typecheck: bare non-negative integer literals are context-polymorphic

NOW.md

@@ -2,6 +2,13 @@
 
 Last updated: 2026-08-08
 
+## typecheck: bare non-negative literals are context-polymorphic (Closes #1923)
+
+- infer_expr pinned every integer literal to I32; in u32-dominant specs every 'let x = 0; x = u32_expr;' raised a false 'cannot assign U32 to I32' -- 27 tri-net specs failed typecheck on exactly this
+- Now: bare non-negative literals infer Unknown (context-polymorphic); explicitly negative literals stay I32; promote_types resolves Unknown+Known to the KNOWN operand ('100 - value' with value: u32 is u32)
+- tri-net sweep: 27 failing specs -> 2 (both 'assign to immutable array element' singletons, tracked with #1919)
+- Unit suite 1537/1537; FROZEN_HASH resealed
+
 ## typecheck: promote call-arity mismatch from warning to hard error (Closes #1921)
 
 - The arity check existed but only warned, and nothing reads warnings (tri-net's hook greps 'Typecheck OK'; gen paths skip typecheck) -- two tri-net specs shipped wrong-arity calls for months (tri-net#323)

bootstrap/src/compiler.rs

@@ -12794,7 +12794,15 @@ fn infer_expr(node: &Node, symbols: &[SymbolEntry], fns: &[FnEntry]) -> TypeInfo
                 return TypeInfo::Str;
             }
             if node.value.parse::<i64>().is_ok() {
-                return TypeInfo::I32;
+                // A bare NON-NEGATIVE integer literal is context-polymorphic:
+                // t27 specs are u32-dominant, and pinning literals to I32 made
+                // every `let x = 0; ... x = u32_expr;` a false type error
+                // (27 tri-net specs failed typecheck on exactly this). Only an
+                // explicitly negative literal commits to a signed type.
+                if node.value.trim_start().starts_with('-') {
+                    return TypeInfo::I32;
+                }
+                return TypeInfo::Unknown;
             }
             if node.value.parse::<f64>().is_ok() {
                 return TypeInfo::F64;
@@ -12851,8 +12859,13 @@ fn promote_types(a: &TypeInfo, b: &TypeInfo) -> TypeInfo {
     if a == b {
         return a.clone();
     }
-    if *a == TypeInfo::Unknown || *b == TypeInfo::Unknown {
-        return TypeInfo::Unknown;
+    // The KNOWN operand determines the type: `100 - value` where value: u32
+    // is a u32 expression, not an unknown one.
+    if *a == TypeInfo::Unknown {
+        return b.clone();
+    }
+    if *b == TypeInfo::Unknown {
+        return a.clone();
     }
     let a_rank = type_rank(a);
     let b_rank = type_rank(b);

bootstrap/stage0/FROZEN_HASH

@@ -1 +1 @@
-8770de8b22dfda24d5c4099b2610c4a5a6efd49e43a19e335f08eeb09f788743
+37b349425f4d6551c495b3357d229b5a6fb2acf6c55712af4d4ecf5271ec5f53

docs/NOW.md

@@ -1,7 +1,14 @@
-# NOW — typecheck: arity mismatch is a hard error (2026-08-08)
+# NOW — typecheck: context-polymorphic integer literals (2026-08-08)
 
 Last updated: 2026-08-08
 
+## typecheck: bare non-negative literals are context-polymorphic (Closes #1923)
+
+- infer_expr pinned every integer literal to I32; in u32-dominant specs every 'let x = 0; x = u32_expr;' raised a false 'cannot assign U32 to I32' -- 27 tri-net specs failed typecheck on exactly this
+- Now: bare non-negative literals infer Unknown (context-polymorphic); explicitly negative literals stay I32; promote_types resolves Unknown+Known to the KNOWN operand ('100 - value' with value: u32 is u32)
+- tri-net sweep: 27 failing specs -> 2 (both 'assign to immutable array element' singletons, tracked with #1919)
+- Unit suite 1537/1537; FROZEN_HASH resealed
+
 ## typecheck: promote call-arity mismatch from warning to hard error (Closes #1921)
 
 - The arity check existed but only warned, and nothing reads warnings (tri-net's hook greps 'Typecheck OK'; gen paths skip typecheck) -- two tri-net specs shipped wrong-arity calls for months (tri-net#323)