go.tools/go/types: remove Strict mode from API

Also: factor out shared code assertableTo.

LGTM=adonovan
R=adonovan, gordon.klaus
CC=golang-codereviews
https://golang.org/cl/139880043

go/types/api.go

@@ -79,18 +79,6 @@ type Importer func(map[string]*Package, string) (*Package, error)
 // A Config specifies the configuration for type checking.
 // The zero value for Config is a ready-to-use default configuration.
 type Config struct {
-	// If Strict is set, the type-checker enforces additional
-	// rules not specified by the Go 1 spec, but which will
-	// catch guaranteed run-time errors if the respective
-	// code is executed. In other words, programs passing in
-	// Strict mode are Go 1 compliant, but not all Go 1 programs
-	// will pass in Strict mode. The additional rules are:
-	//
-	// - A type assertion x.(T) where T is an interface type
-	//   is invalid if any (statically known) method that exists
-	//   for both x and T have different signatures.
-	Strict bool
-
 	// If IgnoreFuncBodies is set, function bodies are not
 	// type-checked.
 	IgnoreFuncBodies bool
@@ -345,8 +333,8 @@ func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, i
 
 // AssertableTo reports whether a value of type V can be asserted to have type T.
 func AssertableTo(V *Interface, T Type) bool {
-	f, _ := MissingMethod(T, V, false)
-	return f == nil
+	m, _ := assertableTo(V, T)
+	return m == nil
 }
 
 // AssignableTo reports whether a value of type V is assignable to a variable of type T.

go/types/check.go

@@ -20,6 +20,19 @@ const (
 	trace = false // turn on for detailed type resolution traces
 )
 
+// If Strict is set, the type-checker enforces additional
+// rules not specified by the Go 1 spec, but which will
+// catch guaranteed run-time errors if the respective
+// code is executed. In other words, programs passing in
+// Strict mode are Go 1 compliant, but not all Go 1 programs
+// will pass in Strict mode. The additional rules are:
+//
+// - A type assertion x.(T) where T is an interface type
+//   is invalid if any (statically known) method that exists
+//   for both x and T have different signatures.
+//
+const strict = false
+
 // exprInfo stores information about an untyped expression.
 type exprInfo struct {
 	isLhs bool // expression is lhs operand of a shift with delayed type-check

go/types/expr.go

@@ -1406,14 +1406,7 @@ Error:
 
 // typeAssertion checks that x.(T) is legal; xtyp must be the type of x.
 func (check *Checker) typeAssertion(pos token.Pos, x *operand, xtyp *Interface, T Type) {
-	// no static check is required if T is an interface
-	// spec: "If T is an interface type, x.(T) asserts that the
-	//        dynamic type of x implements the interface T."
-	if _, ok := T.Underlying().(*Interface); ok && !check.conf.Strict {
-		return
-	}
-
-	method, wrongType := MissingMethod(T, xtyp, false)
+	method, wrongType := assertableTo(xtyp, T)
 	if method == nil {
 		return
 	}

go/types/lookup.go

@@ -293,6 +293,19 @@ func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType b
 	return
 }
 
+// assertableTo reports whether a value of type V can be asserted to have type T.
+// It returns (nil, false) as affirmative answer. Otherwise it returns a missing
+// method required by V and whether it is missing or just has the wrong type.
+func assertableTo(V *Interface, T Type) (method *Func, wrongType bool) {
+	// no static check is required if T is an interface
+	// spec: "If T is an interface type, x.(T) asserts that the
+	//        dynamic type of x implements the interface T."
+	if _, ok := T.Underlying().(*Interface); ok && !strict {
+		return
+	}
+	return MissingMethod(T, V, false)
+}
+
 // deref dereferences typ if it is a *Pointer and returns its base and true.
 // Otherwise it returns (typ, false).
 func deref(typ Type) (Type, bool) {