From 66c4ee168bc0a403a280ff7c35075376775516ce Mon Sep 17 00:00:00 2001
From: agra <alex@swipelab.co>
Date: Sat, 30 May 2026 03:15:07 +0300
Subject: [PATCH] lang F1 6: contextually type pack-fn prefix args (mapper
 lambda)

lowerPackFnCall lowered the runtime prefix args with no target_type, so a
lambda arg (mapper: Closure(...) -> ...) could not infer its param types.
Now set target_type to the param type while lowering each prefix arg. With
the existing value-projection call-arg spread, mapper(..sources.get) works:
the lambda is contextually typed and the projected values spread into the
call. examples/211 ((a,b)=>a+b over two sources -> 42). 246 + unit green.
---
 examples/211-mapper-projection-spread.sx       | 18 ++++++++++++++++++
 src/ir/lower.zig                               |  6 ++++++
 .../expected/211-mapper-projection-spread.exit |  1 +
 .../expected/211-mapper-projection-spread.txt  |  1 +
 4 files changed, 26 insertions(+)
 create mode 100644 examples/211-mapper-projection-spread.sx
 create mode 100644 tests/expected/211-mapper-projection-spread.exit
 create mode 100644 tests/expected/211-mapper-projection-spread.txt

diff --git a/examples/211-mapper-projection-spread.sx b/examples/211-mapper-projection-spread.sx
new file mode 100644
index 0000000..9121340
--- /dev/null
+++ b/examples/211-mapper-projection-spread.sx
@@ -0,0 +1,18 @@
+// Phase 6 — `mapper(..sources.value)`: project a method over a pack and spread
+// the results into a closure call. The mapper lambda's params are contextually
+// typed from the `Closure(...)` parameter even though `apply` is a pack-fn.
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+
+apply :: (mapper: Closure(s64, s64) -> s64, ..sources: VL) -> s64 {
+    return mapper(..sources.get);     // (a, b) => a + b  applied to (s0.get(), s1.get())
+}
+
+main :: () -> s32 {
+    print("{}\n", apply((a, b) => a + b, IntCell.{ v = 40 }, IntCell.{ v = 2 }));  // 42
+    0;
+}
diff --git a/src/ir/lower.zig b/src/ir/lower.zig
index cd2ac80..167d435 100644
--- a/src/ir/lower.zig
+++ b/src/ir/lower.zig
@@ -9195,7 +9195,13 @@ pub const Lowering = struct {
                 if (isPackParam(p)) break;
                 if (ri >= call_node.args.len) break;
                 if (!p.is_comptime) {
+                    // Contextually type the arg from the param (so a lambda arg
+                    // `(x) => …` takes its param types from a `Closure(...)` param).
+                    const saved_tt = self.target_type;
+                    const pty = self.resolveParamType(&p);
+                    if (pty != .unresolved) self.target_type = pty;
                     args.append(self.alloc, self.lowerExpr(call_node.args[ri])) catch return self.builder.constInt(0, .void);
+                    self.target_type = saved_tt;
                 }
                 ri += 1;
             }
diff --git a/tests/expected/211-mapper-projection-spread.exit b/tests/expected/211-mapper-projection-spread.exit
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/tests/expected/211-mapper-projection-spread.exit
@@ -0,0 +1 @@
+0
diff --git a/tests/expected/211-mapper-projection-spread.txt b/tests/expected/211-mapper-projection-spread.txt
new file mode 100644
index 0000000..d81cc07
--- /dev/null
+++ b/tests/expected/211-mapper-projection-spread.txt
@@ -0,0 +1 @@
+42