internal/shader: Make function duplications error

Closes #1430

internal/shader/shader.go

@@ -235,6 +235,13 @@ func (cs *compileState) parse(f *ast.File) {
 			continue
 		}
 
+		for _, f := range cs.funcs {
+			if f.name == n {
+				cs.addError(d.Pos(), fmt.Sprintf("redeclared function: %s", n))
+				return
+			}
+		}
+
 		inParams, outParams := cs.parseFuncParams(&cs.global, fd)
 		var inT, outT []shaderir.Type
 		for _, v := range inParams {

shader_test.go

@@ -251,6 +251,23 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	}
 }
 
+func TestShaderDuplicatedFunctions(t *testing.T) {
+	if _, err := NewShader([]byte(`package main
+
+func Foo() {
+}
+
+func Foo() {
+}
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	return vec4(0)
+}
+`)); err == nil {
+		t.Errorf("error must be non-nil but was nil")
+	}
+}
+
 func TestShaderNoMain(t *testing.T) {
 	if _, err := NewShader([]byte(`package main
 `)); err == nil {