lang: unification: Fix panic in struct/func cmp of partials

This was discovered by user aequitas. I modified his patch slightly, and
added some comments and a test.

lang/unification/simplesolver.go

@@ -154,6 +154,7 @@ func SimpleInvariantSolver(invariants []interfaces.Invariant, logf func(format s
 
 				if typ, exists := solved[eq.Expr1]; exists {
 					// wow, now known, so tell the partials!
+					// TODO: this assumes typ is a list, is that guaranteed?
 					listPartials[eq.Expr1][eq.Expr2Val] = typ.Val
 				}
 
@@ -211,6 +212,7 @@ func SimpleInvariantSolver(invariants []interfaces.Invariant, logf func(format s
 
 				if typ, exists := solved[eq.Expr1]; exists {
 					// wow, now known, so tell the partials!
+					// TODO: this assumes typ is a map, is that guaranteed?
 					mapPartials[eq.Expr1][eq.Expr2Key] = typ.Key
 					mapPartials[eq.Expr1][eq.Expr2Val] = typ.Val
 				}
@@ -281,6 +283,10 @@ func SimpleInvariantSolver(invariants []interfaces.Invariant, logf func(format s
 
 				if typ, exists := solved[eq.Expr1]; exists {
 					// wow, now known, so tell the partials!
+					// TODO: this assumes typ is a struct, is that guaranteed?
+					if len(typ.Ord) != len(eq.Expr2Ord) {
+						return nil, fmt.Errorf("struct field count differs")
+					}
 					for i, name := range eq.Expr2Ord {
 						expr := eq.Expr2Map[name]                            // assume key exists
 						structPartials[eq.Expr1][expr] = typ.Map[typ.Ord[i]] // assume key exists
@@ -351,6 +357,10 @@ func SimpleInvariantSolver(invariants []interfaces.Invariant, logf func(format s
 
 				if typ, exists := solved[eq.Expr1]; exists {
 					// wow, now known, so tell the partials!
+					// TODO: this assumes typ is a func, is that guaranteed?
+					if len(typ.Ord) != len(eq.Expr2Ord) {
+						return nil, fmt.Errorf("func arg count differs")
+					}
 					for i, name := range eq.Expr2Ord {
 						expr := eq.Expr2Map[name]                          // assume key exists
 						funcPartials[eq.Expr1][expr] = typ.Map[typ.Ord[i]] // assume key exists

lang/unification_test.go

@@ -473,6 +473,41 @@ func TestUnification1(t *testing.T) {
 			fail: true,
 		})
 	}
+	{
+		//test "t1" {
+		//	stringptr => getenv("GOPATH", "bug"),	# bad (two args vs. one)
+		//}
+		expr := &ExprCall{
+			Name: "getenv",
+			Args: []interfaces.Expr{
+				&ExprStr{
+					V: "GOPATH",
+				},
+				&ExprStr{
+					V: "bug",
+				},
+			},
+		}
+		stmt := &StmtProg{
+			Prog: []interfaces.Stmt{
+				&StmtRes{
+					Kind: "test",
+					Name: &ExprStr{V: "t1"},
+					Fields: []*StmtResField{
+						{
+							Field: "stringptr",
+							Value: expr,
+						},
+					},
+				},
+			},
+		}
+		values = append(values, test{
+			name: "function, wrong arg count",
+			ast:  stmt,
+			fail: true,
+		})
+	}
 
 	for index, test := range values { // run all the tests
 		t.Run(fmt.Sprintf("test #%d (%s)", index, test.name), func(t *testing.T) {