glsl2: Check that returned expressions match the function return type.

From my reading of the specification, implicit conversions are not
allowed.  ATI seems to agree, though nVidia allows it without warning.

src/glsl/ast_to_hir.cpp

@@ -2151,9 +2151,17 @@ ast_jump_statement::hir(exec_list *instructions,
 	    opt_return_value->hir(instructions, state);
 	 assert(ret != NULL);
 
-	 /* FINISHME: Make sure the type of the return value matches the return
-	  * FINISHME: type of the enclosing function.
-	  */
+	 /* Implicit conversions are not allowed for return values. */
+	 if (state->current_function->return_type != ret->type) {
+	    YYLTYPE loc = this->get_location();
+
+	    _mesa_glsl_error(& loc, state,
+			     "`return' with wrong type %s, in function `%s' "
+			     "returning %s",
+			     ret->type->name,
+			     state->current_function->function_name(),
+			     state->current_function->return_type->name);
+	 }
 
 	 inst = new(ctx) ir_return(ret);
       } else {