gc: Better typechecks and errors in switches.

Allow any type in switch on interface value.
Statically check typeswitch early.

Fixes #2423.
Fixes #2424.

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/5339045

src/cmd/gc/swt.c

@@ -810,8 +810,8 @@ walkswitch(Node *sw)
 void
 typecheckswitch(Node *n)
 {
-	int top, lno;
+	int top, lno, ptr;
-	Type *t;
+	Type *t, *missing, *have;
 	NodeList *l, *ll;
 	Node *ncase, *nvar;
 	Node *def;
@@ -854,21 +854,35 @@ typecheckswitch(Node *n)
 				typecheck(&ll->n, Erv | Etype);
 				if(ll->n->type == T || t == T)
 					continue;
+				setlineno(ncase);
 				switch(top) {
 				case Erv:	// expression switch
 					defaultlit(&ll->n, t);
 					if(ll->n->op == OTYPE)
 						yyerror("type %T is not an expression", ll->n->type);
-					else if(ll->n->type != T && !eqtype(ll->n->type, t))
+					else if(ll->n->type != T && !assignop(ll->n->type, t, nil) && !assignop(t, ll->n->type, nil)) {
-						yyerror("case %lN in %T switch", ll->n, t);
+						if(n->ntest)
+							yyerror("invalid case %N in switch on %N (mismatched types %T and %T)", ll->n, n->ntest, ll->n->type, t);
+						else
+							yyerror("invalid case %N in switch (mismatched types %T and bool)", ll->n, n->ntest, ll->n->type, t);
+					}
 					break;
 				case Etype:	// type switch
 					if(ll->n->op == OLITERAL && istype(ll->n->type, TNIL)) {
 						;
-					} else if(ll->n->op != OTYPE && ll->n->type != T) {
+					} else if(ll->n->op != OTYPE && ll->n->type != T) {  // should this be ||?
 						yyerror("%lN is not a type", ll->n);
 						// reset to original type
 						ll->n = n->ntest->right;
+					} else if(!implements(ll->n->type, t, &missing, &have, &ptr)) {
+						if(have && !missing->broke && !have->broke)
+							yyerror("impossible type switch case: %lN cannot have dynamic type %T"
+								" (wrong type for %S method)\n\thave %S%hT\n\twant %S%hT",
+								n->ntest->right, ll->n->type, missing->sym, have->sym, have->type,
+								missing->sym, missing->type);
+						else if(!missing->broke)
+							yyerror("impossible type switch case: %lN cannot have dynamic type %T"
+								" (missing %S method)", n->ntest->right, ll->n->type, missing->sym);
 					}
 					break;
 				}

test/fixedbugs/bug270.go

@@ -1,21 +0,0 @@
-// $G $D/$F.go
-
-// Copyright 2010 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.
-
-// http://code.google.com/p/go/issues/detail?id=746
-
-package main
-
-type I interface { F() }
-
-type T struct{}
-
-func (T) F() {}
-
-func main() {
-	switch I(T{}).(type) {
-	case interface{}:
-	}
-}

test/fixedbugs/bug340.go

@@ -10,8 +10,8 @@ package main
 
 func main() {
 	var x interface{}
-	switch t := x.(type) { // GC_ERROR "is not a type"
+	switch t := x.(type) {
-	case 0:		// GCCGO_ERROR "expected type"
+	case 0:		// ERROR "type"
 		t.x = 1 // ERROR "type interface \{\}|reference to undefined field or method"
 	}
 }

test/fixedbugs/bug375.go

@@ -0,0 +1,19 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug375
+
+// Copyright 2011 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.
+
+// Issue 2423
+
+package main
+
+func main() {
+	var x interface{} = "hello"
+
+	switch x {
+	case "hello":
+	default:
+		println("FAIL")
+	}
+}

test/switch3.go

@@ -0,0 +1,38 @@
+// errchk $G -e $D/$F.go
+
+// Copyright 2011 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
+
+
+type I interface {
+       M()
+}
+
+func bad() {
+	var i I
+	var s string
+
+	switch i {
+	case s:  // ERROR "mismatched types string and I"
+	}
+
+	switch s {
+	case i:  // ERROR "mismatched types I and string"
+	}
+}
+
+func good() {
+	var i interface{}
+	var s string
+
+	switch i {
+	case s:
+	}
+
+	switch s {
+	case i:
+	}
+}

test/typeswitch3.go

@@ -0,0 +1,20 @@
+// errchk $G -e $D/$F.go
+
+// Copyright 2011 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
+
+
+type I interface {
+       M()
+}
+
+func main(){
+       var x I
+       switch x.(type) {
+       case string:	// ERROR "impossible"
+               println("FAIL")
+       }
+}