InternPool: handle funcZirBodyInst for func_coerced

Closes #18039

src/InternPool.zig

@@ -8301,6 +8301,13 @@ pub fn funcZirBodyInst(ip: *const InternPool, i: Index) Zir.Inst.Index {
             assert(ip.items.items(.tag)[func_decl_index] == .func_decl);
             break :b ip.items.items(.data)[func_decl_index] + zir_body_inst_field_index;
         },
+        .func_coerced => {
+            const datas = ip.items.items(.data);
+            const uncoerced_func_index: Index = @enumFromInt(ip.extra.items[
+                datas[@intFromEnum(i)] + std.meta.fieldIndex(Tag.FuncCoerced, "func").?
+            ]);
+            return ip.funcZirBodyInst(uncoerced_func_index);
+        },
         else => unreachable,
     };
     return @enumFromInt(ip.extra.items[extra_index]);

test/behavior/call.zig

@@ -499,3 +499,24 @@ test "call inline fn through pointer" {
     const f = &S.foo;
     try f(123);
 }
+
+test "call coerced function" {
+    const T = struct {
+        x: f64,
+        const T = @This();
+        usingnamespace Implement(1);
+        const F = fn (comptime f64) type;
+        const Implement: F = opaque {
+            fn implementer(comptime val: anytype) type {
+                return opaque {
+                    fn incr(self: T) T {
+                        return .{ .x = self.x + val };
+                    }
+                };
+            }
+        }.implementer;
+    };
+
+    const a = T{ .x = 3 };
+    try std.testing.expect(a.incr().x == 4);
+}