func _() {