cmd/compile: better error message for assignment mismatches

Keep left-to-right order when referring to the number of
variables and values involved.

Fixes #22159.

Change-Id: Iccca12d3222f9d5e049939a9ccec07513c393faa
Reviewed-on: https://go-review.googlesource.com/68690
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -3426,7 +3426,7 @@ func typecheckas2(n *Node) {
 	}
 
 mismatch:
-	yyerror("cannot assign %d values to %d variables", cr, cl)
+	yyerror("assignment mismatch: %d variables but %d values", cl, cr)
 
 	// second half of dance
 out:

test/fixedbugs/bug289.go

@@ -9,14 +9,14 @@
 package main
 
 func f1() {
-	a, b := f()	// ERROR "cannot assign|does not match"
+	a, b := f()	// ERROR "assignment mismatch|does not match"
 	_ = a
 	_ = b
 }
 
 func f2() {
 	var a, b int
-	a, b = f()	// ERROR "cannot assign|does not match"
+	a, b = f()	// ERROR "assignment mismatch|does not match"
 	_ = a
 	_ = b
 }

test/fixedbugs/bug487.go

@@ -14,8 +14,8 @@ func G() (int, int, int) {
 }
 
 func F() {
-	a, b := G()	// ERROR "cannot assign"
-	a, b = G()	// ERROR "cannot assign"
+	a, b := G()	// ERROR "assignment mismatch"
+	a, b = G()	// ERROR "assignment mismatch"
 	_, _ = a, b
 }