Fixes Issue 7420 - Return ErrorExp on error to avoid duplicate errors.

Force 32bit diagnostics for test-case.

src/optimize.c

@@ -180,6 +180,7 @@ Expression *fromConstInitializer(int result, Expression *e1)
                 !(v->storage_class & STCtemplateparameter))
             {
                 e1->error("variable %s cannot be read at compile time", v->toChars());
+                e->type = Type::terror;
             }
         }
     }

test/fail_compilation/diag7420.d

@@ -0,0 +1,25 @@
+// REQUIRED_ARGS: -m32
+/*
+TEST_OUTPUT:
+---
+fail_compilation/diag7420.d(3): Error: variable x cannot be read at compile time
+fail_compilation/diag7420.d(3):        while evaluating: static assert(x < 4)
+fail_compilation/diag7420.d(4): Error: variable y cannot be read at compile time
+fail_compilation/diag7420.d(4):        while evaluating: static assert(y == "abc")
+fail_compilation/diag7420.d(5): Error: variable y cannot be read at compile time
+fail_compilation/diag7420.d(5):        while evaluating: static assert(cast(ubyte[])y != null)
+fail_compilation/diag7420.d(6): Error: variable y cannot be read at compile time
+fail_compilation/diag7420.d(6):        while evaluating: static assert(cast(int)y[0u] == 1)
+fail_compilation/diag7420.d(7): Error: variable y cannot be read at compile time
+fail_compilation/diag7420.d(7):        while evaluating: static assert(y[0u..1u].length == 1u)
+---
+*/
+
+#line 1
+int x = 2;
+char[] y = "abc".dup;
+static assert(x < 4);
+static assert(y == "abc");
+static assert(cast(ubyte[])y != null);
+static assert(y[0] == 1);
+static assert(y[0..1].length == 1);