diff --git a/compiler/src/dmd/lexer.d b/compiler/src/dmd/lexer.d
index 9cce7c567234..c9c3ed18d7a9 100644
--- a/compiler/src/dmd/lexer.d
+++ b/compiler/src/dmd/lexer.d
@@ -1964,7 +1964,7 @@ class Lexer
     /***************************************
      * Get postfix of string literal.
      */
-    private void stringPostfix(Token* t) pure @nogc
+    private void stringPostfix(Token* t)
     {
         switch (*p)
         {
@@ -1973,6 +1973,13 @@ class Lexer
         case 'd':
             t.postfix = *p;
             p++;
+            // disallow e.g. `@r"_"dtype var;`
+            if (!Ccompile && isalpha(*p))
+            {
+                const loc = loc();
+                error(loc, "identifier character cannot follow string `%c` postfix without whitespace",
+                    p[-1]);
+            }
             break;
         default:
             t.postfix = 0;
@@ -2187,6 +2194,7 @@ class Lexer
         FLAGS flags = (base == 10) ? FLAGS.decimal : FLAGS.none;
         // Parse trailing 'u', 'U', 'l' or 'L' in any combination
         const psuffix = p;
+LIntegerSuffix:
         while (1)
         {
             FLAGS f;
@@ -2195,26 +2203,31 @@ class Lexer
             case 'U':
             case 'u':
                 f = FLAGS.unsigned;
-                goto L1;
+                break;
             case 'l':
-                f = FLAGS.long_;
                 error("lower case integer suffix 'l' is not allowed. Please use 'L' instead");
-                goto L1;
+                goto case;
             case 'L':
                 f = FLAGS.long_;
-            L1:
-                p++;
-                if ((flags & f) && !err)
+                break;
+            default:
+                // disallow e.g. `Foo!5Luvar;`
+                if (!Ccompile && flags >= FLAGS.unsigned && isalpha(*p))
                 {
-                    error("repeated integer suffix `%c`", p[-1]);
-                    err = true;
+                    const loc = loc();
+                    error(loc, "identifier character cannot follow integer `%c` suffix without whitespace",
+                        p[-1]);
                 }
-                flags = cast(FLAGS)(flags | f);
-                continue;
-            default:
-                break;
+                break LIntegerSuffix;
             }
-            break;
+            p++;
+            if ((flags & f) && !err)
+            {
+                error("repeated integer suffix `%c`", p[-1]);
+                err = true;
+            }
+            flags = cast(FLAGS)(flags | f);
+            continue;
         }
         if (base == 8 && n >= 8)
         {
@@ -2591,6 +2604,7 @@ class Lexer
             imaginary = true;
         }
 
+        bool gotSuffix = false;
         switch (*p)
         {
         case 'F':
@@ -2604,7 +2618,7 @@ class Lexer
             if (isWellformedString && !isOutOfRange)
                 isOutOfRange = Port.isFloat64LiteralOutOfRange(sbufptr);
             result = TOK.float64Literal;
-            break;
+            goto LcheckI;
         case 'l':
             if (!Ccompile)
                 error("use 'L' suffix instead of 'l'");
@@ -2616,13 +2630,22 @@ class Lexer
             result = TOK.float80Literal;
             break;
         }
-
+        gotSuffix = true;
+LcheckI:
         if ((*p == 'i' || *p == 'I') && !Ccompile)
         {
             if (*p == 'I')
                 error("use 'i' suffix instead of 'I'");
             p++;
             imaginary = true;
+            gotSuffix = true;
+        }
+        // disallow e.g. `Foo!5fvar;`
+        if (!Ccompile && gotSuffix && isalpha(*p))
+        {
+            const loc = loc();
+            error(loc, "identifier character cannot follow float `%c` suffix without whitespace",
+                p[-1]);
         }
 
         if (imaginary)
diff --git a/compiler/test/fail_compilation/templatesingleparam.d b/compiler/test/fail_compilation/templatesingleparam.d
new file mode 100644
index 000000000000..7e5a9368585b
--- /dev/null
+++ b/compiler/test/fail_compilation/templatesingleparam.d
@@ -0,0 +1,24 @@
+/*
+REQUIRED_ARGS: -vcolumns
+TEST_OUTPUT:
+---
+fail_compilation/templatesingleparam.d(17,14): Error: identifier character cannot follow string `c` postfix without whitespace
+fail_compilation/templatesingleparam.d(18,10): Error: identifier character cannot follow integer `U` suffix without whitespace
+fail_compilation/templatesingleparam.d(22,6): Error: identifier character cannot follow string `d` postfix without whitespace
+fail_compilation/templatesingleparam.d(23,4): Error: identifier character cannot follow float `f` suffix without whitespace
+---
+*/
+class Foo(alias str) {
+  enum STR = str;
+}
+
+class Bar {
+  Foo!q{foo}bb; // OK
+  Foo!q{foo}cc;
+  Foo!2LUNGS;
+}
+
+@`_`int i; // OK
+@`_`dint di;
+@2flong fi;
+@0xFeedObject obj; // not caught