go/types, types2: bail early if we want a type but don't have one

If we do know whether we need a type or not, make use of the
information when we know that we don't have a type and bail out.

Fixes the issue at hand and also improves some other error messages
which now report that we don't have a type instead of reporting a cycle.

For #65344.

Change-Id: I11182efd452c485d89e6c09ead8a647ea05d7318
Reviewed-on: https://go-review.googlesource.com/c/go/+/559335
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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

@@ -48,6 +48,20 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *TypeName, wantType
 	}
 	check.recordUse(e, obj)
 
+	// If we want a type but don't have one, stop right here and avoid potential problems
+	// with missing underlying types. This also gives better error messages in some cases
+	// (see go.dev/issue/65344).
+	_, gotType := obj.(*TypeName)
+	if !gotType && wantType {
+		check.errorf(e, NotAType, "%s is not a type", obj.Name())
+		// avoid "declared but not used" errors
+		// (don't use Checker.use - we don't want to evaluate too much)
+		if v, _ := obj.(*Var); v != nil && v.pkg == check.pkg /* see Checker.use1 */ {
+			v.used = true
+		}
+		return
+	}
+
 	// Type-check the object.
 	// Only call Checker.objDecl if the object doesn't have a type yet
 	// (in which case we must actually determine it) or the object is a
@@ -57,7 +71,7 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *TypeName, wantType
 	// informative "not a type/value" error that this function's caller
 	// will issue (see go.dev/issue/25790).
 	typ := obj.Type()
-	if _, gotType := obj.(*TypeName); typ == nil || gotType && wantType {
+	if typ == nil || gotType && wantType {
 		check.objDecl(obj, def)
 		typ = obj.Type() // type must have been assigned by Checker.objDecl
 	}

src/go/types/typexpr.go

@@ -49,6 +49,20 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *TypeName, wantType bo
 	}
 	check.recordUse(e, obj)
 
+	// If we want a type but don't have one, stop right here and avoid potential problems
+	// with missing underlying types. This also gives better error messages in some cases
+	// (see go.dev/issue/65344).
+	_, gotType := obj.(*TypeName)
+	if !gotType && wantType {
+		check.errorf(e, NotAType, "%s is not a type", obj.Name())
+		// avoid "declared but not used" errors
+		// (don't use Checker.use - we don't want to evaluate too much)
+		if v, _ := obj.(*Var); v != nil && v.pkg == check.pkg /* see Checker.use1 */ {
+			v.used = true
+		}
+		return
+	}
+
 	// Type-check the object.
 	// Only call Checker.objDecl if the object doesn't have a type yet
 	// (in which case we must actually determine it) or the object is a
@@ -58,7 +72,7 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *TypeName, wantType bo
 	// informative "not a type/value" error that this function's caller
 	// will issue (see go.dev/issue/25790).
 	typ := obj.Type()
-	if _, gotType := obj.(*TypeName); typ == nil || gotType && wantType {
+	if typ == nil || gotType && wantType {
 		check.objDecl(obj, def)
 		typ = obj.Type() // type must have been assigned by Checker.objDecl
 	}

src/internal/types/testdata/check/cycles5.go

@@ -161,8 +161,8 @@ var a12 = makeArray()
 func makeArray() (res T12) { return }
 
 // issue #20770
-var r /* ERROR "invalid cycle in declaration of r" */ = newReader()
-func newReader() r
+var r = newReader()
+func newReader() r // ERROR "r is not a type"
 
 // variations of the theme of #8699 and #20770
 var arr /* ERROR "cycle" */ = f()

src/internal/types/testdata/check/cycles5a.go

@@ -161,8 +161,8 @@ var a12 = makeArray()
 func makeArray() (res T12) { return }
 
 // issue #20770
-var r /* ERROR "invalid cycle in declaration of r" */ = newReader()
-func newReader() r
+var r = newReader()
+func newReader() r // ERROR "r is not a type"
 
 // variations of the theme of #8699 and #20770
 var arr /* ERROR "cycle" */ = f()

src/internal/types/testdata/check/decls1.go

@@ -63,7 +63,7 @@ var (
 	t12 complex64 = -(u + *t11) / *&v
 	t13 int = a /* ERROR "shifted operand" */ << d
 	t14 int = i << j
-	t15 math /* ERROR "not in selector" */
+	t15 math /* ERROR "math is not a type" */
 	t16 math.xxx /* ERROR "undefined" */
 	t17 math /* ERROR "not a type" */ .Pi
 	t18 float64 = math.Pi * 10.0

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

@@ -104,7 +104,7 @@ func issue10979() {
 
 // issue11347
 // These should not crash.
-var a1, b1 /* ERROR "cycle" */ , c1 /* ERROR "cycle" */ b1 = 0 > 0<<""[""[c1]]>c1
+var a1, b1, c1 /* ERROR "cycle" */ b1 /* ERROR "b1 is not a type" */ = 0 > 0<<""[""[c1]]>c1
 var a2, b2 /* ERROR "cycle" */ = 0 /* ERROR "assignment mismatch" */ /* ERROR "assignment mismatch" */ > 0<<""[b2]
 var a3, b3 /* ERROR "cycle" */ = int /* ERROR "assignment mismatch" */ /* ERROR "assignment mismatch" */ (1<<""[b3])
 

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

@@ -2,9 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Examples adjusted to match new [T any] syntax for type parameters.
+// Examples from the issue adjusted to match new [T any] syntax for type parameters.
 // Also, previously permitted empty type parameter lists and instantiations
 // are now syntax errors.
+//
+// The primary concern here is that these tests shouldn't crash the type checker.
+// The quality of the error messages is secondary as these are all pretty esoteric
+// or artificial test cases.
 
 package p
 
@@ -39,7 +43,7 @@ type foo9[A any] interface { foo9 /* ERROR "invalid recursive type" */ [A] }
 func _() { var _ = new(foo9[int]) }
 
 // crash 12
-var u /* ERROR "cycle" */ , i [func /* ERROR "used as value" */ /* ERROR "used as value" */ (u, c /* ERROR "undefined" */ /* ERROR "undefined" */ ) {}(0, len /* ERROR "must be called" */ /* ERROR "must be called" */ )]c /* ERROR "undefined" */ /* ERROR "undefined" */
+var u, i [func /* ERROR "used as value" */ /* ERROR "used as value" */ (u /* ERROR "u is not a type" */ /* ERROR "u is not a type" */ , c /* ERROR "undefined" */ /* ERROR "undefined" */ ) {}(0, len /* ERROR "must be called" */ /* ERROR "must be called" */ )]c /* ERROR "undefined" */ /* ERROR "undefined" */
 
 // crash 15
 func y15() { var a /* ERROR "declared and not used" */ interface{ p() } = G15[string]{} }

src/internal/types/testdata/fixedbugs/issue65344.go

@@ -0,0 +1,19 @@
+// Copyright 2024 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
+
+type T1 C /* ERROR "C is not a type" */
+
+// TODO(gri) try to avoid this follow-on error
+const C = T1(0 /* ERROR "cannot convert 0 (untyped int constant) to type T1" */)
+
+type T2 V /* ERROR "V is not a type" */
+
+var V T2
+
+func _() {
+	// don't produce errors here
+	_ = C + V
+}