[dev.typeparams] go/types: factor out constraint satisfaction check

This is a port of CL 322070 to go/types, adjusted for the different
error reporting API.

Change-Id: I75eafe015b5b00554116527ea021e7a5f9e0343b
Reviewed-on: https://go-review.googlesource.com/c/go/+/324759
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/subst.go

@@ -121,20 +121,30 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 
 	// check bounds
 	for i, tname := range tparams {
-		tpar := tname.typ.(*_TypeParam)
-		iface := tpar.Bound()
-		if iface.Empty() {
-			continue // no type bound
-		}
-
-		targ := targs[i]
-
 		// 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)
+}
+
+// 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.
+func (check *Checker) satisfies(pos token.Pos, targ Type, tpar *_TypeParam, smap *substMap) bool {
+	iface := tpar.Bound()
+	if iface.Empty() {
+		return true // no type bound
+	}
+
 	// The type parameter bound is parameterized with the same type parameters
 	// as the instantiated type; before we can use it for bounds checking we
 	// need to instantiate it with the type arguments with which we instantiate
@@ -150,7 +160,7 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 		// TODO(gri) is this what we want? (spec question)
 		if base, isPtr := deref(targ); isPtr && asTypeParam(base) != nil {
 			check.errorf(atPos(pos), 0, "%s has no methods", targ)
-				break
+			return false
 		}
 		if m, wrong := check.missingMethod(targ, iface, true); m != nil {
 			// TODO(gri) needs to print updated name to avoid major confusion in error message!
@@ -171,13 +181,13 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 			} else {
 				check.softErrorf(atPos(pos), 0, "%s does not satisfy %s (missing method %s)", targ, tpar.bound, m.name)
 			}
-				break
+			return false
 		}
 	}
 
 	// targ's underlying type must also be one of the interface types listed, if any
 	if iface.allTypes == nil {
-			continue // nothing to do
+		return true // nothing to do
 	}
 
 	// If targ is itself a type parameter, each of its possible types, but at least one, must be in the
@@ -186,26 +196,25 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 		targBound := targ.Bound()
 		if targBound.allTypes == nil {
 			check.softErrorf(atPos(pos), _Todo, "%s does not satisfy %s (%s has no type constraints)", targ, tpar.bound, targ)
-				break
+			return false
 		}
 		for _, t := range unpackType(targBound.allTypes) {
 			if !iface.isSatisfiedBy(t) {
 				// TODO(gri) match this error message with the one below (or vice versa)
 				check.softErrorf(atPos(pos), 0, "%s does not satisfy %s (%s type constraint %s not found in %s)", targ, tpar.bound, targ, t, iface.allTypes)
-					break
+				return false
 			}
 		}
-			break
+		return false
 	}
 
 	// Otherwise, targ's type or underlying type must also be one of the interface types listed, if any.
 	if !iface.isSatisfiedBy(targ) {
-			check.softErrorf(atPos(pos), _Todo, "%s does not satisfy %s (%s or %s not found in %s)", targ, tpar.bound, targ, under(targ), iface.allTypes)
+		check.softErrorf(atPos(pos), _Todo, "%s does not satisfy %s (%s not found in %s)", targ, tpar.bound, under(targ), iface.allTypes)
-			break
+		return false
-		}
 	}
 
-	return check.subst(pos, typ, smap)
+	return true
 }
 
 // subst returns the type typ with its type parameters tpars replaced by