go/types: systematic detection of missing instantiation

This is a port of CL 346471 to go/types. Additionally, CheckExpr was
updated for the new API to explicitly allow generic expressions.

The error messages in issue39634.go2 are different because go/parser
produces an IndexExpr with BadExpr index value, for backward
compatibility.

Change-Id: I725926de183a016381513fe0e750d1280688ce29
Reviewed-on: https://go-review.googlesource.com/c/go/+/347391
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/builtins.go

@@ -772,7 +772,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		var t operand
 		x1 := x
 		for _, arg := range call.Args {
-			check.rawExpr(x1, arg, nil) // permit trace for types, e.g.: new(trace(T))
+			check.rawExpr(x1, arg, nil, false) // permit trace for types, e.g.: new(trace(T))
 			check.dump("%v: %s", x1.Pos(), x1)
 			x1 = &t // use incoming x only for first argument
 		}

src/go/types/call.go

@@ -89,8 +89,9 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 		check.record(x)
 
 	} else {
-		check.exprOrType(x, call.Fun)
+		check.exprOrType(x, call.Fun, true)
 	}
+	// x.typ map be generic
 
 	switch x.mode {
 	case invalid:
@@ -100,6 +101,10 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 
 	case typexpr:
 		// conversion
+		check.nonGeneric(x)
+		if x.mode == invalid {
+			return conversion
+		}
 		T := x.typ
 		x.mode = invalid
 		switch n := len(call.Args); n {
@@ -128,6 +133,7 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 		return conversion
 
 	case builtin:
+		// no need to check for non-genericity here
 		id := x.id
 		if !check.builtin(x, call, id) {
 			x.mode = invalid
@@ -141,6 +147,7 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 	}
 
 	// ordinary function/method call
+	// signature may be generic
 	cgocall := x.mode == cgofunc
 
 	sig := asSignature(x.typ)
@@ -478,15 +485,11 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 		}
 	}
 
-	check.exprOrType(x, e.X)
+	check.exprOrType(x, e.X, false)
 	if x.mode == invalid {
 		goto Error
 	}
 
-	if x.mode == typexpr {
-		x.typ = check.varType(e.X)
-	}
-
 	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
 	if obj == nil {
 		switch {
@@ -722,7 +725,7 @@ func (check *Checker) use(arg ...ast.Expr) {
 		// The nil check below is necessary since certain AST fields
 		// may legally be nil (e.g., the ast.SliceExpr.High field).
 		if e != nil {
-			check.rawExpr(&x, e, nil)
+			check.rawExpr(&x, e, nil, false)
 		}
 	}
 }
@@ -754,7 +757,7 @@ func (check *Checker) useLHS(arg ...ast.Expr) {
 				}
 			}
 		}
-		check.rawExpr(&x, e, nil)
+		check.rawExpr(&x, e, nil, false)
 		if v != nil {
 			v.used = v_used // restore v.used
 		}

src/go/types/eval.go

@@ -35,9 +35,10 @@ func Eval(fset *token.FileSet, pkg *Package, pos token.Pos, expr string) (_ Type
 	return info.Types[node], err
 }
 
-// CheckExpr type checks the expression expr as if it had appeared at
+// CheckExpr type checks the expression expr as if it had appeared at position
-// position pos of package pkg. Type information about the expression
+// pos of package pkg. Type information about the expression is recorded in
-// is recorded in info.
+// info. The expression may be an uninstantiated parameterized function or
+// type.
 //
 // If pkg == nil, the Universe scope is used and the provided
 // position pos is ignored. If pkg != nil, and pos is invalid,
@@ -91,8 +92,8 @@ func CheckExpr(fset *token.FileSet, pkg *Package, pos token.Pos, expr ast.Expr,
 
 	// evaluate node
 	var x operand
-	check.rawExpr(&x, expr, nil)
+	check.rawExpr(&x, expr, nil, true) // allow generic expressions
-	check.processDelayed(0) // incl. all functions
+	check.processDelayed(0)            // incl. all functions
 	check.recordUntyped()
 
 	return nil

src/go/types/expr.go

@@ -1059,8 +1059,10 @@ const (
 // rawExpr typechecks expression e and initializes x with the expression
 // value or type. If an error occurred, x.mode is set to invalid.
 // If hint != nil, it is the type of a composite literal element.
+// If allowGeneric is set, the operand type may be an uninstantiated
+// parameterized type or function value.
 //
-func (check *Checker) rawExpr(x *operand, e ast.Expr, hint Type) exprKind {
+func (check *Checker) rawExpr(x *operand, e ast.Expr, hint Type, allowGeneric bool) exprKind {
 	if trace {
 		check.trace(e.Pos(), "expr %s", e)
 		check.indent++
@@ -1071,11 +1073,40 @@ func (check *Checker) rawExpr(x *operand, e ast.Expr, hint Type) exprKind {
 	}
 
 	kind := check.exprInternal(x, e, hint)
+
+	if !allowGeneric {
+		check.nonGeneric(x)
+	}
+
 	check.record(x)
 
 	return kind
 }
 
+// If x is a generic function or type, nonGeneric reports an error and invalidates x.mode and x.typ.
+// Otherwise it leaves x alone.
+func (check *Checker) nonGeneric(x *operand) {
+	if x.mode == invalid || x.mode == novalue {
+		return
+	}
+	var what string
+	switch t := x.typ.(type) {
+	case *Named:
+		if isGeneric(t) {
+			what = "type"
+		}
+	case *Signature:
+		if t.tparams != nil {
+			what = "function"
+		}
+	}
+	if what != "" {
+		check.errorf(x.expr, _Todo, "cannot use generic %s %s without instantiation", what, x.expr)
+		x.mode = invalid
+		x.typ = Typ[Invalid]
+	}
+}
+
 // exprInternal contains the core of type checking of expressions.
 // Must only be called by rawExpr.
 //
@@ -1354,7 +1385,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		x.typ = typ
 
 	case *ast.ParenExpr:
-		kind := check.rawExpr(x, e.X, nil)
+		kind := check.rawExpr(x, e.X, nil, false)
 		x.expr = e
 		return kind
 
@@ -1405,7 +1436,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		return check.callExpr(x, e)
 
 	case *ast.StarExpr:
-		check.exprOrType(x, e.X)
+		check.exprOrType(x, e.X, false)
 		switch x.mode {
 		case invalid:
 			goto Error
@@ -1525,14 +1556,14 @@ func (check *Checker) typeAssertion(at positioner, x *operand, xtyp *Interface,
 // If an error occurred, x.mode is set to invalid.
 //
 func (check *Checker) expr(x *operand, e ast.Expr) {
-	check.rawExpr(x, e, nil)
+	check.rawExpr(x, e, nil, false)
 	check.exclude(x, 1<<novalue|1<<builtin|1<<typexpr)
 	check.singleValue(x)
 }
 
 // multiExpr is like expr but the result may also be a multi-value.
 func (check *Checker) multiExpr(x *operand, e ast.Expr) {
-	check.rawExpr(x, e, nil)
+	check.rawExpr(x, e, nil, false)
 	check.exclude(x, 1<<novalue|1<<builtin|1<<typexpr)
 }
 
@@ -1542,16 +1573,18 @@ func (check *Checker) multiExpr(x *operand, e ast.Expr) {
 //
 func (check *Checker) exprWithHint(x *operand, e ast.Expr, hint Type) {
 	assert(hint != nil)
-	check.rawExpr(x, e, hint)
+	check.rawExpr(x, e, hint, false)
 	check.exclude(x, 1<<novalue|1<<builtin|1<<typexpr)
 	check.singleValue(x)
 }
 
 // exprOrType typechecks expression or type e and initializes x with the expression value or type.
+// If allowGeneric is set, the operand type may be an uninstantiated parameterized type or function
+// value.
 // If an error occurred, x.mode is set to invalid.
 //
-func (check *Checker) exprOrType(x *operand, e ast.Expr) {
+func (check *Checker) exprOrType(x *operand, e ast.Expr, allowGeneric bool) {
-	check.rawExpr(x, e, nil)
+	check.rawExpr(x, e, nil, allowGeneric)
 	check.exclude(x, 1<<novalue)
 	check.singleValue(x)
 }

src/go/types/index.go

@@ -16,7 +16,8 @@ import (
 // In that case x represents the uninstantiated function value and
 // it is the caller's responsibility to instantiate the function.
 func (check *Checker) indexExpr(x *operand, e *typeparams.IndexExpr) (isFuncInst bool) {
-	check.exprOrType(x, e.X)
+	check.exprOrType(x, e.X, true)
+	// x may be generic
 
 	switch x.mode {
 	case invalid:
@@ -26,6 +27,7 @@ func (check *Checker) indexExpr(x *operand, e *typeparams.IndexExpr) (isFuncInst
 	case typexpr:
 		// type instantiation
 		x.mode = invalid
+		// TODO(gri) here we re-evaluate e.X - try to avoid this
 		x.typ = check.varType(e.Orig)
 		if x.typ != Typ[Invalid] {
 			x.mode = typexpr
@@ -39,6 +41,12 @@ func (check *Checker) indexExpr(x *operand, e *typeparams.IndexExpr) (isFuncInst
 		}
 	}
 
+	// x should not be generic at this point, but be safe and check
+	check.nonGeneric(x)
+	if x.mode == invalid {
+		return false
+	}
+
 	valid := false
 	length := int64(-1) // valid if >= 0
 	switch typ := under(x.typ).(type) {

src/go/types/stmt.go

@@ -174,7 +174,7 @@ func (check *Checker) suspendedCall(keyword string, call *ast.CallExpr) {
 	var x operand
 	var msg string
 	var code errorCode
-	switch check.rawExpr(&x, call, nil) {
+	switch check.rawExpr(&x, call, nil, false) {
 	case conversion:
 		msg = "requires function call, not conversion"
 		code = _InvalidDefer
@@ -391,7 +391,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		// function and method calls and receive operations can appear
 		// in statement context. Such statements may be parenthesized."
 		var x operand
-		kind := check.rawExpr(&x, s.X, nil)
+		kind := check.rawExpr(&x, s.X, nil, false)
 		var msg string
 		var code errorCode
 		switch x.mode {

src/go/types/testdata/examples/types.go2

@@ -102,6 +102,7 @@ func _() {
 
 // Generic types cannot be used without instantiation.
 var _ T // ERROR cannot use generic type T
+var _ = T /* ERROR cannot use generic type T */ (0)
 
 // In type context, generic (parameterized) types cannot be parenthesized before
 // being instantiated. See also NOTES entry from 12/4/2019.

src/go/types/testdata/fixedbugs/issue39634.go2

@@ -85,6 +85,9 @@ var x T25 /* ERROR without instantiation */ .m1
 
 // crash 26
 type T26 = interface{ F26[ /* ERROR methods cannot have type parameters */ Z any]() }
+// The error messages on the line below differ from types2 because for backward
+// compatibility go/parser must produce an IndexExpr with BadExpr index for the
+// expression F26[].
 func F26[Z any]() T26 { return F26[] /* ERROR operand */ }
 
 // crash 27