lang: funcs: Catch non-specific type build error

If you had ambiguous code, and specified an invalid type, this could
sneak through and become a runtime error, instead of a compile-time
error. We fix this and add a test.

lang/funcs/struct_lookup_func.go

@@ -193,6 +193,14 @@ func (obj *StructLookupFunc) Build(typ *types.Type) (*types.Type, error) {
 	if ix == -1 {
 		return nil, fmt.Errorf("field %s was not found in struct", obj.field)
 	}
+	tF, exists := tStruct.Map[tStruct.Ord[ix]]
+	if !exists {
+		return nil, fmt.Errorf("field %s was not found in struct", obj.field)
+	}
+	// The return value must match the type of the field we're pulling out!
+	if err := typ.Out.Cmp(tF); err != nil {
+		return nil, fmt.Errorf("field %s type error: %+v", obj.field, err)
+	}
 
 	obj.Type = tStruct // struct type
 	obj.Out = typ.Out  // type of return value

lang/interpret_test/TestAstFunc2/unify-template.txtar

@@ -0,0 +1,13 @@
+-- main.mcl --
+import "sys"
+
+$tmplvalues = struct{num => 42, load => $theload,}
+
+$theload bool = sys.load()->x1 # wrong type, make sure the compiler catches it!
+
+file "/tmp/datetime" {
+	state => $const.res.file.state.exists,
+	content => template("num: {{ .num }} seconds\nload average: {{ .load }}\n", $tmplvalues),
+}
+-- OUTPUT --
+# err: errUnify: error setting type: func() { <built-in:_struct_lookup> }, error: field x1 type error: base kind does not match (bool != float)