internal/shader: bug fix: an index must be a constant for vectors

Updates #3011

internal/shader/expr.go

@@ -1150,6 +1150,11 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 		x := exprs[0]
 		t := ts[0]
 
+		if (t.IsFloatVector() || t.IsIntVector()) && idx.Const == nil {
+			cs.addError(e.Pos(), fmt.Sprintf("index must be a constant for the type %s", t.String()))
+			return nil, nil, nil, false
+		}
+
 		var typ shaderir.Type
 		switch t.Main {
 		case shaderir.Vec2, shaderir.Vec3, shaderir.Vec4:

internal/shader/syntax_test.go

@@ -4363,3 +4363,47 @@ func Foo() int {
 		t.Error(err)
 	}
 }
+
+func TestSyntaxIndex(t *testing.T) {
+	// Issue #3011
+	if _, err := compileToIR([]byte(`package main
+
+func Foo() int {
+	var a int
+	var b ivec4
+	return b[a]
+}
+`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+	if _, err := compileToIR([]byte(`package main
+
+func Foo() float {
+	var a int
+	var b mat4
+	return b[a][0]
+}
+`)); err != nil {
+		t.Error(err)
+	}
+	if _, err := compileToIR([]byte(`package main
+
+func Foo() int {
+	const a = 0
+	var b ivec4
+	return b[a]
+}
+`)); err != nil {
+		t.Error(err)
+	}
+	if _, err := compileToIR([]byte(`package main
+
+func Foo() float {
+	const a = 0
+	var b mat4
+	return b[a][0]
+}
+`)); err != nil {
+		t.Error(err)
+	}
+}