better error; clean up lineno in a few places

wreck.mtv=; cat x.go
package main
var x = string.Split()
wreck.mtv=; 6g x.go
x.go:2: type string used as expression
x.go:2: undefined DOT Split on string
x.go:3: illegal types for operand: AS
	undefined
wreck.mtv=;

BUG=1938751
R=ken
OCL=30766
CL=30766

src/cmd/gc/const.c

@@ -84,7 +84,7 @@ convlit1(Node *n, Type *t, int explicit)
 		return;
 	}
 	// avoided repeated calculations, errors
-	if(cvttype(n->type, t)) {
+	if(cvttype(n->type, t) == 1) {
 		n->type = t;
 		return;
 	}
@@ -603,8 +603,7 @@ ret:
 	n->val = v;
 
 	// check range.
-	lno = lineno;
-	lineno = n->lineno;
+	lno = setlineno(n);
 	overflow(v, n->type);
 	lineno = lno;
 

src/cmd/gc/subr.c

@@ -106,7 +106,14 @@ setlineno(Node *n)
 	int32 lno;
 
 	lno = lineno;
-	if(n != N && n->op != ONAME) {
+	if(n != N)
+	switch(n->op) {
+	case ONAME:
+	case OTYPE:
+	case OPACK:
+	case OLITERAL:
+		break;
+	default:
 		lineno = n->lineno;
 		if(lineno == 0) {
 			if(debug['K'])

src/cmd/gc/walk.c

@@ -123,15 +123,7 @@ loop:
 		return;
 
 	more = N;
-	switch(n->op) {
-	case ONAME:	// one only; lineno isn't right for right now
-	case OPACK:
-	case OTYPE:
-	case OLITERAL:
-		break;
-	default:
-		lineno = n->lineno;
-	}
+	setlineno(n);
 
 	switch(n->op) {
 
@@ -253,6 +245,13 @@ loop:
 		fatal("walktype: switch 1 unknown op %N", n);
 		goto ret;
 
+	case OTYPE:
+		if(!n->diag) {
+			n->diag = 1;
+			yyerror("type %T used as expression", n->type);
+		}
+		goto ret;
+
 	case ODCL:
 		goto ret;
 
@@ -1799,8 +1798,12 @@ walkdot(Node *n)
 		n->op = ODOTPTR;
 	}
 
-	if(!lookdot(n, t))
+	if(!lookdot(n, t)) {
+		if(!n->diag) {
+			n->diag = 1;
 			yyerror("undefined DOT %S on %T", n->right->sym, n->left->type);
+		}
+	}
 }
 
 Node*