go/types, types2: better error messages for invalid calls

Rather than reporting "non-function" for an invalid type parameter,
report which type in the type parameter's type set is not a function.

Change-Id: I8beec25cc337bae8e03d23e62d97aa82db46bab4
Reviewed-on: https://go-review.googlesource.com/c/go/+/654475
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -244,19 +244,19 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 
 	// If the operand type is a type parameter, all types in its type set
 	// must have a common underlying type, which must be a signature.
-	// TODO(gri) use commonUnder condition for better error message
+	u, err := commonUnder(x.typ, func(t, u Type) *errorCause {
-	u, err := commonUnder(x.typ, nil)
+		if _, ok := u.(*Signature); u != nil && !ok {
-	sig, _ := u.(*Signature)
+			return newErrorCause("%s is not a function", t)
-	if sig == nil {
-		if err != nil {
-			check.errorf(x, InvalidCall, invalidOp+"cannot call %s: %s", x, err.format(check))
-		} else {
-			check.errorf(x, InvalidCall, invalidOp+"cannot call non-function %s", x)
 		}
+		return nil
+	})
+	if err != nil {
+		check.errorf(x, InvalidCall, invalidOp+"cannot call %s: %s", x, err.format(check))
 		x.mode = invalid
 		x.expr = call
 		return statement
 	}
+	sig := u.(*Signature) // u must be a signature per the commonUnder condition
 
 	// Capture wasGeneric before sig is potentially instantiated below.
 	wasGeneric := sig.TypeParams().Len() > 0

src/go/types/call.go

@@ -246,19 +246,19 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 
 	// If the operand type is a type parameter, all types in its type set
 	// must have a common underlying type, which must be a signature.
-	// TODO(gri) use commonUnder condition for better error message
+	u, err := commonUnder(x.typ, func(t, u Type) *errorCause {
-	u, err := commonUnder(x.typ, nil)
+		if _, ok := u.(*Signature); u != nil && !ok {
-	sig, _ := u.(*Signature)
+			return newErrorCause("%s is not a function", t)
-	if sig == nil {
-		if err != nil {
-			check.errorf(x, InvalidCall, invalidOp+"cannot call %s: %s", x, err.format(check))
-		} else {
-			check.errorf(x, InvalidCall, invalidOp+"cannot call non-function %s", x)
 		}
+		return nil
+	})
+	if err != nil {
+		check.errorf(x, InvalidCall, invalidOp+"cannot call %s: %s", x, err.format(check))
 		x.mode = invalid
 		x.expr = call
 		return statement
 	}
+	sig := u.(*Signature) // u must be a signature per the commonUnder condition
 
 	// Capture wasGeneric before sig is potentially instantiated below.
 	wasGeneric := sig.TypeParams().Len() > 0

src/internal/types/testdata/check/lookup1.go

@@ -11,7 +11,7 @@ func _() {
 		x, aBc int
 	}
 	_ = s.x
-	_ = s /* ERROR "invalid operation: cannot call non-function s.x (variable of type int)" */ .x()
+	_ = s /* ERROR "invalid operation: cannot call s.x (variable of type int): int is not a function" */ .x()
 	_ = s.X // ERROR "s.X undefined (type struct{x int; aBc int} has no field or method X, but does have field x)"
 	_ = s.X /* ERROR "s.X undefined (type struct{x int; aBc int} has no field or method X, but does have field x)" */ ()
 
@@ -26,7 +26,7 @@ func _() {
 	}
 	var s S
 	_ = s.x
-	_ = s /* ERROR "invalid operation: cannot call non-function s.x (variable of type int)" */ .x()
+	_ = s /* ERROR "invalid operation: cannot call s.x (variable of type int): int is not a function" */ .x()
 	_ = s.X // ERROR "s.X undefined (type S has no field or method X, but does have field x)"
 	_ = s.X /* ERROR "s.X undefined (type S has no field or method X, but does have field x)" */ ()
 }
@@ -77,9 +77,9 @@ func _[P any](x P) {
 }
 
 func _[P int](x P) {
-	x /* ERROR "cannot call non-function x (variable of type P constrained by int)" */ ()
+	x /* ERROR "cannot call x (variable of type P constrained by int): int is not a function" */ ()
 }
 
 func _[P int | string](x P) {
-	x /* ERROR "cannot call x (variable of type P constrained by int | string): int and string have different underlying types" */ ()
+	x /* ERROR "cannot call x (variable of type P constrained by int | string): int is not a function" */ ()
 }

src/internal/types/testdata/fixedbugs/issue49482.go

@@ -13,7 +13,7 @@ const P = 2 // declare P to avoid noisy 'undefined' errors below.
 
 // The following parse as invalid array types due to parsing ambiguitiues.
 type _ [P *int /* ERROR "int (type) is not an expression" */ ]int
-type _ [P /* ERROR "non-function P" */ (*int)]int
+type _ [P /* ERROR "cannot call P (untyped int constant 2): untyped int is not a function" */ (*int)]int
 
 // Adding a trailing comma or an enclosing interface resolves the ambiguity.
 type _[P *int,] int

test/fixedbugs/issue17038.go

@@ -6,4 +6,4 @@
 
 package main
 
-const A = complex(0()) // ERROR "cannot call non-function"
+const A = complex(0()) // ERROR "cannot call .* not a function"

test/fixedbugs/issue22822.go

@@ -13,7 +13,7 @@ func F() {
 	slice := []int{1, 2, 3}
 	_ = slice
 	len := int(2)
-	println(len(slice)) // ERROR "cannot call non-function len .type int., declared at LINE-1|expected function|cannot call non-function len"
+	println(len(slice)) // ERROR "cannot call non-function len .type int., declared at LINE-1|expected function|cannot call len"
 	const iota = 1
-	println(iota(slice)) // ERROR "cannot call non-function iota .type int., declared at LINE-1|expected function|cannot call non-function iota"
+	println(iota(slice)) // ERROR "cannot call non-function iota .type int., declared at LINE-1|expected function|cannot call iota"
 }

test/fixedbugs/issue27356.go

@@ -11,9 +11,9 @@ package p
 var a = []int{1,2,3}
 
 func _(len int) {
-	_ =  len(a) // ERROR "cannot call non-function|expected function"
+	_ =  len(a) // ERROR "cannot call|expected function"
 }
 
 var cap = false
-var _ = cap(a) // ERROR "cannot call non-function|expected function"
+var _ = cap(a) // ERROR "cannot call|expected function"