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Fix up type merging and improve error messages a bit.

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -27,6 +27,7 @@

 /* find the most accurate type mapping */

 static Type *tf(Type *t)

 {

+    char buf[1024];

     Type *lu;

     assert(t != NULL);

@@ -37,7 +38,7 @@

             tytab[t->tid] = lu;

         }

-        printf("%s => ", tystr(t));

+        printf("%s => ", tyfmt(buf, 1024, t));

         if (!tytab[t->tid])

             break;

         t = tytab[t->tid];

@@ -141,10 +142,13 @@

 static void matchcstrs(Node *ctx, Type *a, Type *b)

 {

     if (b->type == Tyvar) {

-        if (!b->cstrs)

-            b->cstrs = dupbs(a->cstrs);

-        else if (a->cstrs)

+        /* make sure that if a = b, both have same cstrs */

+        if (a->cstrs && b->cstrs)

             bsunion(b->cstrs, a->cstrs);

+        else if (a->cstrs)

+            b->cstrs = dupbs(a->cstrs);

+        else if (b->cstrs)

+            a->cstrs = dupbs(b->cstrs);

     } else {

         if (!cstrcheck(a, b))

             fatal(ctx->line, "%s incompatible with %s near %s", tystr(a), tystr(b), ctxstr(ctx));

@@ -171,7 +175,6 @@

         b = t;

     }

-    breakhere();

     matchcstrs(ctx, a, b);

     if (a->type != b->type) {

         if (a->type == Tyvar)

@@ -404,10 +407,11 @@

 /* returns the final type for t, after all unifications

  * and default constraint selections */

-static Type *tyfin(Type *t)

+static Type *tyfin(Node *ctx, Type *t)

 {

     static Type *tyint;

     int i;

+    //char buf[1024];

     if (!tyint)

         tyint = mkty(-1, Tyint);

@@ -418,8 +422,10 @@

             return tyint;

     } else {

         for (i = 0; i < t->nsub; i++)

-            t->sub[i] = tyfin(t->sub[i]);

+            t->sub[i] = tyfin(ctx, t->sub[i]);

     }

+    //if (t->type == Tyvar)

+        // fatal(t->line, "underconstrained type %s near %s", tyfmt(buf, 1024, t), ctxstr(ctx));

     return t;

 }

@@ -433,7 +439,7 @@

                 typesub(n->file.stmts[i]);

             break;

         case Ndecl:

-            settype(n, tyfin(type(n)));

+            settype(n, tyfin(n, type(n)));

             if (n->decl.init)

                 typesub(n->decl.init);

             break;

@@ -453,18 +459,18 @@

             typesub(n->loopstmt.body);

             break;

         case Nexpr:

-            settype(n, tyfin(type(n)));

+            settype(n, tyfin(n, type(n)));

             for (i = 0; i < n->expr.nargs; i++)

                 typesub(n->expr.args[i]);

             break;

         case Nfunc:

-            settype(n, tyfin(n->func.type));

+            settype(n, tyfin(n, n->func.type));

             for (i = 0; i < n->func.nargs; i++)

                 typesub(n->func.args[i]);

             typesub(n->func.body);

             break;

         case Nlit:

-            settype(n, tyfin(type(n)));

+            settype(n, tyfin(n, type(n)));

             switch (n->lit.littype) {

                 case Lfunc:     typesub(n->lit.fnval); break;

                 case Larray:    typesub(n->lit.arrval); break;

--- a/parse/parse.h

+++ b/parse/parse.h

@@ -93,6 +93,7 @@

 struct Type {

     Ty type;

     int tid;

+    int line;

     Bitset *cstrs;    /* the type constraints matched on this type */

     size_t nsub;      /* For fnsub, tusub, sdecls, udecls, edecls. */

     Type **sub;       /* sub-types; shared by all composite types */

--- a/parse/type.c

+++ b/parse/type.c

@@ -50,6 +50,7 @@

     t = zalloc(sizeof(Type));

     t->type = ty;

     t->tid = ntypes++;

+    t->line = line;

     tytab = xrealloc(tytab, ntypes*sizeof(Type*));

     tytab[t->tid] = NULL;