cmd/compile: use more specific error message for assignment mismatch

Show a more specifc error message in the form of "%d variables but %v
returns %d values" if an assignment mismatch occurs with a function
or method call on the right.

Fixes #27595

Change-Id: Ibc97d070662b08f150ac22d686059cf224e012ab
Reviewed-on: https://go-review.googlesource.com/135575
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -3341,7 +3341,7 @@ func typecheckas(n *Node) {
 	checkassign(n, n.Left)
 	if n.Right != nil && n.Right.Type != nil {
 		if n.Right.Type.IsFuncArgStruct() {
-			yyerror("assignment mismatch: 1 variable but %d values", n.Right.Type.NumFields())
+			yyerror("assignment mismatch: 1 variable but %v returns %d values", n.Right.Left, n.Right.Type.NumFields())
 			// Multi-value RHS isn't actually valid for OAS; nil out
 			// to indicate failed typechecking.
 			n.Right.Type = nil
@@ -3486,7 +3486,12 @@ func typecheckas2(n *Node) {
 	}
 
 mismatch:
-	yyerror("assignment mismatch: %d variables but %d values", cl, cr)
+	switch r.Op {
+	default:
+		yyerror("assignment mismatch: %d variable but %d values", cl, cr)
+	case OCALLFUNC, OCALLMETH, OCALLINTER:
+		yyerror("assignment mismatch: %d variables but %v returns %d values", cl, r.Left, cr)
+	}
 
 	// second half of dance
 out:

test/fixedbugs/issue26616.go

@@ -6,13 +6,13 @@
 
 package p
 
-var x int = three() // ERROR "1 variable but 3 values"
+var x int = three() // ERROR "assignment mismatch: 1 variable but three returns 3 values"
 
 func f() {
-	var _ int = three() // ERROR "1 variable but 3 values"
-	var a int = three() // ERROR "1 variable but 3 values"
-	a = three()         // ERROR "1 variable but 3 values"
-	b := three()        // ERROR "1 variable but 3 values"
+	var _ int = three() // ERROR "assignment mismatch: 1 variable but three returns 3 values"
+	var a int = three() // ERROR "assignment mismatch: 1 variable but three returns 3 values"
+	a = three()         // ERROR "assignment mismatch: 1 variable but three returns 3 values"
+	b := three()        // ERROR "assignment mismatch: 1 variable but three returns 3 values"
 
 	_, _ = a, b
 }

test/fixedbugs/issue27595.go

+// errorcheck
+
+// Copyright 2018 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
+
+var a = twoResults()       // ERROR "assignment mismatch: 1 variable but twoResults returns 2 values"
+var b, c, d = twoResults() // ERROR "assignment mismatch: 3 variables but twoResults returns 2 values"
+var e, f = oneResult()     // ERROR "assignment mismatch: 2 variables but oneResult returns 1 values"
+
+func twoResults() (int, int) {
+	return 1, 2
+}
+
+func oneResult() int {
+	return 1
+}