fix Issue 13921 - ICE with template instantiation error

src/mtype.c

@@ -6729,6 +6729,11 @@ void TypeInstance::resolve(Loc loc, Scope *sc, Expression **pe, Type **pt, Dsymb
     {
         //printf("s = %s\n", s->toChars());
         s->semantic(sc);
+        if (!global.gag && tempinst->errors)
+        {
+            *pt = terror;
+            return;
+        }
     }
     resolveHelper(loc, sc, s, NULL, pe, pt, ps, intypeid);
     if (*pt)
@@ -6754,7 +6759,8 @@ Type *TypeInstance::semantic(Loc loc, Scope *sc)
     if (!t)
     {
         if (!e && s && s->errors)
-        {   // if there was an error evaluating the symbol, it might actually
+        {
+            // if there was an error evaluating the symbol, it might actually
             // be a type. Avoid misleading error messages.
             error(loc, "%s had previous errors", toChars());
         }

src/template.c

@@ -2618,8 +2618,8 @@ FuncDeclaration *TemplateDeclaration::doHeaderInstantiation(
 
     if (global.endGagging(olderrors))
     {
-        assert(fd->type->ty != Tfunction);
-        return NULL;
+        if (fd->type->ty != Tfunction)
+            return NULL;
     }
     assert(fd->type->ty == Tfunction);
 

test/fail_compilation/ice11086.d

@@ -1,8 +1,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/ice11086.d(11): Error: template instance foo!A template 'foo' is not defined
-fail_compilation/ice11086.d(11): Error: foo!A had previous errors
+fail_compilation/ice11086.d(10): Error: template instance foo!A template 'foo' is not defined
 ---
 */
 

test/fail_compilation/ice13921.d

@@ -0,0 +1,27 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/ice13921.d(13): Error: undefined identifier undefined_identifier
+fail_compilation/ice13921.d(25): Error: template instance ice13921.S!string error instantiating
+---
+*/
+
+struct S(N)
+{
+    void fun()
+    {
+        undefined_identifier;
+        // or anything that makes the instantiation fail
+    }
+
+}
+
+void test(T)(S!T)
+{
+}
+
+void main()
+{
+    S!string g;
+    test(g);
+}