gc: more crash avoidance

Fixes #961.
Fixes #962.

R=ken2
CC=golang-dev
https://golang.org/cl/1903043

src/cmd/gc/const.c

@@ -536,6 +536,12 @@ evconst(Node *n)
 		v = toflt(v);
 		rv = toflt(rv);
 	}
+	if(v.ctype != rv.ctype) {
+		// Use of undefined name as constant?
+		if((v.ctype == 0 || rv.ctype == 0) && nerrors > 0)
+			return;
+		fatal("constant type mismatch %T(%d) %T(%d)", nl->type, v.ctype, nr->type, rv.ctype);
+	}
 
 	// run op
 	switch(TUP(n->op, v.ctype)) {

src/cmd/gc/typecheck.c

@@ -1129,6 +1129,10 @@ reswitch:
 	case ORETURN:
 		ok |= Etop;
 		typechecklist(n->list, Erv | Efnstruct);
+		if(curfn == N) {
+			yyerror("return outside function");
+			goto error;
+		}
 		if(curfn->type->outnamed && n->list == nil)
 			goto ret;
 		typecheckaste(ORETURN, getoutargx(curfn->type), n->list, "return argument");

test/fixedbugs/bug297.go

+// errchk $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.
+
+// Used to crash; issue 961.
+
+package main
+
+type ByteSize float64
+const (
+	_ = iota;   // ignore first value by assigning to blank identifier
+	KB ByteSize = 1<<(10*X) // ERROR "undefined"
+)

test/fixedbugs/bug298.go

+// errchk $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.
+
+package ddd
+
+func Sum() int
+	for i := range []int{} { return i }  // ERROR "return outside function"
+