cmd/compile: improve error message for wrong number of arguments to return

Fixes #4215.
Fixes #6750.

Improves the error message for wrong number of arguments by comparing
the signature of the return call site arguments, versus the function's
expected return arguments.

In this CL, the signature representation of:
+ ideal numbers(TIDEAL) ie float*, complex*, rune, int is
"number" instead of "untyped number".
+ idealstring is "string" instead of "untyped string".
+ idealbool is "bool" instead of "untyped bool".

However, the representation of other types remains as the compiler
would produce.

* Example 1(in the error messages, if all lines were printed):
$ cat main.go && go run main.go
package main

func foo() (int, int) {
  return 2.3
}

func foo2() {
  return int(2), 2
}

func foo3(v int) (a, b, c, d int) {
  if v >= 5 {
    return 1
  }
  return 2, 3
}

func foo4(name string) (string, int) {
  switch name {
  case "cow":
    return "moo"
  case "dog":
    return "dog", 10, true
  case "fish":
    return ""
  default:
    return "lizard", 10
  }
}

type S int
type T string
type U float64

func foo5() (S, T, U) {
  if false {
    return ""
  } else {
    ptr := new(T)
    return ptr
  }
  return new(S), 12.34, 1 + 0i, 'r', true
}

func foo6() (T, string) {
  return "T"
}

./issue4215.go:4: not enough arguments to return, got (number) want (int, int)
./issue4215.go:8: too many arguments to return, got (int, number) want ()
./issue4215.go:13: not enough arguments to return, got (number) want (int, int, int, int)
./issue4215.go:15: not enough arguments to return, got (number, number) want (int, int, int, int)
./issue4215.go:21: not enough arguments to return, got (string) want (string, int)
./issue4215.go:23: too many arguments to return, got (string, number, bool) want (string, int)
./issue4215.go:25: not enough arguments to return, got (string) want (string, int)
./issue4215.go:37: not enough arguments to return, got (string) want (S, T, U)
./issue4215.go:40: not enough arguments to return, got (*T) want (S, T, U)
./issue4215.go:42: too many arguments to return, got (*S, number, number, number, bool) want (S, T, U)
./issue4215.go:46: not enough arguments to return, got (string) want (T, string)
./issue4215.go:46: too many errors

* Example 2:
$ cat 6750.go && go run 6750.go
package main

import "fmt"

func printmany(nums ...int) {
  for i, n := range nums {
    fmt.Printf("%d: %d\n", i, n)
  }
  fmt.Printf("\n")
}

func main() {
  printmany(1, 2, 3)
  printmany([]int{1, 2, 3}...)
  printmany(1, "abc", []int{2, 3}...)
}
./issue6750.go:15: too many arguments in call to printmany, got (number, string, []int) want (...int)

Change-Id: I6fdce78553ae81770840070e2c975d3e3c83d5d8
Reviewed-on: https://go-review.googlesource.com/25156
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/typecheck.go

@@ -2685,12 +2685,12 @@ notenough:
 			// Method expressions have the form T.M, and the compiler has
 			// rewritten those to ONAME nodes but left T in Left.
 			if call.Op == ONAME && call.Left != nil && call.Left.Op == OTYPE {
-				yyerror("not enough arguments in call to method expression %v", call)
+				yyerror("not enough arguments in call to method expression %v, got %s want %v", call, nl.retsigerr(), tstruct)
 			} else {
-				yyerror("not enough arguments in call to %v", call)
+				yyerror("not enough arguments in call to %v, got %s want %v", call, nl.retsigerr(), tstruct)
 			}
 		} else {
-			yyerror("not enough arguments to %v", op)
+			yyerror("not enough arguments to %v, got %s want %v", op, nl.retsigerr(), tstruct)
 		}
 		if n != nil {
 			n.Diag = 1
@@ -2701,13 +2701,49 @@ notenough:
 
 toomany:
 	if call != nil {
-		yyerror("too many arguments in call to %v", call)
+		yyerror("too many arguments in call to %v, got %s want %v", call, nl.retsigerr(), tstruct)
 	} else {
-		yyerror("too many arguments to %v", op)
+		yyerror("too many arguments to %v, got %s want %v", op, nl.retsigerr(), tstruct)
 	}
 	goto out
 }
 
+// sigrepr is a type's representation to the outside world,
+// in string representations of return signatures
+// e.g in error messages about wrong arguments to return.
+func (t *Type) sigrepr() string {
+	switch t {
+	default:
+		return t.String()
+
+	case Types[TIDEAL]:
+		// "untyped number" is not commonly used
+		// outside of the compiler, so let's use "number".
+		return "number"
+
+	case idealstring:
+		return "string"
+
+	case idealbool:
+		return "bool"
+	}
+}
+
+// retsigerr returns the signature of the types
+// at the respective return call site of a function.
+func (nl Nodes) retsigerr() string {
+	if nl.Len() < 1 {
+		return "()"
+	}
+
+	var typeStrings []string
+	for _, n := range nl.Slice() {
+		typeStrings = append(typeStrings, n.Type.sigrepr())
+	}
+
+	return fmt.Sprintf("(%s)", strings.Join(typeStrings, ", "))
+}
+
 // type check composite
 func fielddup(n *Node, hash map[string]bool) {
 	if n.Op != ONAME {

test/fixedbugs/issue4215.go

+// errorcheck
+
+// Copyright 2016 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 main
+
+func foo() (int, int) {
+	return 2.3 // ERROR "not enough arguments to return, got \(number\) want \(int, int\)"
+}
+
+func foo2() {
+	return int(2), 2 // ERROR "too many arguments to return, got \(int, number\) want \(\)"
+}
+
+func foo3(v int) (a, b, c, d int) {
+	if v >= 0 {
+		return 1 // ERROR "not enough arguments to return, got \(number\) want \(int, int, int, int\)"
+	}
+	return 2, 3 // ERROR "not enough arguments to return, got \(number, number\) want \(int, int, int, int\)"
+}
+
+func foo4(name string) (string, int) {
+	switch name {
+	case "cow":
+		return "moo" // ERROR "not enough arguments to return, got \(string\) want \(string, int\)"
+	case "dog":
+		return "dog", 10, true // ERROR "too many arguments to return, got \(string, number, bool\) want \(string, int\)"
+	case "fish":
+		return "" // ERROR "not enough arguments to return, got \(string\) want \(string, int\)"
+	default:
+		return "lizard", 10
+	}
+}
+
+type S int
+type T string
+type U float64
+
+func foo5() (S, T, U) {
+	if false {
+		return "" // ERROR "not enough arguments to return, got \(string\) want \(S, T, U\)"
+	} else {
+		ptr := new(T)
+		return ptr // ERROR "not enough arguments to return, got \(\*T\) want \(S, T, U\)"
+	}
+	return new(S), 12.34, 1 + 0i, 'r', true // ERROR "too many arguments to return, got \(\*S, number, number, number, bool\) want \(S, T, U\)"
+}
+
+func foo6() (T, string) {
+	return "T", true, true // ERROR "too many arguments to return, got \(string, bool, bool\) want \(T, string\)"
+}

test/fixedbugs/issue6750.go

+// errorcheck
+
+// Copyright 2016 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 main
+
+import "fmt"
+
+func printmany(nums ...int) {
+	for i, n := range nums {
+		fmt.Printf("%d: %d\n", i, n)
+	}
+	fmt.Printf("\n")
+}
+
+func main() {
+	printmany(1, 2, 3)
+	printmany([]int{1, 2, 3}...)
+	printmany(1, "abc", []int{2, 3}...) // ERROR "too many arguments in call to printmany, got \(number, string, \[\]int\) want \(...int\)"
+}