go/types, types2: don't look up fields or methods when expecting a type

As we have seen many times, the type checker must be careful to avoid
accessing named type information before the type is fully set up. We
need a more systematic solution to this problem, but for now avoid one
case that causes a crash: checking a selector expression on an
incomplete type when a type expression is expected.

For golang/go#57522

Change-Id: I7ed31b859cca263276e3a0647d1f1b49670023a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/461577
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/types2/call.go

@@ -447,7 +447,7 @@ var cgoPrefixes = [...]string{
 	"_Cmacro_", // function to evaluate the expanded expression
 }
 
-func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *Named) {
+func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *Named, wantType bool) {
 	// these must be declared before the "goto Error" statements
 	var (
 		obj      Object
@@ -559,6 +559,25 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *Named) {
 		goto Error
 	}
 
+	// Avoid crashing when checking an invalid selector in a method declaration
+	// (i.e., where def is not set):
+	//
+	//   type S[T any] struct{}
+	//   type V = S[any]
+	//   func (fs *S[T]) M(x V.M) {}
+	//
+	// All codepaths below return a non-type expression. If we get here while
+	// expecting a type expression, it is an error.
+	//
+	// See issue #57522 for more details.
+	//
+	// TODO(rfindley): We should do better by refusing to check selectors in all cases where
+	// x.typ is incomplete.
+	if wantType {
+		check.errorf(e.Sel, NotAType, "%s is not a type", syntax.Expr(e))
+		goto Error
+	}
+
 	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
 	if obj == nil {
 		// Don't report another error if the underlying type was invalid (issue #49541).

src/cmd/compile/internal/types2/expr.go

@@ -1587,7 +1587,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		return kind
 
 	case *syntax.SelectorExpr:
-		check.selector(x, e, nil)
+		check.selector(x, e, nil, false)
 
 	case *syntax.IndexExpr:
 		if check.indexExpr(x, e) {

src/cmd/compile/internal/types2/typexpr.go

@@ -256,7 +256,7 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 
 	case *syntax.SelectorExpr:
 		var x operand
-		check.selector(&x, e, def)
+		check.selector(&x, e, def, true)
 
 		switch x.mode {
 		case typexpr:

src/go/types/call.go

@@ -450,7 +450,7 @@ var cgoPrefixes = [...]string{
 	"_Cmacro_", // function to evaluate the expanded expression
 }
 
-func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *Named) {
+func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *Named, wantType bool) {
 	// these must be declared before the "goto Error" statements
 	var (
 		obj      Object
@@ -563,6 +563,25 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *Named) {
 		goto Error
 	}
 
+	// Avoid crashing when checking an invalid selector in a method declaration
+	// (i.e., where def is not set):
+	//
+	//   type S[T any] struct{}
+	//   type V = S[any]
+	//   func (fs *S[T]) M(x V.M) {}
+	//
+	// All codepaths below return a non-type expression. If we get here while
+	// expecting a type expression, it is an error.
+	//
+	// See issue #57522 for more details.
+	//
+	// TODO(rfindley): We should do better by refusing to check selectors in all cases where
+	// x.typ is incomplete.
+	if wantType {
+		check.errorf(e.Sel, NotAType, "%s is not a type", ast.Expr(e))
+		goto Error
+	}
+
 	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
 	if obj == nil {
 		// Don't report another error if the underlying type was invalid (issue #49541).

src/go/types/expr.go

@@ -1568,7 +1568,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		return kind
 
 	case *ast.SelectorExpr:
-		check.selector(x, e, nil)
+		check.selector(x, e, nil, false)
 
 	case *ast.IndexExpr, *ast.IndexListExpr:
 		ix := typeparams.UnpackIndexExpr(e)

src/go/types/typexpr.go

@@ -256,7 +256,7 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 
 	case *ast.SelectorExpr:
 		var x operand
-		check.selector(&x, e, def)
+		check.selector(&x, e, def, true)
 
 		switch x.mode {
 		case typexpr:

src/internal/types/testdata/check/cycles0.go

@@ -45,7 +45,7 @@ type (
 
 	// pointers
 	P0 *P0
-	PP *struct{ PP.f /* ERROR no field or method f */ }
+	PP *struct{ PP.f /* ERROR PP.f is not a type */ }
 
 	// functions
 	F0 func(F0)

src/internal/types/testdata/check/decls0.go

@@ -63,7 +63,7 @@ type (
 
 
 type (
-	p1 pi.foo /* ERROR "no field or method foo" */
+	p1 pi.foo /* ERROR "pi.foo is not a type" */
 	p2 unsafe.Pointer
 )
 
@@ -189,10 +189,10 @@ func f4() (x *f4 /* ERROR "not a type" */ ) { return }
 // TODO(#43215) this should be detected as a cycle error
 func f5([unsafe.Sizeof(f5)]int) {}
 
-func (S0) m1 (x S0 /* ERROR illegal cycle in method declaration */ .m1) {}
-func (S0) m2 (x *S0 /* ERROR illegal cycle in method declaration */ .m2) {}
-func (S0) m3 () (x S0 /* ERROR illegal cycle in method declaration */ .m3) { return }
-func (S0) m4 () (x *S0 /* ERROR illegal cycle in method declaration */ .m4) { return }
+func (S0) m1 (x S0.m1 /* ERROR S0.m1 is not a type */ ) {}
+func (S0) m2 (x *S0.m2 /* ERROR S0.m2 is not a type */ ) {}
+func (S0) m3 () (x S0.m3 /* ERROR S0.m3 is not a type */ ) { return }
+func (S0) m4 () (x *S0.m4 /* ERROR S0.m4 is not a type */ ) { return }
 
 // interfaces may not have any blank methods
 type BlankI interface {

src/internal/types/testdata/check/issues0.go

@@ -97,7 +97,7 @@ func issue10979() {
 		nosuchpkg /* ERROR undefined: nosuchpkg */ .Nosuchtype
 	}
 	type I interface {
-		I.m /* ERROR no field or method m */
+		I.m /* ERROR I.m is not a type */
 		m()
 	}
 }

src/internal/types/testdata/fixedbugs/issue39634.go

@@ -19,7 +19,7 @@ func(*ph1[e,e /* ERROR redeclared */ ])h(d /* ERROR undefined */ )
 // func t2[T Numeric2](s[]T){0 /* ERROR not a type */ []{s /* ERROR cannot index */ [0][0]}}
 
 // crash 3
-type t3 *interface{ t3.p /* ERROR no field or method p */ }
+type t3 *interface{ t3.p /* ERROR t3.p is not a type */ }
 
 // crash 4
 type Numeric4 interface{t4 /* ERROR not a type */ }

src/internal/types/testdata/fixedbugs/issue57522.go

@@ -0,0 +1,24 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// A simplified version of the code in the original report.
+type S[T any] struct{}
+var V = S[any]{}
+func (fs *S[T]) M(V.M /* ERROR "V.M is not a type" */) {}
+
+// Other minimal reproducers.
+type S1[T any] V1.M /* ERROR "V1.M is not a type" */
+type V1 = S1[any]
+
+type S2[T any] struct{}
+type V2 = S2[any]
+func (fs *S2[T]) M(x V2.M /* ERROR "V2.M is not a type" */ ) {}
+
+// The following still panics, as the selector is reached from check.expr
+// rather than check.typexpr. TODO(rfindley): fix this.
+// type X[T any] int
+// func (X[T]) M(x [X[int].M]int) {}
+

test/fixedbugs/issue18392.go

@@ -10,5 +10,5 @@ type A interface {
 	// TODO(mdempsky): This should be an error, but this error is
 	// nonsense. The error should actually mention that there's a
 	// type loop.
-	Fn(A.Fn) // ERROR "type A has no method Fn|A.Fn undefined"
+	Fn(A.Fn) // ERROR "type A has no method Fn|A.Fn undefined|A.Fn is not a type"
 }