lang: funcs: core: fmt: Fix printf unification bug

We also add a backup fix to avoid a panic in case we ever hit a new
unification bug that lets something through, we can at least turn it
into a runtime issue. This adds a test as well.

lang/funcs/core/fmt/printf_func.go

@@ -199,6 +199,11 @@ func (obj *PrintfFunc) Unify(expr interfaces.Expr) ([]interfaces.Invariant, erro
 					invariants = append(invariants, invar)
 				}
 
+				// catch situations like `printf("%d%d", 42)`
+				if len(cfavInvar.Args) <= i+1 {
+					return nil, fmt.Errorf("more specifiers (%d) than values (%d)", len(typList), len(cfavInvar.Args)-1)
+				}
+
 				// add the relationships to the called args
 				invar = &interfaces.EqualityInvariant{
 					Expr1: cfavInvar.Args[i+1],
@@ -557,6 +562,11 @@ func compileFormatToString(format string, values []types.Value) (string, error)
 		}
 		inType = false // done
 
+		if ix >= len(values) {
+			// programming error, probably in type unification
+			return "", fmt.Errorf("more specifiers (%d) than values (%d)", ix+1, len(values))
+		}
+
 		// check the type (if not a variant) matches what we have...
 		if typ == types.TypeVariant {
 			if values[ix].Type() == nil {

lang/interpret_test/TestAstFunc2/printfunificationerr0.txtar

@@ -0,0 +1,5 @@
+-- main.mcl --
+import "fmt"
+test fmt.printf("%d%d", 42) {} # should not pass, missing second int
+-- OUTPUT --
+# err: errUnify: only recursive solutions left