From fd854d70d81fb2e1922d799dd31f8fdc44dc19ea Mon Sep 17 00:00:00 2001
From: Khaled Hosny <khaled@aliftype.com>
Date: Sat, 27 May 2023 02:50:15 +0300
Subject: [PATCH] =?UTF-8?q?[feaLib]=20Simplify=20variable=20scalars=20that?=
 =?UTF-8?q?=20don=E2=80=99t=20vary?=
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If a variable scalar does not vary (i.e. all values are the same), we
can simplify it and use a simple value instead. Arguably users shouldn’t
be using variable scalar in this case, but it helps when the feature
code is auto generated so each feaLib users doesn’t have to do the
check done here themselves.
---
 Lib/fontTools/feaLib/builder.py                   | 3 +++
 Tests/feaLib/data/variable_scalar_valuerecord.fea | 1 +
 Tests/feaLib/data/variable_scalar_valuerecord.ttx | 9 ++++++++-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/Lib/fontTools/feaLib/builder.py b/Lib/fontTools/feaLib/builder.py
index 6eaff843a..e0e102864 100644
--- a/Lib/fontTools/feaLib/builder.py
+++ b/Lib/fontTools/feaLib/builder.py
@@ -1613,6 +1613,9 @@ class Builder(object):
             )
 
         varscalar.axes = self.axes
+        if not varscalar.does_vary:
+            return varscalar.default, None
+
         default, index = varscalar.add_to_variation_store(
             self.varstorebuilder, self.model_cache, self.font.get("avar")
         )
diff --git a/Tests/feaLib/data/variable_scalar_valuerecord.fea b/Tests/feaLib/data/variable_scalar_valuerecord.fea
index bf9a26b7d..0b4026544 100644
--- a/Tests/feaLib/data/variable_scalar_valuerecord.fea
+++ b/Tests/feaLib/data/variable_scalar_valuerecord.fea
@@ -2,4 +2,5 @@ languagesystem DFLT dflt;
 feature kern {
     pos one 1;
     pos two <0 (wght=200:12 wght=900:22 wdth=150,wght=900:42) 0 0>;
+    pos three <0 (wght=200:12 wght=900:12 wdth=150,wght=900:12) 0 0>;
 } kern;
diff --git a/Tests/feaLib/data/variable_scalar_valuerecord.ttx b/Tests/feaLib/data/variable_scalar_valuerecord.ttx
index 338b72213..e3251f691 100644
--- a/Tests/feaLib/data/variable_scalar_valuerecord.ttx
+++ b/Tests/feaLib/data/variable_scalar_valuerecord.ttx
@@ -76,7 +76,7 @@
       <Lookup index="0">
         <LookupType value="1"/>
         <LookupFlag value="0"/>
-        <!-- SubTableCount=2 -->
+        <!-- SubTableCount=3 -->
         <SinglePos index="0" Format="1">
           <Coverage>
             <Glyph value="one"/>
@@ -97,6 +97,13 @@
             </YPlaDevice>
           </Value>
         </SinglePos>
+        <SinglePos index="2" Format="1">
+          <Coverage>
+            <Glyph value="three"/>
+          </Coverage>
+          <ValueFormat value="2"/>
+          <Value YPlacement="12"/>
+        </SinglePos>
       </Lookup>
     </LookupList>
   </GPOS>