diff --git a/Makefile b/Makefile
index 664ff72..a769559 100644
--- a/Makefile
+++ b/Makefile
@@ -99,7 +99,7 @@ sextest:
 SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features \
                     feature-flags lambdas compound-literals closures fixpoint \
                     wildcards inference type-shapes unnamed-params \
-                    closure-signatures
+                    closure-signatures c99
 
 # Multi-module linking is checked end to end; see tests/modules/Makefile.
 check-modules: sexc
diff --git a/infer.scm b/infer.scm
index 64b19d5..3ba37b0 100644
--- a/infer.scm
+++ b/infer.scm
@@ -509,10 +509,12 @@
      ((eq? a b) #t)
      ((unknown-type? a) #t)
      ((unknown-type? b) #t)
-     ((and (tvar? a) (tvar? b) (tvar-rigid? b) (not (tvar-rigid? a)))
-      (bind-tvar! a b form))
-     ((tvar? a) (bind-tvar! a b form))
-     ((tvar? b) (bind-tvar! b a form))
+     ;; Whichever side is free takes the binding, so that a rigid
+     ;; variable is solved *to* rather than solved, in either order.
+     ;; Both rigid and distinct is the mismatch `eq?' above let through.
+     ((and (tvar? a) (not (tvar-rigid? a))) (bind-tvar! a b form))
+     ((and (tvar? b) (not (tvar-rigid? b))) (bind-tvar! b a form))
+     ((or (tvar? a) (tvar? b)) (type-mismatch a b form))
      ;; A typedef unifies as what it stands for. Its name survives in
      ;; whichever side is printed later, since neither side is rebuilt.
      ((alias-type? a) (unify (alias-expansion a) b form))
diff --git a/semen.scm b/semen.scm
index c547e25..c47866f 100644
--- a/semen.scm
+++ b/semen.scm
@@ -693,13 +693,19 @@
 
 (define +closure-env-bytes+ 16)
 
-;;; +closure-env-bytes+ for maximum capacity, max-align-t for
-;;; effectiveness, hence union
+;;; +closure-env-bytes+ for maximum capacity, and an alignment wide
+;;; enough for anything that fits in them, hence union. `max_align_t'
+;;; would say that in one word, but it is C11 and the target is C99, so
+;;; the widest built-ins say it instead: a union is aligned for the
+;;; strictest of its members.
 (define +closure-env-type+ 'ƛenv)
 
 (define (closure-env-declaration)
-  `(union ,+closure-env-type+ ((align max-align-t)
-                               (bytes (¤ char ,+closure-env-bytes+)))))
+  `(union ,+closure-env-type+ ((bytes (¤ char ,+closure-env-bytes+))
+                               (align-integer (long long))
+                               (align-real (long double))
+                               (align-pointer (* void))
+                               (align-code (fn ((* void)) void)))))
 
 (define +closure-structs+ (make-hash-table))
 (define +closure-forwards+ (make-hash-table))
diff --git a/tests/infer.scm b/tests/infer.scm
index 587fe1e..2d13c64 100644
--- a/tests/infer.scm
+++ b/tests/infer.scm
@@ -183,7 +183,17 @@
         (test-assert "an ordinary variable binds to it instead"
                      (unify a r #f))
         ;; An unsolved variable resolves to itself.
-        (test-assert "it is still open" (tvar? (resolve r))))))
+        (test-assert "it is still open" (tvar? (resolve r))))
+      ;; ...and the same the other way round: it is which side is free
+      ;; that decides, not which side was written first.
+      (let ((r (fresh-rigid-tvar))
+            (a (fresh-tvar)))
+        (test-assert "rigid first binds the free one" (unify r a #f))
+        (test-assert "to the parameter itself" (eq? r (resolve a))))
+      (let ((r1 (fresh-rigid-tvar))
+            (r2 (fresh-rigid-tvar)))
+        (test-error "two parameters do not unify with each other"
+                    (unify r1 r2 #f)))))
 
   (test-group "constraints"
     (test #t (entails? 'numeric (parse-type 'int)))
diff --git a/tests/sex-programs/c99.sex b/tests/sex-programs/c99.sex
new file mode 100644
index 0000000..f65cd37
--- /dev/null
+++ b/tests/sex-programs/c99.sex
@@ -0,0 +1,28 @@
+(compilation "-- -std=c99 -pedantic-errors")
+(input)
+(output "c99: 42")
+(return 0)
+
+;;; The closure environment is part of the ABI, so its union is declared
+;;; in every translation unit whether or not one is used. That put
+;;; whatever it was written with into every program: `max_align_t' named
+;;; the alignment in one word, and made C11 the floor for a program with
+;;; no closure in it at all.
+;;;
+;;; The widest built-ins say the same thing -- a union is aligned for the
+;;; strictest of its members -- and say it in C99.
+;;;
+;;; Closures themselves still want C11 for the `_Static_assert' that
+;;; checks the captures fit, so this program keeps clear of them.
+
+(include stdio.h)
+
+(struct point ((x int) (y int)))
+
+(fn area ((p (struct point))) int
+  (return (* (. p x) (. p y))))
+
+(pub fn main () int
+  (var p (struct point) #((struct point) : 6 7))
+  (printf "c99: %d\n" (area p))
+  (return 0))